Look at this clear jar. Salt was stirred into the water until you could not see a single grain. Has the salt gone for good, or is it still in there somehow?

Hands up: gone forever, still there, or not sure?

Here is a second jar. You can still see the sand grains in the water. That is a mixture: the materials are together but keep their own properties.

Our first jar looks clear. When salt disappears into water but is still there, that is a solution.

Two puzzles for your group. First: when salt dissolves and the water looks clear, where has the salt gone? Second: if we pour sandy water through filter paper, what will be caught and what will run through?

Agree one prediction for each puzzle and be ready to say why.

Watch the set-up carefully. First we will try filtering: pouring sandy water through paper with tiny holes so undissolved bits are caught. Then we will pour a little clear salt solution into shallow dishes for the windowsill. That starts evaporation, when water slowly dries away and can leave dissolved solid behind.

Before anything is poured: what do you think will be left in the filter paper, and what will look different in the dishes after a few days?

In your group you will recover the sand today and set up a waiting experiment for the salt. Filtering and slow windowsill evaporation are the methods we use.

1. Get the sand back. Fold the filter paper, sit it in the funnel, and pour your sandy water through slowly. Look at what is left in the paper and what is in the cup below.

Talk as you work: which material came back straight away, and which one will take days? How will you know the sand recovery worked?