O Level Notes : Chemistry - Acids and Bases

Acid is a substance which produces hydrogen ions, H+, when it is dissolved in water. All acids contain hydrogen ions, but not all compounds that contain hydrogen are acids, e.g. NH3.


Acid is a substance which produces hydrogen ions, H+, when it is dissolved in water.

All acids contain hydrogen ions, but not all compounds that contain hydrogen are acids, e.g. NH3. This is because NH3 does not produce hydrogen ions in water. It is the hydrogen ions produced that are responsible for the properties of acids.


Properties of Acids:

  1. Acids have a sour taste.
  2. They have a pH below 7.
  3. They are generally corrosive in nature. The lower the pH, the more corrosive they are.
  4. They turn blue litmus paper red.
  5. They dissolve in water to form solutions that conduct electricity.
  6. Acids react with reactive metals to form hydrogen and a salt.

metal + acid salt + hydrogen


  1. Acids react with carbonates to form a salt, carbon dioxide and water.

carbonate + acid salt + carbon dioxide + water


  1. Acids react with metal oxides and hydroxides to form a salt and water.

metal oxide + acid salt + hydrogen


Acids only show the properties of acids when they are dissolved in water. This is because acids dissociate in water to produce the hydrogen ions which are responsible for the acidic properties.




A base is any metal oxide or hydroxide. This means that a base contains either oxide ions O2- or hydroxide ions OH-.

We can also define a base as a substance that reacts with an acid to give a salt and water only.

base + acid salt + water


For example,


CuO (s) + H2SO4 (aq) CuSO4 (aq) + H2O (l)


In this case, copper (II) sulphate CuSO4 is the salt produced.


Another example,


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


In this case, sodium chloride NaCl is the salt produced.


Note in the above cases, the oxide ions or the hydroxide ions from the bases react with the hydrogen ions from the acids to form water. This reaction is known as neutralization, and it is represented by the ionic equation below.

            OH- (aq) + H+ (aq) H2O (l)


It occurs between alkalis and acids.




An alkali is a base that is soluble in water. In an aqueous solution, an alkali produces hydroxide ions OH-.

Most bases are insoluble in water. They are not considered as alkalis. Some common examples of alkalis include sodium hydroxide NaOH, potassium hydroxide KOH, calcium hydroxide Ca(OH)2 and aqueous ammonia NH3.


Properties of Alkalis:

  1. Alkalis have a bitter taste and a soapy feel.
  2. They have a pH above 7.
  3. The higher the PH, the more corrosive they are.
  4. They turn red litmus paper blue.
  5. All alkalis produce hydroxide ions when dissolved in water.
  6. All alkalis can react with water to form salt and water only. This is known as neutralization. We have already discussed this.
  7. Alkalis heated with ammonia salts give off ammonia gas.

alkali + ammonium salt ammonia + salt + water


  1. Alkalis can react with a solution of one metal salt to give a metal hydroxide and another metal salt.

alkali + salt (of metal A) metal hydroxide + salt (of metal B)


Uses of Acids:


Sulphuric acid:

  • In making detergents
  • In making fertilizers
  • In car batteries


Hydrochloric acid:

  • In leather processing
  • For cleaning metals


Ethanoic acid:

  • (In vinegar) To preserve food
  • In making adhesives such as glue


Uses of Bases:


Ammonia solution:

  • In window cleaning solutions.
  • In fertilizers.


Calcium oxide:

  • In neutralizing acidic soil


Sodium hydroxide:

  • In making soaps and detergents


Concentration and Strength:


The term concentration tells us how much a substance is dissolved in 1 dm3 of the solution. This is changeable, of course. By adding more of the substance, we increase its concentration. By adding more of the solvent, we decrease the concentration.


The term strength refers to how easily an acid or an alkali dissociates when dissolved in water. This is a property that is not changeable.


Types of Oxides:


An oxide is a compound of oxygen and another element. Most oxides can be grouped into four types: acidic oxides, basic oxides, amphoteric oxides, and neutral oxides.


Acidic Oxides:

Non-metals may form acidic oxides. Most acidic oxides dissolve in water to form an acid. These oxides do not react with acids. However, they react with alkalis to form salt and water.

An example of acidic oxide is sulphur trioxide SO3. it is a gas which, when dissolved in water, forms Sulphuric acid H2SO4.


Basic Oxides:

The oxides of metals are basic oxides. Most basic oxides are insoluble in water. Those that are soluble are called alkalis.

Basic oxides are solids at room temperature. They react with acids to form a salt and water. They have no reaction with bases or alkalis.

An example of basic oxide is calcium oxide. It dissolves in water to form calcium hydroxide, while it reacts with hydrochloric acid or any other acid to form calcium chloride or any other salt, in addition with water.


Amphoteric Oxides:

Amphoteric oxides are metallic oxides that react with both acids and bases to form salts and water. Meaning, amphoteric oxides are oxides that contain both the abilities of acids and alkalis. If an amphoteric oxide is reacted with an acid, it will show the properties of an alkali and a neutralization reaction occurs. Similarly, if it is reacted with an alkali, it will show acidic properties and here, too, a neutralization reaction will occour.

Examples of amphoteric oxides are aluminium oxide, zinc oxide, lead (II) oxide, etc.


Neutral Oxides:

Some non-metals form oxides that show neither basic nor acidic properties. These oxides are called neutral oxides. They are insoluble in water. Some examples are carbon monoxide and nitric oxide. Water itself is also a neutral oxide.