When a material breaks into pieces too small to see and spreads all through a liquid, we say it dissolves.

Does salt dissolve in water?

Yes, it does: the salt is still there, in pieces too small to see.

The salt water is a solution: a mixture where you cannot see the parts.

Does chalk dust dissolve in water?

No, it does not: the bits stay bits, and they settle on the bottom.

Now Imani tries a spoonful of sugar, and then a spoonful of sand, each in a glass of cold water.

Does sugar dissolve in water? Yes, it does.

Does sand dissolve in water? No, it does not: the grains settle on the bottom.

The four glasses after stirring, with the answer under each: salt yes, chalk no, sugar yes, sand no.