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Rock salt is natural salt that is dug out of the ground. In the UK there are salt mines in Cheshire and much of this salt is crushed and used to 'grit' the roads in winter when they have ice and snow on them, helping to keep them clear. Rock salt is the remains of prehistoric seas and is a mixture of salt together with sand and rock.
In the first experiment, Robert is trying to extract some pure salt from a lump of rock salt; he is hoping that he will have enough to sprinkle on his fish and chips!
