Some substances dissolve when you mix them with water. When a substance dissolves, it looks like it disappears. But in fact it has just mixed with the water to make a transparent (see-through) liquid called a solution.
When you mix sugar with water, the sugar dissolves to make a transparent solution. Salt dissolves in water too.
Heat can help some substances dissolve faster in water. Salt, for example, will dissolve quicker in hot water than in cold water.
Substances that dissolve in water are called soluble substances.
Substances that do not dissolve in water are called insoluble substances.
When you mix sand or flour with water, they do not dissolve.
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