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Sour-tasting substances, such as lemon juice or vinegar from fermentation of fruit into wine, have been known as acids for thousands of years.

Another group of household substances, bases, have been known since ancient civilisations too. Residues from fires, such as potash, soda and lime, were categorised as being slippery or soap-like to touch. These are bases – the chemical opposites of acids. Bases that dissolve in water are known as alkalis.

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Why group these substances?

Grouping substances into acids or alkalis gives valuable information about how each substance can be expected to react or its potential uses.

As human beings we have a tendency to sort things, from colours by an artistic designer to types of plant by a botanist. Grouping can make tasks easier – for example sorting different sizes of school shirt helps when selecting a suitable size for use. It is useful to categorise substances as acids or bases.

While acids and bases have been grouped for thousands of years, understanding their behaviour and properties has developed more recently, advancing over the last few centuries, as we shall consider in more detail later.

Substances can be grouped like this when we consider everyday household substances.

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