Question

Why we can easily break a piece of chalk but cant do the same with plank of wood?

Answer 1

This is because the chalk particles are loosely combined whereas the particles present in wood are closely packed. So we can easily break a piece of chalk but can't do the same with a plank of wood.

Answer 2

Air is a gas, it has more space in between particles. But solid block of wood is rigid, hence it is difficult to move our hands through it.