Exercise 3 solution

One workable plan (my own; other orders also work):

1. Magnet. Pass a magnet over the dry mixture; the iron filings stick to it and are pulled out. Why it works: iron is attracted to a magnet and sand and salt are not (the chapter's iron-and-sulfur example uses the same idea).

2. Dissolve. Stir the remaining sand and salt into warm water. Why it works: the salt dissolves and the sand does not. (This property of salt and sand is from general knowledge.)

3. Filter. Pour the mixture through a coffee filter or clean cloth. The sand stays behind; the salt solution passes through. Rinse and dry the sand.

4. Evaporate. Heat the salt solution gently until the water has gone, leaving the salt. Why it works: evaporation is one of the methods the Chemical substance article names for separating mixtures.

Each step is a physical process, so no new substance forms and each component keeps its own properties. Filtering is not named in the article's list (chromatography, distillation, evaporation); it is a standard method I have added.