Question

Chalk markings can not be easily rubbed off from the black-board if kept for a longer time.What do you conclude by this???

Answer 1

Hello..... Chalk markings react with moisture present in the atmosphere. So they stick to the surface of black board. That's why it can't be rubbed off easily.

Answer 2

Chalk marking can't be easily rubbed off from the black board if kept for a longer time because the black board (as a adsorbent ) have some forces to attract molecular species.