Materials Required:
l Blackboard
l Piece of chalk
l Glass window
l An old cloth
Procedure:
You have probably seen your teacher write with chalk many times, but have you ever thought about the chalk itself? Do you know what makes chalk stick to a surface? For the answer, try this experiment.
- If you are at school or have a black-board at home, draw a line on the blackboard with a piece of chalk. Do you have any trouble? Of course not.
- Now go to a window and rub the piece of chalk on the glass. Where is the mark? Very little chalk sticks to the glass.
- Rub the piece of chalk on an old cloth. You will find that some chalk marks will stick to the cloth, but seeing letters is difficult. Try rubbing the chalk on other substances, then read the explanation below.
This Is What Happens:
When you rub a piece of chalk against something, the chalk breaks apart into separate particles at the point of contact. Each of these tiny particles carries an electrical charge, and it is this charge that determines what substance the particles will cling to—a blackboard surface and chalk particles will cling together tightly; a glass surface, however, provides a poor area for the chalk particles to stick. A cloth is better than glass, but still not as good as a blackboard.
