Science corner

16th June 2006 at 01:00
Ray Oliver and the sharp science of vinegar

The attraction of home brew to the impecunious student community is obvious. But when home brewers turn to wine making, disaster may strike.

Many litres of wine end up as something less palatable - wine vinegar.

Unless the fermenting wine is isolated from micro-organisms in the air, the alcohol in the wine is converted into acetic acid (ethanoic acid).

This conversion was studied by Lavoisier, who was later executed in the French Revolution. Lavoisier demonstrated that alcohol absorbed oxygen when it changed into vinegar. He was right - this is an example of an oxidation reaction. The word vinegar means sour wine. Vinegar was known to the Romans and soldiers are believed to have drunk water mixed with a little vinegar. Few children today would drink this mixture more than once.

Compare wine, malt and distilled vinegar for the experiments that follow.

Some old books of household hints recommend using vinegar in cleaning.

Vinegar contains between 5 per cent and 12 per cent acetic acid. Children can experiment with it to clean glasses. Add 5ml vinegar to 1 litre of warm water and use it to wash some glasses. It should improve the surface shine.

Vinegar should never be used to clean marble work tops or limestone tiles. The reason why can be seen if you drop small pieces of chalk rock or limestone into a tube of vinegar. There is rapid fizzing as the rock reacts and carbon dioxide gas is released. Try the same experiment with crushed egg shells or sea shells. The chemical that is common to all these materials is calcium carbonate. When the fizzing stops, test with an indicator to check that all the acid has been used up. Next, filter the mixture and leave the solution to evaporate - crystals of calcium acetate (ethanoate) will be left. Extend the study by asking children to research the uses of vinegar at home. Pickled onions and many sauces should list vinegar as an ingredient.

Finally, why is pure acetic acid called glacial acid? It is because, on a cold day, it freezes solid like ice.

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