NCERT Class – 9 Science solution: Chapter 2 Is Matter Around Us Pure-Part 10

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Question 37:

A group of students took an old shoe box and covered it with a black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample contained in a beaker/tumbler in the box as shown in the Fig. They were amazed to see that milk taken in the tumbler was illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it?

Result for shoe box

Shoe Box

Result for shoe box

  1. Explain why the milk sample was illuminated. Name the phenomenon involved.

  2. Same results were not observed with a salt solution. Explain.

  3. Can you suggest two more solutions which would show the same effect as shown by the milk solution?

Answer:

  1. Milk is a colloid and would show Tyndall effect.

  2. Salt solution is a true solution and would not scatter light.

  3. Detergent solution, sulphur solution.

Question 38:

Classify each of the following, as a physical or a chemical change. Give reasons.

  1. Drying of a shirt in the sun.

  2. Rising of hot air over a radiator.

  3. Burning of kerosene in a lantern.

  4. Change in the colour of black tea on adding lemon juice to it.

  5. Churning of milk cream to get butter.

Answer:

Physical changes:

  • Drying of a shirt in the sun

  • Rising of hot air over a radiator

  • Churning of milk cream to get butter

Chemical changes:

  • Burning of kerosene in a lantern

  • Change in the colour of black tea on adding lemon juice to it

Image of Physical Change and Chemical Change

Image of Physical Change and Chemical Change

Image of Physical Change and Chemical Change

Question 39:

During an experiment the students were asked to prepare a (Mass/Mass) solution of sugar in water. Ramesh dissolved of sugar in of water while Sarika prepared it by dissolving of sugar in water to make of the solution.

  1. Are the two solutions of the same concentration?

  2. Compare the of the two solutions.

Answer:

No.

Solution made by Ramesh

Solution made by Sarika

The solution prepared by Sarika has a higher mass % than that prepared by Ramesh.