I have been asked a difficult question by an older primary pupil. He wanted to know, “Why is oil black?”. But some kinds of oil aren’t black. And when oil is black, why?

There are really two parts to this question. The first is: how do we see colours, so how do we recognise that green is green or blue is blue? The second is: what makes oil black when it appears black?

In answer to the first question, in terms that a primary child might understand, light is made up of all the colours. So when what looks likes a single beam of light is shone through a glass prism, it splits into the various colours of the rainbow (a demonstration that always amazes children). But when it hits something that is green, for example, only the green waves are reflected back to us. The other frequencies are absorbed.

The second question depends on what kind of oil we are talking about. Oil comes from different sources - from the earth or from living matter. Almost always it contains additional material. Olive oil, for example, carries the flavour of olives, which is why we like cooking with it. Engine oil is a yellow or greenish colour when we put it into a car engine, but is black when we drain it out several thousand miles later, because it is full of impurities, such as specks of carbon from burned petrol. Crude oil is black when it comes out of the ground, before it has been refined, for similar reasons. Some oil products have colouring added to them, so they could be almost any colour, or even transparent.

If your children have posed a tricky question, send it to Diane Hofkins at the TES.