Acid and Bases

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What Is This Module About?

Have you ever wondered why your mother tells you not to drink soft drinks on an emptystomach? Or why your throat hurts when you throw up? Have you experienced hyperacidity?What do you do to relieve the pain? Have you ever heard of acid rain?

You can learn the answers to these questions in this module.

Acids and bases are important substances. Most of the materials that you use contain acidsand bases. There are also plenty of materials that are manufactured with the use of acids andbases. Different acids and bases also have substantial effects on you and your surroundings.Hence, it is important that you learn to recognize what acids and bases are.

This module contains three lessons. These are:

Lesson 1 – What Are Acids and Bases?

Lesson 2 – What Is pH?

Lesson 3 – How Do Acids and Bases Combine?

What Will You Learn From This Module?

After studying this module, you should be able to:

♦ enumerate the properties of acids and bases;

♦ differentiate between an acid and a base;

♦ relate pH to the acidity or basicity of a solution;

♦ describe what happens in a neutralization reaction;

♦ identify the products of a neutralization reaction; and

♦ name some uses of acids and bases in everyday life.

Wait!

Before you start studying this module, make sure that you have already read the followingmodules:

♦ Matter 1A: Forms, Properties and Changes

♦ Matter 1B: Forms, Properties and Changes

♦ Chemical Bonds: Forces That Link

♦ Chemical Reactions Around Us

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Let’s See What You Already Know

Before you continue reading this module, take the following test first to find out how wellyou know the topics to be discussed.

Encircle the letter of the correct answer.

1. Which of the following is an acid?

a. soft drink c. baking sodab. soap d. milk of magnesia

2. Which of the following best describes a base?

a. foul odor c. slippery to touchb. sour taste d. all of the above

3. Sodium hydroxide is a Brønsted-Lowry base because __________.

a. it accepts protons c. it releases H+ in water b. it donates protons d. it releases OH– in water

4. Which of the following is an acid?

a. oxide c. chlorideb. acetate d. hydronium

5. Which of the following is a product of a neutralization reaction?

a. acid c. waterb. base d. none of the above

6. Which of the following best describes a strong acid?

a. contains a hydroxyl groupb. contains two or more hydrogen atomsc. dissociates completely in waterd. slightly dissociates in water

7. Which of the following is present in batteries that are used in cars, trucks and otherautomobiles?

a. phenol c. cyanic acidb. acetic acid d. sulfuric acid

8. What is the neutral pH?

a. 2 c. 7b. 4 d. 9

9. At what pH is a substance considered corrosive?

a. 1 c. 7b. 4 d. 9

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10. If the amount of hydronium ions is increased, the pH will _____________.

a. increase c. remain the sameb. decrease d. none of the above

Well, how was it? Do you think you fared well? Compare your answers with those in theAnswer Key on page 35 to find out.

If all your answers are correct, very good! This shows that you already know much aboutthe topics in this module. You may still study the module to review what you already know. Whoknows, you might learn a few more new things as well.

If you got a low score, don’t feel bad. This means that this module is for you. It will help youunderstand some important concepts that you can apply in your daily life. If you study this modulecarefully, you will learn the answers to all the items in the test and a lot more! Are you ready?

You may go now to the next page to begin Lesson 1.

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LESSON 1

What Are Acids and Bases?

Do you know what acids and bases are? How can you distinguish an acid from a base? Youcan tell the difference between an acid and a base through their properties. If you know whatthese properties are, you will be able to identify even the acids and bases in your home.

In this lesson, you will learn what the properties of acids and bases are. You will also findout what acids and bases are commonly found in your home.

Let’s Try This

You will need the following materials:

vinegar dishwashing liquid/paste bath soap orange juice kalamansi toothpaste

Taste a small sample of each of the materials in the list. Make sure that you immediately spitout the inedible materials (namely, the bath soap, dishwashing liquid/paste and toothpaste) rightafter tasting them. Touch each material and feel its texture.

Let’s Think About This

1. What are the taste and texture of each of the materials that you tested? Write these inthe table below.

Material Taste Texture

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2. Based on the table on page 4, classify the materials into two groups, those with bittertaste and slippery texture, and those with sour taste and are not slippery.

_______________________ _______________________

_______________________ _______________________

_______________________ _______________________

_______________________ _______________________

Compare your answers with those in the Answer Key on page 35.

Let’s Learn

Most foods contain acids. Examples of these are citrus fruits like lemon, orange anddalandan. Acids taste sour. Do you now know why you are not to drink soft drinks when youhaven’t eaten anything yet? Soft drinks contain carbonic acid. When carbonic acid combines withthe other acids inside your stomach, you experience stomach pain. Now, why do you think yourthroat hurts when you throw up? When you vomit, the bile that you throw up containshydrochloric acid and other acids.

Coffee granules taste bitter, but when you add water, how do they taste? They taste slightlysour. This is because coffee also contains an amount of acid. Some substances exhibit their acidicproperty when they are dissolved in water. Coffee is one such substance. Another example ismilk. Powdered milk becomes slightly acidic when it is dissolved in water. But in this case, wecannot easily say that milk tastes sour. So, there must be some other ways of determining if asubstance is an acid or not.

Here are more examples of foods that contain acids.

How about bases? Most bases taste bitter and are slippery to touch. They are the maincomponents of soaps and other cleansing agents. Like acids, some substances become basic onlywhen they are dissolved in water. Baking soda is one such substance. When it is dissolved inwater, it becomes slippery to touch.

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Here are some examples of materials that contain bases. Do you recognize them?

Acids taste sour; bases taste bitter. However, you cannot use taste as a means ofdetermining whether a substance is an acid or a base. This is because there are some acids andbases that are corrosive, that is, touching or tasting them would be harmful. What then is a safeway of determining if a substance is an acid or a base?

You can use an indicator for this purpose. An indicator is a substance that changes in colorin the presence of an acid or a base. Litmus is a common indicator. It is a naturally occurring dyeextracted from lichen. Lichen is a type of vegetable. Strips of paper dipped in litmus can be usedas indicators.

We can use litmus paper to prove that a substance is an acid or a base. Litmus paper isused in hospitals to determine whether fecal (waste) matter for analysis is acidic or basic. This isbecause normal fecal matter should be slightly basic.

Remember the following:

Acids turn blue litmus paper to red.

BRA — Blue to Red, Acid

Bases turn red litmus paper to blue.

RBB — Red to Blue, Base

shampoo

soap

lotion

7


1. Write down other examples of acids that you can see at home.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

2. Write down other examples of bases that you can find at home.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

3. Camote tops in sinigang change color when tamarind extract is added. Camote topsare an example of an _______________.


Let’s Learn

Why do acids and bases have their distinct properties and characteristics? Among the firstto explain the behavior of acids and bases was Svante August Arrhenius. His theory, which iscalled the Arrhenius ionic theory, states that an acid is a substance that gives hydrogen ions(H+) on dissociation (release of ions) in water, and a base is a substance that gives hydroxyl ions(OH–) on dissociation in water. Thus, hydrochloric acid (HCl) is an acid and sodium hydroxide(NaOH) is a base. Let us see why:

HCl H+ + Cl–

NaOH Na+ + OH–

In the presence of water, HCl dissociates into two ions, hydrogen ion and chloride. Thus, itis considered an acid.

In the presence of water, NaOH dissociates into two ions, sodium ion and hydroxyl ion.Thus, it is considered a base.

In the light of recent knowledge on Arrhenius’s free hydrogen ion, it has been found thathydrogen ion cannot exist in aqueous (water) solution. The hydrogen ion must be completelysurrounded by water. Therefore, it is actually a hydronium ion that is formed, not a hydrogen ion.Look at the following dissociation reaction:

H2O H+ + OH–

2H2O H

3O+ + OH–

Notice the double-headed arrows in both chemical equations. This indicates that thereactions are reversible, that is, they can take place the other way around. For example, thehydronium ion and hydroxyl ion can combine to form water molecules.

H2O

H2O

8

We can also say that an Arrhenius acid is a substance that upon dissociation can increasethe concentration of the solvent cation, H

3O+. Likewise, an Arrhenius base is a substance that can

increase the concentration of the solvent anion, OH–.

In the Arrhenius ionic theory, the role played by water as solvent is central to acid-basebehavior. However, it was proven later that there are acid-base reactions that occur among gasesand in solvents that cannot be ionized, such as benzene.

Before we go any further, let us first recall the definitions of the terms mentioned in thepreceding section. Can you recall what an ion is? An ion is an atom that has either gained or lostan electron. A positive ion is called a cation, while a negative ion is called an anion.

Danish chemist Johannes Brønsted and English chemist Thomas Lowry each offered a setof new definitions of an acid and a base.

According to the Brønsted-Lowry theory, an acid is any substance that donates a protonto another substance, and a base is any substance that can accept a proton from any othersubstance. In other words, an acid is a proton donor while a base is a proton acceptor.

This redefinition does not really contradict the Arrhenius definition. An Arrhenius acid is aBrønsted-Lowry acid that has dissociated in water by transferring its proton to the solvent.

H+A– + H2O A– + H

3O

An Arrhenius base is also a Brønsted-Lowry base because it accepts a proton when itreacts with an acid. The difference is that it is no longer the only base that can exist in water. Anyspecies, molecular or ionic, as long as it can accept a proton, whether in solution or in a gas-phase reaction, is considered a base.

H+Cl– + NH3

Cl– + NH4+

H3O+ + Cl– H

2O + HCl

Let’s Study and Analyze

Let us take a look at the dissociation of nitric acid (HNO3) in water:

HNO3

H+ + NO3–

Take note that HNO3 gives up the proton H+.

Therefore, HNO3 is a Brønsted-Lowry acid.

H+ then combines with water:

H+ + H2O H

3O+

base(proton acceptor)

base(proton acceptor)

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The water molecule accepted the proton from HNO3.

Therefore, H2O can also act as a base because it follows the definition of a Brønsted-Lowry

base.

Let’s Review

Hydrochloric acid (HCl) dissociates in water in the following manner:

HCl + H

2O

H

3O+

+ Cl–

What do you think will happen to the hydrogen in HCl?__________________________________________________________________________________________________________________________________________

Which is the acid? Why?__________________________________________________

Which is the base? Why?__________________________________________________

Compare your answers with the following:

The hydrogen in HCl was transferred to H2O.

Thus, the acid is HCl because it donated a proton to water.

The base is H2O because it accepted a proton from HCl.

Let’s Learn

Hydrogen carbonate ion (HCO3–) is one of the species present in lakes and rivers. It

dissociates in water to form a carbonate ion (CO32–) and a hydronium ion (H

3O+).

HCO3– + H

2O H

3O+ + CO

32–

Notice that HCO3– lost its hydrogen to form CO

32–. H

2O, on the other hand, gained the

hydrogen to form H3O+.

Which of them is the base? If you answered H2O, then you’re right! This is because H

2O

received the hydrogen ion, which is the proton. HCO3– is the acid because it released the proton.

As you can see from the previous examples, bases are not limited to substances containingthe hydroxyl group. Any substance that can accept a proton is considered a base. Examples ofthis are:

♦ ammonia (NH3)

♦ metal oxides, such as sodium oxide (Na2O) and magnesium oxide (MgO)

♦ anions, such as chloride (Cl–) and cyanide (CN–)

acid

base

10

There are still many things you need to know about acids and bases. Before you proceed tothe next lesson, do this simple activity first.

Let’s Try This

You can make a homemade acid-base indicator. Just do this simple activity to find out howyour indicator works.

Get six substances in your home that you think may contain acids or bases. These mayinclude baking soda, soft drink, butter, pineapple, shampoo and milk.

Try to get three or more of the following:

cosmos flowers pink gumamela flowerssantan flowers black beansyellow bell flowers mayana leaves

In a bowl, press the santan flowers with a spoon. Add a little water until you get at least onetablespoon of the extract. The extract will be your indicator.

Get a piece of white paper and a sheet of plastic cover/cellophane. Draw six small circles onthe paper and write the name of each of the materials you will test near each circle. Cover thepaper with the plastic cover/cellophane. See the diagram below.

1. On each circle, put three drops of the material to be tested. Dissolve a small amount ofthe solids in water before you put them on the circles.

2. Add a drop of the santan extract on each circle. Observe whether there will be achange in color of the extract.

Follow the same procedure for extracting an indicator from each of the plant parts listed.Test each indicator on all the materials. Classify the materials into two groups based on the colorchanges in the indicators.

To determine which group is the acid or the base, do the taste test (except for the shampoo)and determine the texture of each material. Make a table similar to that in “Let’s Try This” onpage 4.

pineapple milk baking soda

butter soft drink shampoo

white paper

cellophane

ss

11

If you want to know if you got the right results, compare the color changes of yourindicators with the expected colors in the table below:

If you did not get the same results, it may be because the acids and bases in your materialsare not easily detected.

Show your results to your Instructional Manager or Facilitator.

Let’s See What You Have Learned

A. Identify the acids and the bases in the following reactions:

1. HCN + H2PO

4– CN– + H

3PO

4

2. HS– + OH– S2– + H2

3. H2O + NH

2– OH– + NH

3

4. H2O + HCl H

3O+ + Cl–

5. NH3 + H

2O NH

4+ + OH–

B. Answer the following questions:

1. What is the difference between the Arrhenius ionic theory and the Brønsted-Lowry theory?_______________________________________________________________________________________________________________________________________________________________________________________

2. Why do you need to use an indicator when testing if a substance is an acid or abase?_______________________________________________________________________________________________________________________________________________________________________________________

Compare your answers with those in the Answer Key on pages 35 and 36. Did you get aperfect score? If you did, that’s very good! If you did not, don’t worry. Just review the parts ofthis lesson that you did not understand very well. Afterward, you may move on to Lesson 2.

Indicator Acid

Gumamela Pink/colorless Light

Santan Pale orange Yello

Cosmos Yellow orange Red

Yellow bell Light yellow Dark

Mayana Light pink Light

Black beans Light yellow Dark

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Let’s Remember

♦ Acids have a sour taste. They turn blue litmus paper to red.

♦ Bases have a bitter taste and are slippery to touch. They turn red litmus paper to blue.

♦ An Arrhenius acid donates a hydrogen ion or hydronium ion while an Arrhenius basedonates a hydroxyl ion.

♦ A Brønsted-Lowry acid donates a proton while a Brønsted-Lowry base accepts aproton.

♦ An indicator is a substance that changes color in the presence of an acid or a base.

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LESSON 2

What Is pH?

How can you know how acidic or basic a substance is? For example, when do youconsider rain acidic? How will you know if a substance is corrosive without touching it?

These questions will be answered in this lesson.

In this lesson, you will find out what pH is and how it is used to measure how acidic or basica substance is. You will also find out what instruments or tools are used for measuring pH. Inaddition, you will also find out the pH of some common materials. Are you eager to learn allabout pH? Read on!


Even rain can be acidic. How could this happen? Gases such as sulfur dioxide (SO2),

nitrogen dioxide (NO2) and carbon dioxide (CO

2) that are emitted from vehicles and factories

can cause rain to become acidic. When SO2, NO

2 and CO

2 go up into the atmosphere, they

eventually become sulfuric acid (H2SO

4), nitric acid (HNO

3) and carbonic acid (HCO

3). These

acids combine with water droplets in the clouds. These water droplets then become acid rain.

Air pollution

Traffic exhaust and smoke fromfactories, power stations and firesare carried into the air.

Acidified clouds

Pollutants combine with atmosphericmoisture to form sulfurous, sulfuric, nitricand carbonic acids.

Acid rain

Damaged forest

Uneven, sparse foliageunable to performphotosynthesisefficiently.

Deposition

Some particles do not mixwith moisture, but comedown as “acid rain” in aharmful process calleddeposition.

Damaged root systems areunable to collect nutrients orsupport trees in windstorms.

Dead lake

Acid disrupts thedelicate balanceof the lakeecosystem andeventually kills allorganisms.

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Acid rain damages trees and foliages. It combines with waters from rivers and lakes andkills fishes and other aquatic organisms. Acid rain also damages structures that are made ofmarble and other artifacts. Shown below is a statue before and after it was exposed to acid rain.


1. Do you live in a place where acid rain falls? Why do you say so?________________________________________________________________________________________________________________________________________________________

2. How do you think acid rain can affect you?________________________________________________________________________________________________________________________________________________________

3. How can you help prevent acid rain?________________________________________________________________________________________________________________________________________________________

Have your Instructional Manager or Facilitator check your answers.

Let’s Learn

pH of Acids and Bases

How can you determine if the rain that falls on your area is acidic or not? What if the sourtaste of the acid is not easily detected? This is where the concept of pH comes in. So how do wedefine pH? pH stands for “power of hydrogen.”

Can you see a connection between acidity and pH? Do you remember that acids arecapable of releasing hydronium ions in water? Well, pH is the measure of the amount ofhydronium ions present in a solution.

A marble statue before and after exposure to acid rain

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We can understand this concept better by getting the pH of pure water through thedissociation of water. Water is said to be amphiprotic or amphoteric, meaning it can act both asan acid and a base. It can release a proton in the presence of a weaker acid and it can accept aproton in the presence of a stronger acid.

The hydrogen in water constantly transfers from one molecule of water to another. Themolecules of water react this way:

H2O + H

2O H

3O+ + OH–

Water dissociates very slightly. Not all water molecules give up their hydrogen ions insolution. Because of this, a small amount of H

2O is still present in the solution. The hydrogen ions

that are released combine with the molecules of H2O to form H

3O+. Each H

3O+ will then combine

with another to form H2O molecules and OH–. Therefore, tests and computations can be made to

determine the amount of H3O+ in the solution.

Computations show that in 1 liter of pure water, there is 0.0000001 mole of H3O+, which is

also the amount of OH– ions because they have a 1:1 ratio in the balanced equation for thedissociation reaction. Another way of writing this value is through scientific notation:

1 × 10–7 or 10–7 mole of H3O+ per liter of water

1 × 10–7 or 10–7 mole of OH– per liter of water

Because the amount of positive ions and negative ions are equal in the solution, pure water issaid to be neutral, meaning it carries no electric charge and is therefore neither an acid nor abase. This is because the cations cancel out the anions in the solution. This is like having an equalnumber of boys and girls in a room. Let’s say the boys are negatively charged and the girls arepositively charged. You cannot say that there are more boys than girls. In the same way, youcannot say that pure water is an acid or a base.

To make the values simpler, when water has a concentration of 1 × 10–7 or 10–7 mole ofH

3O+ ions per liter of water, the water is said to have pH 7.

Aside from pH, the concept of pOH is also used. In this case, it is the concentration ofhydroxide ions that is measured.

Can we therefore say that water has pOH 7? Yes, of course because we have equalamounts of hydronium ions and hydroxyl ions in the solution.

Therefore we can add the pH and the pOH of pure water to come up with:

pH + pOH = 14

We can use this formula to get the pH of a substance if the pOH is given. We can also usethis to get the pOH if the pH is given.

What do you think will happen to the pH of pure water in the clouds when sulfuric acidcombines with the water? Will the pH of the resulting rainwater be higher or lower than that ofpure water? Let’s find out.

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Remember that pH is dependent on the concentration of hydronium ions present in thesolution. So when an acid is added to water, the concentration of hydronium ions will increase.The value may change from 0.0000001 mole to 0.000001 mole of hydronium ions. In scientificnotation, the value will change from 1 × 10–7 to 1 × 10–6. The pH will then be 6.

So, will the pH of the acidic rainwater be higher or lower than that of pure water? It will belower.

How about pOH? At the start, we had equal amounts of hydronium and hydroxyl ions inthe solution. But the amount of hydronium ions increased, therefore the amount of hydroxide ionsdecreased. If the pH of rainwater is 6, what could be its pOH?

From the formula.

pH + pOH = 14

we can derive the formula for pOH.

We just subtract the pH value from 14.

pOH = 14 – pH

= 14 – 6

= 8

Therefore, the pOH of rainwater is 8.

If the pOH of urine is 9, what must be its pH?

To answer this question, we just subtract the pOH value from 14.

pH = 14 – pOH

= 14 – 9

= 5

The pH of urine is 5.

Let’s Review

1. The pH of wine is 3. What must be its pOH?_________________________________________________________________

2. The pOH of detergent is 5. What is its pH?

_________________________________________________________________

Are your answers the same as the following?

1. If the pH of wine is 3, then its pOH is 14 – 3 = 11.

2. If the pOH of detergent is 5, then its pH is 14 – 5 = 9.

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Study the following diagram.

Write increase or decrease.

1. What happens to the pOH of a substance when pH is increased? ______________

2. What happens to the pH if pOH is increased? ______________

Based on the graph and on the calculations, when you increase the pH, the pOH willdecrease. And if you increase the pOH, the pH will decrease. When you add the pH and thepOH of one solution, the sum should always be 14.

Let’s Learn

A substance is said to be acidic if the amount of hydronium ions is more than 1 × 10–7 or ifthe pH is less than 7. A substance is basic if the amount of hydronium ions is less than 1 × 10–7 orif the pH is more than 7. To understand this a lot better, do the activity on the next page.

A. pH = 8 pOH = 6

B. pH = 7 pOH = 7

C. pH = 6 pOH = 8

Comparison of the pH and pOH in solution

A B C

pH

pOH

18


Imagine yourself standing on a bridge connecting two barangays, Barangay Mahalimuyakand Barangay Maaliwalas. You’re facing Barangay Mahalimuyak and behind you is BarangayMaaliwalas.

1. If you walk forward, where will you be? ________________________

2. If you walk backward, where will you be? ________________________

Are your answers the same as these?

1. If you walk forward, you will be in Barangay Mahalimuyak.

2. If you walk backward, you will be in Barangay Maaliwalas.

Let’s Learn

Now, think of the bridge as the pH of a neutral substance, which is pH 7. If you go forward,the substance will become basic. The pH of a base can range from 8 to 14. If you go backward,the substance will become acidic. The pH range of an acid is from 1 to 6.

Putting it simply:

pH = 7 — neutral because the concentration of hydronium ions and the concentration ofhydroxyl ions are equal

pH < 7 — acidic solution because the concentration of hydronium ions is greater than that ofthe hydroxyl ions

pH > 7 — basic solution because the concentration of the hydronium ions is less than that ofthe hydroxyl ions

Barangay Mahalimuyak Barangay Maaliwalas

ss

19


Using the preceding explanation, complete this number line. Write the values that wouldcorrespond to acidic and basic pH.

The number line that you completed above is the pH scale. A more detailed pH scale willlook like this:

Notice that the pH scale can be used in expressing how acidic or how basic a substance is.Therefore, there is no need for a pOH scale. But of course, you must also be familiar with theconcept of pOH.

So how do we measure pH?

Let’s Learn

Measuring pH

We can get the pH of a substance if the concentration of the ions in the solution is given. Butwe must calculate the pH. There are other ways of getting the pH without solving for it. The mostcommon materials used are the pH paper and pH meter.

The pH paper is a strip of paperthat can change colors. The coloredportion is very sensitive to changes inpH. If you dip the paper in an acidic orbasic solution, the color of the paper willchange. You can check the pH of thesolution by comparing the new color ofthe paper with a standard range ofcolors and their corresponding pHvalues.

ss

increasing acidity

neutral pH

decreasing acidity

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

s

s s

pH = 7

pH < 7.0 pH > 7.0

ss

acidic pH neutral pH basic pH

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If you want to get an accurate value of pH,you can use a pH meter. This instrumentconsists of a voltmeter connected to an electrodedipped in the solution. The pH reading will showon the instrument.

This is what a pH meter looks like:

Why do you need to get the pH of asubstance? You already learned that differentsubstances have different pH values. The pHvalue can tell you how strong an acid or a baseis. Substances with pH values lower than 2 andhigher than 12 are corrosive. Examples of substances that are corrosive are toxic wastes andconcentrated acids and bases. Knowing the pH of a substance will also tell you how safe it is touse that substance. You surely would not use a soap that has a very high pH, would you? Itwould surely be harmful to your skin. You would want to use a soap with a pH that is nearer thatof your own body.


Many of the materials that we encounter at home contain acids and bases. Here are some ofthem and their corresponding pH values.

Corrosive

Lemon juiceVinegar

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Acidrain

Beer

Tomatojuice

Tapwater

Corrosive

SodaWine

MilkSaliva

Blood

Detergentss

Householdammonias

Urine

21

1. Which substance is the most acidic?_________________________________________________________________

2. Which substances are neutral or nearly neutral?_________________________________________________________________

3. Which substance is the most basic?

_________________________________________________________________

Based on the table on the previous page, the most acidic substance is lemon juice. Why?Because lemon juice has the lowest pH. The substances that are neutral or can be neutral are tapwater, urine and saliva because they are all near the pH 7 mark. Recall that the neutral pH is 7.Lastly, the most basic substance is the household ammonia because it has the highest pH.


A. Fill in the blanks.

1. Adding acid to pure water will ___________ the concentration of the hydroniumions and ________ the pH of water.

2. Adding a base to pure water will __________ the concentration of thehydronium ions and _____________ the pH of water.

3. A substance is corrosive if it has a pH of ______ or _______.

4. A substance is an acid if it has a pH _________ 7.

5. A substance is a base if it has a pH _________ 7.

6. A ______________ is an instrument for accurately measuring the pH of asolution.

7. Blood is __________ because it has a pH that is almost equal to 7.

B. Name the ways by which you can tell if a substance is an acid or a base.

______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Compare your answers with those in the Answer Key on page 36. Did you get a perfectscore? If you did, that’s very good! If you did not, don’t worry. Just review the parts of thislesson that you did not understand very well. Afterward, you may proceed to Lesson 3.

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Let’s Remember

♦ The pH of a solution is a measure of the concentration of hydronium ions in thatsolution while the pOH is a measure of the concentration of hydroxyl ions in thesolution.

♦ The higher the hydronium ion concentration, the lower the pH.

♦ If the pH is equal to 7, the solution is neutral. If the pH is less than 7, the solution isacidic. If the pH is greater than 7, the solution is basic.

♦ The pH paper and pH meter are used in getting the pH of a substance.

♦ Substances that have pH lower than 2 and higher than 12 are corrosive.

23

LESSON 3

How Do Acids and Bases Combine?

What happens when you eat too much, drink too much soft drink or do not eat anything atall? Sometimes you suffer from hyperacidity. What do you do when you experience hyperacidity?Usually, you take an antacid to relieve the pain, right? What do you think happens inside yourstomach when you take an antacid? How is the antacid able to relieve the pain?

You will learn the answers to these questions and some more as you study this lesson.

This lesson will tell you what happens when an acid reacts with a base. You will find outwhat a strong acid, weak acid, strong base and weak base are. Are you ready to learn more?Read on!


Look at the following pictures:

1. What happened to the two couples?_________________________________________________________________

2. Who ended up with each other?

_________________________________________________________________

2

Raymund + Hazel Raymund – Hazel

Glenn + Kate Glenn – Kate

ss

+

+

Glenn – Hazel Glenn + Hazel

s

+

1

Raymund – Kate Raymund + Kate

s

+

24

The first couple separated. This is also what happened to the second couple. Raymund andHazel, and Glenn and Kate ended up together.

This kind of thing is something you see in the movies and sometimes even in real life. But didyou know that this kind of separation and combination can be compared to reactions betweenacids and bases?

Let’s Learn

Let’s apply the analogy of the two couples to the reaction between an acid and a base. Letus assume that the first couple is an acid. The boy is a hydrogen ion and the girl is a chloride ion.The second couple will then be a base. The boy is a sodium ion and the girl is a hydroxyl ion.

In the same manner that each of the two couples separated, the acid and the basedissociate in water. Do you remember the term dissociation? An acid or a base is said todissociate if it forms ions in solution. Another term that is used for dissociation is ionization.

Like the couples in the illustration above, we can write the dissociation of the acid and thebase in this way:

Raymund-Kate Raymund + Kate

HCl H+ + Cl–

Glenn-Hazel Glenn + Hazel

NaOH Na+ + OH–

If you combine the acid and base, the positive ion of the acid will combine with the negativeion of the base:

Raymund + Hazel Raymund-Hazel

H+ + OH– H2O

Likewise, the positive ion from the base will combine with the negative ion from the base:

Glenn + Kate Glenn-Kate

Na+ + Cl– NaCl

Glenn – Hazel Glenn + Hazel

s

+

Raymund – Kate Raymund + Kate

s

+

ACID BASE

25

We can write the reaction as:

Raymund + Kate + Glenn + Hazel Raymund-Hazel + Glenn-Kate

H+ + Cl– + Na+ + OH– H+ OH– + Na+ Cl–

The overall reaction will then be:

HCl + NaOH H2O + NaCl

The products are water and sodium chloride (NaCl), a salt. A salt consists of a metalcation and a monoatomic (with one atom) or polyatomic (with more than one atom) anion.

The combination of HCl and NaOH is a neutralization reaction. Why is it calledneutralization? It is a neutralization reaction because water is a product of the reaction. Water isneutral because the concentration of hydronium ions and that of hydroxyl ions are equal.

Not all acids and bases form water when combined. Only those acids and bases thatcompletely dissociate in solution, as well as some oxides, undergo neutralization reactions. Whatdo we call these acids and bases that completely dissociate in water?

An acid that completely dissociates in water is a strong acid.

All the hydrogen ions in the solution are converted to hydronium ions.

A base that completely dissociates in water is a strong base.

If we have strong acids and bases, we also have weak acids and bases.

A weak acid dissociates slightly in water. This means only a fractionof the acid molecules release hydrogen ions in water.

A weak base is one that dissociates slightly in water.

Examples of weak acids are acetic acid, which is found in vinegar, and citric acid, which isfound in citrus fruits.

Ammonia is an example of a weak base. It is commonly found in cleansing agents and hairdyes.

26

We are going to study the reactions between strong acids and bases and strong acids andoxides. Here is a list of all the strong acids and bases:

STRONG ACIDS STRONG BASES

Formula Name Formula Name

HClO4

perchloric acid LiOH lithium hydroxide

H2SO4 sulfuric acid* NaOH sodium hydroxide

HBr hydrobromic acid KOH potassium hydroxide

HCl hydrochloric acid RbOH rubidium hydroxide

HNO3 nitric acid CsOH cesium hydroxide

H3PO4 phosphoric acid TlOH thallium (II) hydroxide

HF hydrofluoric acid Ca(OH)2

calcium hydroxide

Hl hydroiodic acid Sr(OH)2 strontium hydroxide

Ba(OH)2 barium hydroxide

Now let’s predict the products of the neutralization reaction between nitric acid (HNO3) and

potassium hydroxide (KOH). KOH is also used in the manufacture of soap.

HNO3

+ KOH ?

Let us first write the ions that will be formed from the dissociation of HNO3 and KOH:

HNO3

H+ + NO3–

KOH K+ + OH–

Combining and exchanging the ions we get:

H+ + NO3– + K+ + OH– H+ OH– + K+ NO

3–

Because the reaction is a neutralization reaction, the products are water and a salt.

HNO3 + KOH H

2O + KNO

3

Take note that what we did was exchange the ions. The positive ion of the acid which is H+

combined with the negative ion of the base, which is OH–, to form water. The positive ion of thebase, K+, combined with the negative ion of the acid, NO

3–, to form potassium nitrate (KNO

3), a

salt.

*Only the first proton in sulfuric acid completely dissociates in water. The first product of dissociation,HSO4

–, is a weak acid.

27


Sulfuric acid is very corrosive. It is commonly used in batteries for cars, trucks and otherautomobiles. It is also used as a dehydrating agent (one that has the ability to remove water).

We can neutralize sulfuric acid (H2SO

4) by adding a strong base such as strontium

hydroxide [Sr(OH)2] in the solution. A neutralization reaction is possible because both are

included in the list of strong acids and bases. The neutralization reaction is as follows:

H2SO

4 + Sr(OH)

2 ?

H2SO

4 is completely dissociated in water:

H2SO

4 H+ + HSO

4–

The dissociation of Sr(OH)2 is:

Sr(OH)2

Sr2+ + OH–

Combining all the ions in the solution, we get:

H+ + HSO4– + Sr2+ + OH– ?

To get the products, we just exchange the ions from the acid and the base. Take note that apositive ion should combine with a negative ion:

H+ + HSO4– + Sr2+ + OH– H+ OH– + Sr2+ HSO

4–

The overall reaction will then be:

H2SO

4 + Sr(OH)

2 H

2O + Sr(HSO

4)

2

To balance the reaction, we put 2 before H2SO

4 and 2 before H

2O:

2H2SO

4 + Sr(OH)

2 2H

2O + Sr(HSO

4)

2

28

Let’s Review

Predict the products of the neutralization reaction between perchloric acid (HClO4), used in

bleaching agents and lithium hydroxide (LiOH).

1. Write the reactants first:

_________________________________________________________________

2. Write the equation for the dissociation of HClO4: ____________________________

3. Write the equation for the dissociation of LiOH: ____________________________

4. Now write all the ions formed on the reactant side and the new pairs of ions on theproduct side:

_________________________________________________________________

5. Write the overall reaction:

_________________________________________________________________


Let’s Read

Antacids are drugs that reduce the amount of excess acid in the digestive tract. The acidpresent in your stomach is hydrochloric acid (HCl). Small amounts of hydrochloric acid aresecreted by cells in the lining of your stomach to aid in food digestion.

Hyperacidity occurs when there is excess HCl secreted by the cells or when you take in toomuch food that contains acids, like soft drink and coffee.

How do antacids relieve the pain of hyperacidity? Antacids contain magnesium carbonate.Magnesium carbonate (MgCO

3), in the presence of heat (∆), decomposes to magnesium oxide

(MgO) and carbon dioxide (CO2) in the following reaction:

MgCO3 ∆ MgO + CO

2

29

The magnesium oxide will then react with the HCl in your stomach. Remember that evenoxides can act as bases because they can accept protons from acids. The neutralization reactionbetween magnesium oxide and hydrochloric acid is:

Mg2+ + O2– + 2H+ + 2Cl– H2O + MgCl

2

or simply

MgO + 2HCl H2O + MgCl

2

Now you can see that the antacid neutralizes the acid in your stomach and thus relieves thepain.

Let’s Review

1. List three foods that you must avoid eating when you experience hyperacidity.___________________________________________________________________________________________________________________________________________________________________________________________________

2. Why is magnesium oxide (MgO) considered a base?___________________________________________________________________________________________________________________________________________________________________________________________________

Compare your answers with the following:

1. When you experience hyperacidity, you should avoid eating foods that contain toomuch acid. Some of these are citrus fruits, mangoes and guavas. You should also avoiddrinking coffee, soft drinks and fruit juices. Why? Because the acid content of thesefoods will only add to the excess acid in your stomach.

2. Magnesium oxide is considered a base because it can accept a proton from an acid.

Here’s something to keep in mind:

Hyperacidity should be treated immediately. Regular occurrence of hyperacidity may resultin ulcer, which occurs when the acid slowly destroys the lining of the stomach.

30

Let’s Review

Calcium carbonate (CaCO3), a component of marble, decomposes to form carbon dioxide

(CO2) and calcium oxide (CaO) when exposed to acid rain that contains nitric acid (HNO

3).

CaCO3 ∆ CaO + CO

2

A neutralization reaction will then occur between nitric acid and calcium oxide. Completethe neutralization reaction:

Ca2+ + O2– + 2H+ + 2NO3

– _________ + Ca2+ NO3–

or

CaO + 2HNO3

________ + ___________

What should be written on the lines? On the first line, you just pair up the hydrogen ion andthe oxide ion (H+ O2–).

Then write the compounds formed by the reaction on the next two lines. The products areH

2O and Ca(NO

3)

2.


Complete the following neutralization reactions:

1. HNO3 + Ba(OH)

2

2. H3PO

4 + NaOH

3. HClO4 + LiOH

4. HBr + Ca(OH)2

5. HI + Mg(OH)2

6. HCl + Sr(OH)2

7. KOH + H2SO

4

8. HF + Mg(OH)2

9. RbOH + HBr

10. CsOH + H3PO

4

Compare your answers with those in the Answer Key on pages 36 and 37. Did you geteverything right? If you did, that’s very good! If you did not, just review the parts of the lessonthat you did not understand very well before you move on to the next part of this module.

31

Let’s Remember

♦ A strong acid completely dissociates in water. All the hydrogen ions are converted tohydronium ions.

♦ A strong base is one that completely dissociates in water.

♦ A weak acid is one that slightly dissociates in water.

♦ A weak base slightly dissociates in water.

♦ A neutralization reaction occurs between a strong acid and a strong base or astrong acid and some metal oxides.

♦ Water and salt are the products of a neutralization reaction.

Well, this is the end of the module. Congratulations for finishing it. Did you enjoying studyingit? Did you learn a lot from it? The following is a summary of its main points to help youremember them better.

Let’s Sum Up

This module tells us that:

♦ Acids are substances that are capable of giving up a proton to other substances. Theyusually taste sour and can turn blue litmus paper to red.

♦ Bases are substances that accept a proton from other substances. They taste bitterand are slippery to touch. They can turn red litmus paper to red.

♦ An indicator is a substance that changes color in the presence of an acid or a base.

♦ A strong acid completely dissociates in water. All the hydrogen ions are converted tohydronium ions.

♦ A strong base is one that completely dissociates in water.

♦ A weak acid is one that slightly dissociates in water.

♦ A weak base slightly dissociates in water.

♦ A neutralization reaction occurs between a strong acid and a strong base or astrong acid and some metal oxides.

♦ Water and salt are the products of a neutralization reaction.

32

What Have You Learned?

A. Encircle the letter of the correct answer.

1. Which of the following is not true about lemon juice?

a. It tastes sour.b. It has an acid content.c. It has a pH below 7.d. It turns red litmus paper to blue.

2. Nerissa used too much muriatic acid on her kitchen sink. Which of the followingshould she use to neutralize the acid and remove the smell?

a. milkb. saltc. vinegard. baking soda

3. Perchloric acid (HClO4) is used as a bleaching agent. It dissociates in water to

form:

HClO4 + H

2O H

3O+ + ClO

4–

Which among the four species is the strongest acid?

a. HClO4

b. H2O

c. H3O+

d. ClO4–

4. Which of the following best describes a Brønsted-Lowry base?

a. accepts protonsb. accepts hydrogen ionsc. contains a hydroxyl groupd. releases hydroxyl ions in water

5. Phenolphthalein is a substance that changes color when it is added to a soapsolution. Phenolphthalein is an example of a/an _____________.

a. acidb. basec. indicatord. litmus paper

6. Which of the following is most effective in determining how acidic or basic asubstance is?

a. litmus paperb. pH paperc. pH meterd. phenolphthalein

33

7. Which of the following is commonly used in the manufacture of soap?

a. HCl c. KOHb. HNO

3d. NaCl

8. When nitric acid (HNO3), a strong acid, and sodium hydroxide (NaOH), a

strong base, combine, they will form _________________.

a. H+ + NO3–

b. Na+ + OH–

c. H2O + NaNO

3

d. H2O + NaOH

9. Which of the following substances is the most corrosive?

a. detergent, pH = 9.5b. acid rain, pH = 4c. household ammonia, pH = 11d. concentrated HCl, pH = 1.5

10. If the pH of soda is 3, what is its pOH?

a. 3b. 5c. 7d. 11

B. Fill in the blanks.

1. An acid changes ________ litmus paper to _______.

2. A base changes _______ litmus paper to _______.

3. The ____________ is an instrument for accurately measuring the pH of asolution.

4. If the pOH of vinegar is 9, then its pH is _________, which means that it is a/an___________.

5. If the pH of milk is 5, then its pOH is ________, which means that it is a/an__________.

6. A/An ______________ base is one that forms hydroxyl ions in water.

7. Ca(OH) 2 + ____________ 2H

2O + CaCl

2

8. ___________ + H3PO

4 H

2O + NaH

2PO

4

9. MgO + 2HI _____________ + ______________

10. _________ and _______ are the products of a neutralization reaction.


34

If your score is:

17–20 Great! You learned a lot from this module. You may now study a new module.

11–16 Good! Just review the items that you did not get right.

6–10 Review the parts of the module that you did not understand very well.

0–5 You should study the whole module again.

35

Answer Key

A. Let’s See What You Already Know (pages 2–3)

1. a 6. c

2. c 7. d

3. a 8. c

4. d 9. a

5. c 10. b

B. Lesson 1

Let’s Think About This (pages 4–5)

1.

2. Vinegar, kalamansi and orange juice are grouped together because they alltaste sour and are not slippery to touch.

Bath soap, dishwashing liquid and toothpaste all taste bitter and are allslippery to touch so they should all be grouped together.

Let’s Think About This (page 7)

1. The following are possible answers: Muriatic acid, tamarind, guava, pineapplejuice and tomato sauce are some examples of acids found at home.

2. Baking soda, lye, toothpaste, shampoo, detergent and bath soap are bases thatcan be found at home.

3. Camote tops are an example of an indicator.

Let’s See What You Have Learned (page 11)

A. 1. The acid is HCN and the base is H2PO

4–.

2. The acid is HS– and the base is OH–.3. The acid is H

2O and the base is NH

2–.

4. The acid is HCl and the base is H2O.

5. The acid is H2O and the base is NH

3.

Material Taste Texture

Vinegar Sour Not slippery

Bath soap Bitter Slippery

Kalamansi Sour Not slippery

Dishwashing liquid/paste

Bitter Slippery

Orange juice Sour Not slippery

Toothpaste Bittersweet Slippery

36

B. 1. According to the Arrhenius theory, an acid is a substance that giveshydrogen ions on dissociation in water and a base is a substance that giveshydroxyl ions upon dissociation in water; according to the Brønsted-Lowrytheory, on the other hand, an acid is a proton donor while a base is a protonacceptor. The Brønsted-Lowry definition actually covers the Arrhenius acidand base.

2. You need to use an indicator when testing if a substance is an acid or a basebecause some acids and bases are harmful and testing them by touch ortaste would be dangerous.

C. Lesson 2


A. 1. increase; decrease2. decrease; increase3. lower than 2; higher than 124. lower than5. higher than6. pH meter7. neutral

B. You can tell if a substance is an acid or a base by tasting it, touching it, dippinglitmus paper or any other indicator in it, getting its pH by using pH paper or a pHmeter.

D. Lesson 3

Let’s Review (page 28)

1. HClO4 + LiOH

2. HClO4

H+ + ClO4–

3. LiOH Li+ + OH–

4. H+ + ClO4– + Li+ + OH– H+ OH– + Li+ ClO

4–

5. HClO4 + LiOH H

2O + LiClO

4


1. 2HNO3 + Ba(OH)

22H

2O + Ba(NO

3)

2

2. H3PO

4 + NaOH H

2O + NaH

2PO

4

3. HClO4 + LiOH H

2O + LiClO

4

4. 2HBr + Ca(OH)2

2H2O + CaBr

2

5. 2HI + Mg(OH)2

2H2O + MgI

2

6. 2HCl + Sr(OH)2

SrCl2

+ 2H2O

37

7. KOH + H2SO

4 KHSO

4 +

H

2O

8. HF + Mg(OH)2

MgF2

+ 2H

2O

9. RbOH + HBr RbBr + H2O

10. CsOH + H3PO

4 CsH

2PO

4 +

H

2O

C. What Have You Learned? (pages 32–33)

A. 1. (d) Lemon juice is an acid. It turns blue litmus paper to red and not theother way around.

2. (d) Baking soda is the only base among the options. A base is needed toneutralize an acid.

3. (a) HClO4 is the strongest acid because it is included in the list of strong

acids, that is, all the hydrogen ions are converted to hydronium ions.

4. (a) A Brønsted-Lowry base accepts protons.

5. (c) Phenolphthalein is an indicator. An indicator changes color in thepresence of an acid or a base. Soap solution is basic, hence it changesthe color of phenolphthalein.

6. (c) A pH meter can accurately measure the pH of a solution and thusdetermine whether the solution is an acid or a base.

7. (c) KOH is used in making soaps because it’s a base.

8. (c) Nitric acid and sodium hydroxide will form water (H2O) and sodium

nitrate (NaNO3). Because the reactants are a strong acid and a strong

base the products are water and a salt.

9. (d) Concentrated HCl with a pH of 1.5 is the most corrosive. Corrosivesubstances are those with a pH lower than 2 or higher than 12.

10. (d) pOH = 14 – pH

= 14 – 3

= 11

B. 1. blue; red 2. red; blue 3. pH meter 4. 5; acid 5. 9; base6. Arrhenius

7. 2HCl 8. NaOH 9. MgI

2 ; H

2O

10. Water; salt

38

Glossary

Acid rain Rainwater that contains acids such as sulfuric, nitric, sulfurous and carbonic acidsare formed from factory and vehicular emissions.

Amphiprotic A substance that can act both as an acid and as a base.

Amphoteric See amphiprotic.

Anion A negative ion.

Arrhenius acid A substance that gives hydrogen ions upon dissociation in water.

Arrhenius base A substance that gives hydroxyl ions upon dissociation in water.

Brønsted-Lowry acid A proton donor.

Brønsted-Lowry base A proton acceptor.

Cation A positive ion.

Corrosive Capable of destroying something through chemical action; a property ofsubstances with a pH higher than 12 or lower than 2.

Dissociation The process in which a molecule separates into its component ions; alsocalled ionization.

Extract A substance prepared from another substance.

Hydronium An ion formed from a water molecule and a hydrogen ion (H3O+).

Hydroxyl The ion formed from hydrogen and oxygen (OH–).

Indicator A substance that changes color in the presence of an acid or a base.

Ion An electrically charged particle.

Neutralization The reaction between an acid and a base that results in the formation ofwater and a salt.

pH Stands for “power of hydrogen”; the concentration of hydronium ions in a solution.

pOH The concentration of hydroxyl ions in a solution.

Proton A positively charged particle (+1) with a mass of 1.00723 units; its symbol is H+.

.

39

References

Atkins, W. and L.L. Jones. Chemical Principles: The Quest for Insight. U.S.A.: Freemanand Company, 2000.

Blei, I. General, Organic and Biochemistry: Connecting Chemistry to Your Life.U.S.A.: Freeman and Company, 2000.

Mapa, Amelia P. and Trinidad Felipe. Science and Technology III: Chemistry. QuezonCity: SD Publications Inc., 1999.

Nebel, B.J. and R.T. Wright. Environmental Science. 7th ed. U.S.A.: Prentice-Hall,2000.

Documents

Acid and Bases