Why chalk does not dissolve in water?

Answer 1

Chalk is made of calcium carbonate which is insoluble in water. The chemical bonds are to strong. It does dissolve in acids.
Many minerals do not dissolve in water.

(It would be better to say that many do not dissolve easily, but we are not discussing why it take millions of years to dissolve granite.)

In chemistry, one would say that a mineral dissolves readily if it can be broken into parts that become ions and the hydration energy of the ions is larger than the cohesive energy of the constituents in the solid. (More generally, it can be any subunit or whole molecules which are hydrated. Sugar, for instance, dissolves into whole molecules.)

There are two kinds of chalk. The mineral Chalk is a porous form of limestone composed of calcium carbonate. Blackboard chalk is calcium sulfate.

In both versions of chalk there is low solubility in water and there are some additional details that are interesting if you study chemistry because some forms of the substance are "hydrated" and that is an important aspect in their water chemistry.

The bottom line is that things dissolve in water when the water molecule will adhere strongly to the different subunits of the molecules of the solid and it will not dissolve if the adhesion between between particles of the solid is strong enough and that is the case for chalk.

Answer 2

Because it is hard