Acids, Bases, Alkalis and Neutralisation Reactions

The common mineral acids are

Hydrochloric acid - HCl _(aq)

Sulfuric acid - H₂SO_{4 (aq)}

Nitric acid - HNO_{3 (aq)}

Phosphoric acid - H₃PO_{4 (aq)}

It is important you learn them by name and formula. Notice they are always aqueous (dissolved in water)

A base is a chemical which neutralises an acid and there are lots of different types of bases. Most bases are insoluble compounds, such as metal oxides and metal carbonates, but another type of base which is soluble is called an 'alkali'.

The most common alkali is sodium hydroxide (NaOH_(aq)), but any metal hydroxide can be considered an alkali.

(An unusual example of an alkali that doesn't end in hydroxide is ammonia (NH₃).)

Acids and bases can be thought of as the 'opposite' of each other and so they react together.

All alkalis are bases, but not all bases are alkalis.

Click here to go to the Key Stage 3 bitesize revision material on acids/bases/alkalies/indicators.

When an acid reacts with an alkali, in the most simple case of a neutralisation reaction, we produce a salt and also give off water.

acid + alkali --> salt + water

e.g. sulfuric acid + sodium hydroxide --> sodium sulfate + water

The ending of the salt's name always comes from the acid we start with....

Sulfuric acid makes 'sulfate' salts

Nitric acid makes 'nitrate' salts

Phosphoric acid makes 'phosphate' salts

Hydrochloric acid makes 'chloride' salts (the annoying one...doesn't end in ate!)

How do we know that we have added the right volume of acid to alkali to make the solution neutral?

Sadly, this is not that easy! If we add too much acid, there will be some acid left over and the pH would be acidic... if we add too much alkali the reverse will be true... so we would be sat there adding drop by drop of acid and alkali to try and get a neutral solution (i.e. green with universal indicator). But there is another problem too! In order to get our solid salt crystals back at the end of the experiment, we have to evaporate the water away from the solution. If we have added universal indicator in there as well, the crystals we get at the end will be stained green... where they should actually be white if pure.