The diagram above shows the pH scale, and the pH that acidic and alkaline substances may have. Here are some facts about acids, bases and alkalis:
- Neutral substances have a pH of 7.
- A base is the opposite of an acid.
- An alkali is a base which dissolves in water.
- A base neutralises an acid.
- The most common bases are metal oxides and metal hydroxides.
- You can tell from the name whether a substance is a base or an acid, as acids always have the word 'acid' at the end.
You can see examples of names and a neutralisation reaction below:
sulfuric acid + sodium hydroxide sodium sulfate + water
A neutralisation reaction always produces a salt and water. The salt is made from the metal of the base and the non-metal of the acid. In the example, above the salt - sodium sulfate - is made of sodium (the metal from the base) and sulfur compound (the non-metal of the acid). There are three main acids you will use:
- nitric acid, which produces nitrates
- sulfuric acid, which produces sulfates
- hydrochloric acid, which produces chlorides
When an alkali is added to the acid, the pH rises until it becomes 7. It then rises above 7, when more alkali is added. When an acid is added to an alkali, the pH starts high, but decreases as the acid neutralises the alkali. It becomes neutral and then falls below 7, turning acidic.
There are three general equations for neutralisation reactions:
acid + oxide salt + water
acid + hydroxide salt + water
acid + carbonate salt + water + carbon dioxide
Oxides contain oxygen, hydroxides contain the hydroxide group (OH-) and carbonates contain the carbonate group (CO3-2).
We will now answer some questions on neutralisation and acids and alkalis.