How much will dissolve
Chemistry · Key Stage 2 · The School
Stir a spoon of salt into a glass of water and it disappears. Keep adding and eventually a layer sits on the bottom and stays there however long you stir. The water has taken all it can hold at that temperature, and the solution is saturated.
Use the same volume of water, the same temperature, and add the solid a level spoon at a time, stirring until it stops disappearing. Count the spoons. Repeat for a different solid and compare. Change the water volume or temperature and the number changes, which is why both are held still.
Most solids dissolve more readily in hot water than cold — more sugar goes into a hot drink than a cold one before it stops disappearing. Stirring does not change the limit; it only gets you there faster, by keeping fresh water in contact with the solid.
The salt is still present and still salt, spread through the water too finely to see. Weigh the water before and the solution after and the mass has gone up by the mass of the salt. That is the evidence that dissolving is a change you can reverse, unlike burning.
Imagine a class finds the salt limit for 100 ml of water at 20°C. That water has a mass of about 100 g. They add salt 10 g at a time and stir each time. After 10 g, 20 g and 30 g, every grain vanishes. The fourth 10 g is too much, and salt stays on the bottom. They filter it out, dry it and weigh it: about 4 g. So 40 g went in and 4 g is left. That means 40 - 4 = 36 g dissolved, and the water is saturated. The solution weighs about 100 + 36 = 136 g, which is the water plus the salt that dissolved. The Wikipedia solubility table gives 35.89 g of salt per 100 ml of water at 20°C. These made-up numbers were chosen to match it.