Acid and basesBases

Prior knowledge

What happened the tooth?

• Our teeth is composed of calcium carbonate.

• Recall: Acids react with metal carbonates to give salt, water and carbon dioxide.

What will we learn today?• What is a base?• What are alkalis?• Physical properties of alkalis?• Chemical properties of alkalis?• Characteristics and properties of the Universal indicator.

Bases• A base is a substance that reacts with an acid to form salt and

water only.• General equation ( acid + base)

Base + acid Salt + Water• Recall: Metal oxides and metal hydroxides are examples of

bases.• An alkali is a base that is soluble in water

This is the main focus

Examples of basesBases (metal oxides and hydroxides)

Bases that are soluble in water (alkalis)

Bases that are insoluble in water

Sodium oxide, Na2O Magnesium oxide, MgO

Potassium oxide, K2O Magnesium hydroxide, Mg(OH)2

Sodium hydroxide, NaOH Zinc oxide, ZnO

Potassium hydroxide, KOH Copper(II) oxide, CuO

Ammonia solution, NH3(aq)

Physical Properties of alkalis• Bitter taste

• Soapy feel

• Turn red litmus paper blue.

• Turn Universal indicator Blue/violet

Alkali

Chemical properties of alkalis (I)• Alkalis react with acids to form a salt and water.• This reaction is known as neutralisation.• Example

NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)

• H+(aq) +OH-(aq) H2O(l)

Sodium hydroxide

Hydrochloric acid

Sodium chloride

Water

Chemical properties of alkalis (II)

Sodium hydroxide

• Alkalis produce hydroxide ions when dissolved in water.

• Example

NaOH(aq) Na+(aq) + OH-(aq)Sodium ions Hydroxide ions

Chemical properties of alkalis (III)• Alkalis, when heated with ammonium salts, releases ammonia

gas.• Example

NaOH(aq) + NH4Cl(s) NaCl(aq) +H2O(l) +NH3(g)Heat

Chemical properties of alkalis (IV)• Alkalis react with some salt solution to form precipitates of

metal hydroxides.• Example

2NaOH(aq) + FeSO4(aq) Fe(OH)2(s) +Na2SO4(aq)

Strong Acid vs Weak Acid

Strong acid

• Strong acid is one that undergoes complete dissociation to form ions in water.

• In a solution of strong acid, all the acid molecules dissociate to become ions in water.

• ExamplesSulfuric acid, Hydrochloric acid, Nitric acid

Weak acid

• Weak acid is one that undergoes partial dissociation to form H+

ions in water. • In a solution of a weak acid, few acid molecules dissociate to

become ions in water. (Most of the acid molecules remain as acid molecules in water.)

• ExamplesEthanoic acid, Citric acid

The pH scale• The pH scale is used to illustrate if a solution is acidic, neutral

or alkaline.• The pH scale ranges from 0 to 14.• The lower the pH value of a solution the more acidic it is.• The higher the pH value of a solution the higher is its

alkalinity.

Understanding the scale

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

Neutral

Acidity increases<<<<<<<<<<<<<

Alkalinity increases>>>>>>>>>>>>>>>

Strong acidse.g. HCl

weak alkalis e.g. NH3(aq)

Thinking point1.What is the pH range for acidic solutions?2.What is the pH range for alkaline solutions?3.What is the pH/ pH range for neutral solutions?4.What can be a possible value of a strong acid and weak alkali?

Universal indicator• An universal indicator measures the pH of a solution.• A different pH would result in a different colour change for the

universal indicator.

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

Red

Green

PurpleblueOrange

yellow Bluish-green

A coloured version

What have we learnt today?• What is a base?• The chemical properties of bases.• The physical properties of bases.• The neutralisation process.• Characteristics and properties of the universal indicators.