What is dissolving?
Some substances dissolve when you mix them with water.
When a substance dissolves, it might look like it has disappeared, but in fact it has just mixed with the water to make a transparent (see-through) liquid called a solution.
Substances that dissolve in water are called soluble substances. When you mix sugar with water, the sugar dissolves to make a transparent solution. Salt is soluble in water too.
Substances that do not dissolve in water are called insoluble substances. When you mix sand or flour with water, they do not dissolve.