Chemistry GCSE - C4a (Acids and Bases)

A solution's pH can be measured using Universal Indicator. This indicator changes colour to show how acidic or alcali a solution is, eg:

• 1-4 (red) = strong acid
• 5 (orange) and 6 (yellow) = weak acid
• 7 (green) = neutral
• 8 (blue) and 9 (blue-purple) = weak alkali
• 10-14 (purple) = strong alkali

A solution can also be sested using litmus paper to find if a solution is acidic (pink), or alkali (blue).

The three common labatory acids are:

• Hydrochloric Acid
• Sulfiric Acid
• Nitric Acid

When they react with an acid, the salt changes the name slightly, meaning:

• Hydrochloric --> Chloride
• Sulfuric --> Sulfate
• Nitric --> Nitrate

All acids in solution in water contain hydrogen ions, H+.When an acid is neutralised by an alkali, the OH- ion in the alkali reacts with the hydrogen ion to form water.

H+ + OH- --> H2O

A base is a substance which neutralises an acid. They are solids, but can dissolve in water to form an alkali. Bases always have a metal part (eg. Sodium) and an non-metal part (eg. Hydroxide). They together form Sodium Hydroxide from the example substances.

When an alkali is added bit by bit to an acid, it gradually neutralises the acid. When an acid is added to an alkali, the acid neutralises the alkali. Adding an alkali increases the pH and adding an acid reduces the pH.

This reaction is called neutralisation and always creates a salt and water:

acid + base --> salt + water

i.e.: Sulfuric Acid + Sodium Hydroxide --> Sodium Sulfate + Water

The metal part of the alkali forms the beginning of the salt (sodium) and the second part is the acid (sulfuric --> sulfate)

Metal oxides and metal hydroxides are bases, and will neutralise acids, ie.:

Sulfuric Acid + Copper (II) Oxide --> Copper (II) Sulfate + Water

Hydrochloric Acid + Copper (II) Hydroxide --> Copper Cloride + Water

Ammonia is a base. When ammonia gas is bubbled through an acid, neutralisation takes place.

Acids can also be neutralised by carbonates. For example carbon dioxide is also made as well as the salt and the base:

Hydrochloric Acid + Calcium Carbonate --> Calcium Chloride + Carbon Dioxide + Water