Question

Why is it easy to break a piece of chalk but difficult for a coal?

Answer 1

We can break a piece of chalk easily because particles of matter in a chalk are somewhat loosely packed. Though it is a solid but then also its particles are loosely packed and on application of force it easily breaks. 
While in the case of an coal piece, the particles of matter are very very tightly packed and it requires great great force to break it up into pieces.