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Chemistry · Key Stage 2 · The School
Dissolved is not gone
When salt dissolves in water it forms a SOLUTION. The salt has not disappeared and has not been destroyed — its particles have spread out among the water particles, too small to see. You can prove it two ways: the solution tastes salty, and the mass of the solution equals the mass of the water plus the salt. Nothing was lost.
What makes it faster
Three things speed dissolving up: STIRRING, which keeps fresh water in contact with the solid; HEAT, because faster-moving particles separate the solid more quickly; and SMALLER PIECES, because more surface is exposed. None of these change how much can dissolve in the end — only how quickly it gets there.
Getting it back
Because dissolving is a physical change, it can be reversed. Leave the solution somewhere warm and the water evaporates, leaving the salt behind as crystals. That is EVAPORATION used as a separation method, and it is how sea salt has been produced for thousands of years. A substance that could not be recovered would tell you a chemical change had happened instead.
Weigh a beaker of water: 200 g. Weigh 10 g of salt. Total 210 g. Stir the salt in until it disappears completely, then weigh the beaker again: 210 g. The salt did not go anywhere — the balance says so. Now leave the beaker on a warm windowsill for a few days and the salt reappears as crystals as the water evaporates. Mass conserved, then the substance recovered: two pieces of evidence for the same thing.