(1) Some materials can dissolve in water. They will disappear in water. Example: sugar, salt, instant coffee powder and milk.
(2) When a material dissolves in water, we get a solution.
(3) Some materilas cannot dissolve in water. These materials do not dissapear in water. Example: sand, coin, key and leaf.
(4) You can check whether something dissolves in water by
(a) tasting the mixture
(b) filtering the mixture