When the Magnet Stops Working

Take a jar. Put in a spoon of grey iron filings and a spoon of yellow sulfur powder, and stir them together.

Look closely and you can still see grey specks and yellow specks. Hold a magnet against the side and the iron climbs the glass towards it, leaving the sulfur behind. Everything you learnt last lesson works perfectly: two parts, one difference, magnetic separation.

Now heat that same jar. It begins to glow — and it keeps glowing after you take the flame away, which is your clue from lesson 17 that something chemical is happening. When it cools, what is in the jar is a single black solid. No grey specks. No yellow specks.

Run the magnet over it again.

Nothing happens.

The iron has not gone anywhere. Every atom of it is still in that jar. But it is no longer iron — it has joined with the sulfur into a new substance, iron sulfide, and iron sulfide is not magnetic. The method that worked five minutes ago has stopped working, and no filtering, heating or distilling will bring the iron back.

That is the difference between a mixture and a compound, and you just watched it happen.

By the end of this lesson you'll be able to:

• say what an element, a compound and a mixture each are
• use two questions to classify any substance you are given
• explain why a compound has properties nothing like the elements inside it
• write the symbols for the first twenty elements, and say why six of them look wrong