Acids, Alkalis and

Bases

Learning ObjectivesBy the end of the lesson, you should

be able to:

a) Give the definition of a strong and weak acid, and the properties of it.

b) Identify the products of reactions between Acid and metals, carbonates, bases.

c) Give the definition of a strong and weak Alkali, and the properties of it.

d) Identify the products of reactions between Bases and Ammonia salts

e) State the reactivity series and colour changes of each indicators.

Acids An acid is a substance which produces

hydrogen ions as the only positive ions when it is dissolved in water.

E.g. HCl molecules dissolve in water to give hydrogen ion and chloride ion.

[Examples] Strong Acids

Common Name Chemical Name Chemical Formula

Hydrochloric Acid

Hydrogen Chloride

HCl

Nitric Acid Hydrogen Nitrate

HNO3

Sulfuric Acid Hydrogen Sulfate H2SO4

Strength of an Acid The strength of an acid depends on its degree

of dissociation/ ionization in water to form hydrogen ions (H+).

A strong acid is one that ionizes completely in water to produce hydrogen ions (H+). There

are no molecules left. The solution contains a high concentration of ions : Good electrical

conductor.

HCl (aq) → H+ (aq) + Cl– (aq)

A weak acid is one that ionizes partially in water to produce few hydrogen ions (H+). Most of

the acid molecules remain as molecules. The solution contains a low concentration of ions :

Poor electrical conductor.

Properties of AcidsAn acid, when dissolved in water, forms a

colourless solution. Solution of acids have the following properties:

a) Sour taste

b) Change the colour of indicators (E.g. Turns blue litmus paper red)

c) Contains hydrogen ions and conduct electricity

d) Reacts with metals, carbonates and bases.

*Note: Acids only behave as acids when they are dissolved in water.

Reaction with metalsAcids react with metals to produce a salt and

hydrogen

E.g. 2HCl (aq) + Zn (s) → ZnCl2 (aq) + H2 (g)

Observations:

a) Effervescence observed. Colourless, odourless gas evolved which extinguished a light wooden splint with a “pop” sound.

b) Grey solid reduced in size (when in excess). Grey solid dissolves to form a colourless solution.

c) Heat is liberated.

Reaction with carbonatesAcids react with carbonates to produce a salt,

carbon dioxide and water

E.g. Na2CO3 (s) + H2SO4 (aq) → Na2SO4 (aq)+

CO2 (g)+ H2O (l)

Observations:

a) Effervescence observed. Colourless, odourless gas evolved which reacts with calcium hydroxide to form a white precipitate.

b) White solid dissolves to form a colourless solution.

Reaction with basesAcids react with bases (metal oxides and

hydroxides) to produce a salt and water

E.g. MgO (s) + H2SO4 (aq) → MgSO4 (aq)+ H2O (l)

Observations:

a) White solid dissolves to form a colourless solution.

b) Heat is evolved. No visible change.

Uses of acids

Sulfuric Acid: Ethanoic Acid:

To make Detergents and Fertilizers

To preserve food

In car batteries as battery acid

treatment of the sting of box jellyfish

Oil refining, wastewater processing, and chemical synthesis.

Used in photographic films

IndicatorsIndicators Observations

Litmus paper An acid turns blue litmus paper red

Methyl Orange Used to identify strong acidsColour change at pH 4Strong acids turn methyl orange redAlkalis turn methyl orange yellow

Phenolphthalein Used to identify strong alkalisColour change at pH 9Strong alkali turn phenolphthalein pinkPhenolphthalein remains colourless in acid.

Universal Indicator

In neutral solutions (pH 7) , universal indicator remains green. Strong acid: redStrong alkali: violet

Bases and AlkalisDefinition of base:

Bases are the oxides or hydroxides of metal that reacts with an acid to form salt and

water only.

Definition of alkali:

An alkali is a metal hydroxide which is soluble in water and produces hydroxide ions (OH–)

in water. All alkalis are bases but not all bases are alkalis.

Insoluble base:

Copper (II) oxide, aluminium oxide, zinc oxide, zinc hydroxide.

Soluble base/alkali:

Sodium oxide, Potassium oxide, Sodium hydroxide, Potassium hydroxide,

Ammonium Hydroxide

The strength of an alkali depends on its degree of dissociation/ ionization in water to

form hydroxide ions (OH–) . The properties are due to hydroxide ions.

A strong alkali is one that ionizes completely in water to produce hydroxide ions (OH–) . There

are no molecules left. The common strong alkalis are sodium hydroxide and potassium

hydroxide.

KOH(aq) → K+ (aq) + OH– (aq)

A weak alkali is one that ionizes partially in water to produce few hydroxide ions (OH–). Most of the alkali molecules remain as molecules.

Strength of an alkali

Properties of basesa) Alkalis have a soapy feel and bitter taste

b) Alkalis can change the colour of indicators (e.g. turn red litmus paper blue)

c) Bases can react with acids to form salt and water only- Neutralization

Example:

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

Reaction with Ammonium SaltsBases react with ammonium salts to produce

a salt, ammonia and water.

E.g. NaOH (aq) + NH4Cl (s) → NaCl (aq) + NH3 (g) + H2O (l)

Observations:

a) A colourless, pungent gas is evolved, that turns damp red litmus paper blue.

Uses of bases and alkalisMagnesium Hydroxide:

- In toothpaste to neutralize the acids on our teeth produced by bacteria

Ammonia Solution:- Used for cleaning glass windows

THE END