Class 9 Science


Exemplar Problem

Long Answer Type Questions

Part 2

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 Fig.illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it?

Tyndall Effect

(a) Explain why the milk sample was illuminated. Name the phenomenon involved.

Answer: Since, milk is a colloid and when light scattered from the particles of colloids, it is illuminated, thus light was illuminated when passed through the milk. This is known as Tyndall Effect.

(b) Same results were not observed with a salt solution. Explain.

Answer: For scattering of light the size of particles should be large enough. Since the particles of solution are not enough to scattered the beam of light, hence same result were not observed.

(c) Can you suggest two more solutions which would show the same effect as shown by the milk solution?

Answer: Soap bubbles and fog are the colloids, hence same effect, i.e. scattering of light is shown by these. This is known as Tyndall effect.

Question 38: Classify each of the following, as a physical or a chemical change. Give reasons.

(a) Drying of a shirt in the sun.

Answer: Drying of shirt in the sun is a Physical change. Since in this change no new substance is formed.

(b) Rising of hot air over a radiator.

Answer: Since, in rising of hot air over a radiator no new substance is formed, hence it is a Physical change.

(c) Burning of kerosene in a lantern.

Answer: While burning of kerosene in a lantern carbon dioxide, and water vapour is formed, hence it is a Chemical change.

(d) Change in the colour of black tea on adding lemon juice to it.

Answer: In this change a new substance is formed, hence it is a Chemical change.

(e) Churning of milk cream to get butter.

Answer: While churning of milk cream to get butter, no new substance is formed, hence it is a Physical change.

Question 39: 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

Answer: No, the two solutions have different concentrations.

(b) Compare the mass % of the two solutions.

Answer: We know;
Mass % of Solution `=(text{Mass of soute})/(text{Mass of solution})xx100`

For first solution:
Mass of solute = 10 gram
Mass of solution = 100 gram + 10 gram = 110 gram

Hence; Mass % of solution`=10/110xx100=9.99%`

For second solution:
Mass of solute = 10 gram
Mass of solution = 100 gram

Hence; Mass % of solution `=10/100xx100=10%`

Mass percent of first solution: Mass percent of second solution `= 9.99 : 10`

Question 40: You are provided with a mixture containing sand, iron filings, ammonium chloride and sodium chloride. Describe the procedures you would use to separate these constituents from the mixture?

Answer: The given mixture can be separated using the following process.

Magnetic Separation: Using magnetic separation the iron fillings can be separated from the given mixture.

In this a magnet is hover just above the mixture, since iron is a magnetic substance it is attracted by magnet and stuck with it. By this first of all iron fillings are separated.

Sublimation: After the separation of iron fillings, ammonium chloride is separated by the process of sublimation.

Since, ammonium chloride is a sublimate and it turns into vapour directly without changing into liquid, thus when the mixture is sublimated, the ammonium chloride is deposited over the inner wall of funnel leaving the sodium chloride and sand in the watch glass. Ammonium chloride is separated by scratching from the inner wall of the funnel.

Filtration: Now the left mixture of sand and sodium chloride is put in water, after stirring the sodium chloride is dissolved in water. The solution is separated by the process of filtration. The sand left over the filter paper is separated out.

Vapourisation: By the process vaporization, the liquid so obtained is vapouried and crystals of ammonium chloride can be obtained.

Hence, by using the methods of magnetic separation, sublimation, filteration and vapourisation and crystallization the component of given mixture of sand, iron fillings, ammonium chloride and sodium chloride can be separated.

Question 41: 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 + 100g of water
(b) 0.11g of NaCl + 100g of water
(c) 0.0l g of NaCl + 99.99g of water
(d) 0.10 g of NaCl + 99.90g of water

Answer: (c) 0.01 g of NaCl + 99.99 g of water

Question 42: Calculate the mass of sodium sulphate required to prepare its 20% (mass percent) solution in 100g of water?

Answer: In a 20% solution containing 100 g water; the mass percentage of water = 100 – 20 = 80%

∴ 80% of solution is 100 gm

∴ 100% of solution is `100/80` gm

∴ 20% of solution is `100/80xx20 = 25` gm

Hence; to prepare 20% (w/w) solution in 100 gram of water 25 gram of sodium sulphate is needed.