Question

Why the marks of chalk on the blackboard are unable to be wiped out easily after a long time?

Answer 1

It is so because little bit particles of chalk gets diffused into the blackboard and hence are difficult to remove.
You can even try it.

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Answer 2

Explanation:

The marks of chalk on the blackboard are unable to be wiped out easily after a long time because of diffusion of chalk particles.

As chalk is a solid and solids have a slow diffusion process. When we write on the blackboard using a chalk then the particles of chalk diffuse on the board and occupy the vacant spaces of the blackboard.

So, when kept for a longer period of time the marks of chalk are unable to be wiped out easily.