Question

Why it is difficult to clean a blackboard if it is left for a few days? Even after cleaning some dim impression of chalk is left in the blackboard.

Answer 1

 it is difficult to clean a blackboard if it is left for a few days be cause the chalk particles are good example of diffusion.diffusion is rare in solids if it occurs it is not easy to remove those diffused partices

Answer 2

BECAUSE ,

DIFFUSION IS RARE IN SOLIDS, THEREFORE UT'S DIFFICULT TO CLEAN/ RUB A BLACKBOARD, WHICH HASN'T RUBBED FOR FEW or SEVERAL DAYS.