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.2.4. 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? 

(a) Explain why the milk sample was illuminated. Name the phenomenon involved.
(b) Same results were not observed with a salt solution. Explain.
(c) Can you suggest two more solutions which would show the same effect as shown by the milk solution?

Ans. (a) Milk is a colloid and would show Tyndall effect.
(b) Salt solution is a true solution and would not scatter light.
(c) Detergent solution, sulphur solution. 
 

Classify each of the following, as a physical or a chemical change. Give reasons.
(a) Drying of a shirt in the sun.
(b) Rising of hot air over a radiator.
(c) Burning of kerosene in a lantern.
(d) Change in the colour of black tea on adding lemon juice to it.
(e) Churning of milk cream to get butter.

Ans. Physical changes —(a), (b), (e)
Chemical changes— (c), (d) 

During an experiment the students were asked to prepare a 10% (Mass/Mass) solution of sugar in water. Ramesh dissolved 10g of sugar in 100g of water while Sarika prepared it by dissolving 10g of sugar in water to make 100g of the solution.
(a) Are the two solutions of the same concentration
(b) Compare the mass % of the two solutions.

ANS (a) No

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

Components of given mixture can be separated by following methods:
(i) By using a magnet: Hovering a magnet over the mixture will result in iron fillings getting stuck to the magnet. Thus, iron will be separated.
(ii) Sublimation: The remaining mixture is heated in a china dish. Ammonium chloride is a sublime and hence it will evaporate without undergoing the liquid phase. Crust of ammonium chloride can be collected by placing and inverted funnel on top of china dish.
(iii) Sedimentation, decantation and filtration: The remaining mixture is dissolved in water. The mixture is allowed to settle for some time. Sand, being insoluble in water, settles at the bottom. Liquid is decanted in another beaker. Then, the liquid is filtered to remove any trace of sand in it.
(iv) Evaporation: The liquid is now a solution of salt in water. This is heated in a beaker so that water evaporated. Once all the water evaporates, we get salt in the beaker. 

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

Arun has prepared 0.01% (by mass) solution of sodium chloride in water. Which of the following correctly represents the composition of the solutions?
(a) 1.00 g of NaCl + 100 g of water
(b) 0.11 g of NaCl + 100 g of water
(c) 0.01 g of NaCl + 99.99 g of water
(d) 0.10 g of NaCl + 99.90 g of water

Let the mass of sodium sulphate required be = x g
The mass of solution would be = (x +100) g
x g of solute in (x+ 100) g of solution