Chemicals EverywhereWe live in a chemical world. Every kind of substance that youcan think of is made of a type of chemical or mixture ofchemicals. Water is a chemical, and the air you breathe is amixture of chemicals. The ink in your pen, hair dyes andbleaches, the lead-lined cover that protects you during a dentalX-ray, and life-saving medicines are all made of chemicals, too(Figure 6.1).

A quick look around your home will reveal an amazing varietyof chemicals in your cupboards and on your shelves. In thebathroom, you will find water, soap, shampoo, deodorant, andtoothpaste — all chemicals. In the storage area, you might findcleaning products, such as ammonia and bleach, and perhapspainting and gardening products. In your kitchen, you might findtable salt, baking soda, and baking powder. Each of these productscontains one or more chemical compounds.

CompoundsA compound is a pure substance made up of two or more elementsthat are chemically combined. For example, water is a compoundconsisting of hydrogen and oxygen. Hydrogen peroxide is also a

210 UNIT B Atoms, Elements, and Compounds

Here is a summary of what youwill learn in this section:

• Compounds are composed oftwo or more elements thatcombine in a specific ratio.

• Ionic compounds form whenmetallic and non-metallicelements combine chemically.

• Molecular compounds formwhen non-metallic elementscombine chemically.

How Compounds Form

Figure 6.1 Hydrogen peroxide is a chemical compound that may be used to createhighlights in hair.

6.1

Hydrogen peroxideStrong oxidizer. Keep out of contact with finely divided

reducing agents and metals. Can only be stored for

prolonged periods of time if stabilized with a little

phosphoric acid and stored in amber bottles.

0Flammability

4Health

Special

5Reactivity

OXIDIZER

Stability:

3

Storage:

7

CORROSIVE

Figure 6.2 People using a chemicalsuch as hydrogen peroxide in theirworkplace must, by law, be trained inthe meaning of all the safety symbolson the label.

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compound of hydrogen and oxygen but with completely differentproperties than water (Figure 6.2 on the previous page). Some ofthe differences between water and hydrogen peroxide are listed inTable 6.1. Hydrogen peroxide is a blue liquid that can dissolve inwater to form a solution, which is commonly available inpharmacies. In certain concentrations, it can be used on skin to killbacteria or in hair as a bleaching agent.

All compounds have properties that make them potentiallyuseful as well as hazardous. If a highly concentrated solution ofhydrogen peroxide gets on the skin, it will cause a chemical burn.Even water, if consumed in huge volumes, can make a personsick. Understanding the properties of compounds gives us theknowledge to make use of compounds safely and responsibly.

211Elements combine to form ionic compounds and molecular compounds.

B20 Quick Lab

Water and Hydrogen Peroxide (Teacher Demonstration)

PurposeTo observe properties of water and hydrogen peroxide

Procedure

1. Your teacher will dip one piece of blue litmuspaper into water and another into hydrogenperoxide solution. Observe the litmus paper.

2. Your teacher will dip one piece of cobalt chloridepaper into water and another into hydrogenperoxide solution. Observe the cobalt chloridepaper.

3. Observe as your teacher pours water into abeaker and then stirs in some potassium iodidecrystals.

4. Your teacher will place the graduated cylinder inthe basin and then pour 20 mL of hydrogenperoxide solution and 3 drops of dish soap intothe graduated cylinder. Observe as your teacheradds a small scoop of potassium iodide crystals.

5. Step 4 will produce bubbles of gas. Observe asyour teacher places a glowing splint into the gas.

Question

6. Describe three differences in the properties ofwater and hydrogen peroxide that you observed.

Water (H2O)

• colourless liquid• boils at 100°C • stable in strong sunlight• dissolves the chemical

potassium iodide• does not bleach pigments

Hydrogen peroxide (H2O2)

• blue liquid• boils at 150.2°C• breaks down in light • reacts with the chemical

potassium iodide• strong bleaching agent

Table 6.1 Properties of Waterand Hydrogen Peroxide

CAUTION: Hydrogen peroxide is corrosive to skin.Potassium iodide will stain skin and clothing.

• blue litmus paper

• two 400-mL beakers

• water

• hydrogen peroxidesolution

• cobalt chloride paper

• scoopula

• stirring rod

• potassium iodidecrystals

• 250-mL graduatedcylinder

• basin

• dish soap

• matches

• wooden splint

Materials & Equipment

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Staying on Topic

Writers support their topicsentence by adding sentenceswith details that relate to thetopic. Each new sentenceseems to flow logically from thewords and ideas in the previousone. Notice how the paragraphabout salt seems unifiedbecause all the sentencessupport our understanding ofthe topic.

During Writing Two Types of CompoundsA small change in the way the atoms combine can make a bigdifference in the chemical and physical properties of compounds.Although millions of compounds have been discovered, almost allof them can be classified as one of two types: ionic or molecular.

Ionic CompoundsCommon table salt is familiar to most people as a white substancecomposed of tiny crystals. As discussed in Chapter 5, sodiumchloride forms when a very reactive metal — sodium — is placedin a container with a poisonous, yellow-green non-metal —chlorine gas (Figure 6.3). When these two chemical elements arecombined, the sodium metal explodes in a bright orange flame. Asthe sodium burns, a white, coarse-grained powder is produced.This new substance has properties that are very different fromthe properties of sodium and chlorine. The substance is table salt,or sodium chloride (NaCl).

Sodium chloride is called an ionic compound. Ioniccompounds are pure substances usually consisting of at least onemetal and one non-metal. Most ionic compounds share thefollowing properties:

• have high melting points

• form crystals, which are very regular arrangements ofparticles

• dissolve in water to form solutions that conduct electricity

212 UNIT B Atoms, Elements, and Compounds

Figure 6.3 Sodium (a), is a metal. Sodium combines with chlorine gas in a violent reaction (b). The compound that forms is sodiumchloride, NaCl (c).

(a) (b) (c)

Suggested Activity •B21 Quick Lab on page 215

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All ionic compounds are solids at room temperature. In fact,table salt will not melt until it is heated to 800°C. When sodiumchloride is dissolved in water or melted, it will conduct electricity.Investigations of this property led to the study of electrochemicalcells, which can convert chemical energy into electricity.

Forming Ionic CompoundsWhile combining, each atom changes into an ion. Ions form whenone or more electrons move from a metal atom over to a non-metal atom. This process is shown in Figure 6.4.

As the figure shows, a sodium atom loses an electron by givingit to a chlorine atom. This produces a positive sodium ion and anegative chloride ion.

Positive and negative ions attract each other, so in an ioniccompound, all the positive ions are attracted to all the negativeions. A connection between atoms or ions is known as a bond.The attractions between ions are called ionic bonds. The ionicbonds cause the ions to group together in an alternating patterncalled a crystal arrangement (Figure 6.5).

213Elements combine to form ionic compounds and molecular compounds.

Cl-

1-

Na+ClNa

1+

Figure 6.4 A sodium atom becomes a positive ion when it loses an electron to chlorine. By gainingan electron, chlorine becomes chloride, a negative ion. Together, they form an ionic compound.

CI-Na+

Figure 6.5 The crystals in this tablesalt are held together by ionicbonds.

WORDS MATTER

The word “ion” comes from a Greekword meaning to go or to wander.

Learning Checkpoint

1. What is the main difference between a compound and an element?

2. What two kinds of elements join together to form an ionic compound?

3. What is an ionic bond?

4. How must atoms change so that they can join to form an ionic compound?

5. What physical property or properties could you use to identify an ioniccompound?

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Molecular CompoundsWhen non-metals combine, a pure substance called a molecularcompound is formed. In molecular compounds, the atoms shareelectrons to form small groups, called molecules. Most molecularcompounds share the following properties:

• can be solids, liquids, or gases at room temperature

• usually good insulators but poor conductors of electricity

• have relatively low boiling points

Examples of molecular compounds include table sugar, hydrogenperoxide, and water.

How can the same two elements (hydrogen and oxygen)combine to form compounds as different as water and hydrogenperoxide? Water is made from joining two atoms of hydrogen toone atom of oxygen. This forms a water molecule, and this is thesmallest possible amount of water that can exist. We oftenrepresent this as H2O, where the subscript “2” indicates that twoatoms of hydrogen are included. Hydrogen peroxide is formedwhen four atoms — two each of hydrogen and oxygen — join toform a single molecule. This molecule is represented as H2O2.

There are a number of ways to represent the atoms in amolecule. In every representation of water in Figure 6.6, twohydrogen atoms are connected to an oxygen atom in the middle.The Bohr diagram of water shows that each hydrogen atomshares a pair of electrons with the oxygen atom (a). In the ball-and-stick model, a stick represents each bond between atoms (b).In the third diagram, the areas in which the atoms overlaprepresents the connections that holds them together (c).

214 UNIT B Atoms, Elements, and Compounds

Suggested Activity •B23 Quick Lab on page 216

Figure 6.6 Representations of a water molecule: (a) a Bohr diagram; (b) a ball-and-stick model, in which the sticks represent connectionsbetween atoms; (c) a diagram with overlap between atoms to show how they are connected

(a) (b) (c)

Hydrogen peroxide is consideredto be a more environmentallyfriendly alternative to chlorinebleaches. It is currently used inmany paper-processing facilities toproduce white paper, instead ofusing the more harmful chlorine.The chemical activity of thehydrogen peroxide changes thecolour of the fibre in paper towhite. With a partner, create apamphlet to advertise one of themany other uses of hydrogenperoxide. Begin your research atScienceSource.

Take It Further

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215Elements combine to form ionic compounds and molecular compounds.

B21 Quick Lab

Adding sugar to your tea, or salt to your soup, notonly changes the way your food tastes, it will dissolveto form a solution. That solution may or may not beable to conduct an electric current.

PurposeTo compare the conductivity of different compounds:sodium chloride (table salt) and sucrose (tablesugar)

Procedure

1. Use the marker to label three 100-mL beakersas water, sucrose, and sodium chloride.

2. Fill each beaker with 50 mL of tap water. Set thebeaker labelled “water” aside.

3. Add one small spoonful of sucrose to the secondbeaker. Use the spoon to stir until the sucrose isdissolved.

4. Use a clean spoon to add one small spoonful ofsodium chloride to the third beaker. Use thespoon to stir until the sodium chloride isdissolved.

5. When your solutions are prepared, use a battery-operated conductivity tester to determine whetherany of the three samples conduct electricity.

Questions

6. Sodium chloride and sucrose are both shinywhite crystals, and both dissolve in water. Whatevidence shows which of these crystals is anionic compound?

7. Suggest why it is often unsafe to have highvoltage electricity near water. Refer to the resultsof your experiment in your answer.

Salt and Sugar

Figure 6.7 A conductivity tester

B22 Skill Builder Activity

In this activity, you will examine the components of amolecular model kit to prepare you to build simplemodels. In the kit, balls represent atoms and sticksrepresent connections between the atoms. Carbon isrepresented by a black ball, oxygen by a red ball,chlorine by a green ball, and hydrogen by a whiteball. The number of holes in each ball represents thenumber of connections the atom can make withother atoms.

1. Examine the model atoms of carbon, oxygen,chlorine, and hydrogen. Build as manyconnections as you can between the differentmodel atoms.

2. Describe what you observe about the differentmodels you built.

Molecular Model Kits

• three 100-mL beakers

• marker

• tap water

• 2 small spoons

• sucrose (table sugar)

• sodium chloride(table salt)

• battery-operatedconductivity tester

Materials & Equipment

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216 UNIT B Atoms, Elements, and Compounds

B23 Quick Lab

Chemists use models to gain information about theshape of a molecule. The shape of a molecule is agood predictor of its properties. In this activity, youwill build ball-and-stick models of simple moleculesusing a molecular model kit.

PurposeTo represent the molecules of some commonsubstances

Procedure

1. Work in a small group to use the molecularmodel kit. Your teacher will provide specificinformation about how to use your particular kit.

2. There are two guidelines that you must followwhen building molecular models:

• Each molecule is complete when all the ballsare connected in such a way that all the holesare filled and every connector ends in a hole.

• It is possible in some cases for more than oneconnection to exist between the same twoatoms.

3. For each of the following, build the model andthen make a sketch of it.

(a) H2O (water)

(b) H2 (hydrogen gas)

(c) O2 (oxygen gas)

(d) CH4 (methane, also called natural gas)

(e) CH2Cl2 (a solvent used as a degreaser)

(f) C2H4 (starter material for makingpolyethylene plastic)

(g) C3H8 (propane, a camp fuel)

(h) HCl (hydrogen chloride, present in stomachacids)

(i) H2O2 (hydrogen peroxide)

(j) CO2 (carbon dioxide)

4. The following molecules can each be assembledin two different ways. Build and sketch each:

(a) C2H60

(b) C3H7Cl

Questions

5. Identify the two molecules that representedelements rather than compounds.

6. How do the positions of the holes in each kind ofmodel atom produce a molecule that has athree-dimensional shape?

Building Molecular Models

Figure 6.8 A ball-and-stick model of glucose, a type ofsugar. Glucose can be straight, as shown here, or can twistinto a ring structure.

• molecular model kit

Materials & Equipment

Key ActivityDI

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Key Concept Review1. What is the difference between a compound

and a mixture?

2. What are the two major types ofcompounds? How do the bonds differ ineach type?

3. Give an example of a molecular compound.

4. Give an example of a substance heldtogether by ionic bonds.

5. What types of elements join to formmolecular compounds? Name three suchelements.

6. (a) What is the total number of atoms in awater molecule?

(b) How many elements are in a watermolecule?

Connect Your Understanding7. How is it possible for two different

compounds, such as water and hydrogenperoxide, to both be made of the same twoelements?

8. Examine the following table of propertiesfor two unknown compounds. Which ismost likely an ionic compound and whichis most likely a molecular compound?Explain your reasoning.

9. Use the diagram below to answer thefollowing questions.

(a) What type of diagram is shown?

(b) What type of compound is shown?

(c) Which elements does this compoundcontain?

10. Describe how a salt crystal holds together.

11. Is it possible to wash dishes without usingchemicals? Explain.

12. Can compounds have different propertiesthan their elements have? Explain, using anexample.

Reflection13. Describe three types of models used in this

section to represent compounds. Whichmodel did you find most useful and why?

14. Name two compounds that you werefamiliar with before reading this section.Now that you have completed this section,what have you learned about these twocompounds?

For more questions, go to ScienceSource.

217Elements combine to form ionic compounds and molecular compounds.

2-2+

6.1 CHECK and REFLECT

Property Compound X Compound Y

Boiling point (°C) 82 1550

Melting point (°C) −90 455

Conductivity in solution poor good

Question 9

Properties of Unknown Compounds

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