Question

• Take some full-cream milk in a test tube.
• Centrifuge it by using a centrifuging machine for two minutes. If a centrifuging machine is not available in the school, you can do this activity at home by using a milk churner, used in the kitchen.
• If you have a milk dairy nearby, visit it and ask (i) how they separate cream from milk and (ii) how they make cheese (paneer) from milk.

1. What do you observe on churning the milk?
2. Explain how the separation of cream from milk takes place.

NCERT Class 9th Science Ch 2 Is Matter Around Us Pure

Answer 1

1.Centrifugation
2.Adding lemon juice to milk.

1.The cream settles down.

Answer 2

1) When milk is churned, the cream separates from the milk:

EXPLANATION:

Centrifugal force separates the fat or cream from the milk. The cream floats above the milk because cream has low density as compared to milk. The milk left is skimmed milk.

2) The process which is used to separate milk from cream is called centrifugation.

EXPLANATION:

In this process, milk is rotated in a centrifugal machine which causes the dense particles to settle down in the machine while the lighter ones float up.

Cream is lighter than milk and so accumulates on top. The cream can thus be collected without disturbing milk below in the machine.