Some solids, when added to a liquid, appear to disappear. There are everyday examples of this: salt in a boiling pan of vegetables, sugar in tea of coffee. The solid doesn't disappear, it dissolves and forms a solution, but how can this be proved? A simple understanding of evaporation and condensation is required. When a liquid reaches its boiling point, it evaporates forming a gas. When the gas cools, it condenses, reforming into a liquid. This is called a physical, rather than a chemical change.
So, how do we prove that a solid, dissolved into a liquid doesn't just disappear or evaporate with the liquid?
- You will need an old clean pan, a clean hand mirror, a pair of gloves and some salt
- Cover the bottom of the pan with about 2cm of water
- Stir some salt into the water until it is dissolved
- You have a salt solution in the pan
- Bring the salt solution to the boil
- As the solution boils, it will begin to evaporate giving off steam. (Water vapour.).
- Wearing gloves to protect your fingers from being scolded, hold the mirror over the steam
- The steam will condense on the mirror, reforming into a liquid
- Taste the condensation
Does it taste of salt? Of course not. The salt does not evaporate, but remains in the solution. If the pan is allowed to boil dry, salt crystals will form in the bottom of the pan when all of the water has evaporated. This proves that solids don't just disappear when they dissolve in a liquid.
You could try this experiment with sugar and get the same results. Another good experiment is to repeat, but instead of using foodstuffs, use a water-colour paint in the water. Wipe the mirror with a white tissue paper instead of tasting. The tissue paper will be damp, but there will be no trace of colour on it.
Steam is extremely hot and can scald if precautions are not taken. Do not attempt this experiment unless you are prepared to work safely.
