Class 9 Science

# Mixture

## NCERT Exemplar Problems

### Short Answer Type

#### Part 1

Question: 10. Suggest separation technique(s) one would need to employ to separate the following mixtures.

(a) Mercury and water

Answer: The technique of Filtration

(b) Potassium chloride and ammonium chloride

(c) Common salt, water and sand

Answer: Sedimentation, decantation, filtration and evaporation

(d) Kerosene oil, water and salt

Answer: First filtration using separating funnel then after, vapourisation.

Question: 11. Which of the tubes in Figure given here (a) and (b) will be more effective as a condenser in the distillation apparatus?

Answer: Figure (a) will be more effective, because of containing fractionating column.

Question: 12. Salt can be recovered from its solution by evaporation. Suggest some other technique for the same?

Answer: To separate salt from its solution crystallization can also be used.

Question: 13. The ‘sea-water’ can be classified as a homogeneous as well as heterogeneous mixture. Comment.

Sea water is the mixture of many salts, water and other many impurities. Apart from these many gases are also dissolved in sea water. Because of salt and some other bigger size of impurities sea water is classified as heterogeneous mixture. Because of mixture of several gases in sea water it is also classified as homogeneous mixture.

Question: 14. While diluting a solution of salt in water, a student by mistake added acetone (boiling point 56°C). What technique can be employed to get back the acetone? Justify your choice.

Answer: Since, the boiling point of acetone is 560C and boiling point of water is 1000C, and for distillation the minimum difference in temperature should be 2500C. Thus by the process of distillation acetone can be separated.

Question: 15. What would you observe when

(a) a saturated solution of potassium chloride prepared at 60°C is allowed to cool to room temperature.

Answer: Since the solution is a saturated solution, and prepared at 600C which is above the room temperature, therefore, while it is allowed to cool at room temperature some of the potassium chloride will settle down at the bottom, because saturation decreases with decrease in temperature.

(b) an aqueous sugar solution is heated to dryness.

Answer: When an aquous solution of sugar is heated to dryness, the sugar will be left behind in the container after the vaporization of water. The sugar left in the container may be charred because of more heating.

(c) a mixture of iron filings and sulphur powder is heated strongly.

Answer: When a mixture of iron filling and sulphur power is heated strongly ferrous sulphide will be formed.

Question: 16. Explain why particles of a colloidal solution do not settle down when left undisturbed, while in the case of a suspension they do.

Answer: Particles in colloid is very small compare to that of the suspension. This makes the colloids more stable than suspension.

Because of the larger size of particles they settle down when suspension is left undisturbed. While in the case of colloid they do not settle as the particles are very small.

Question: 17. Smoke and fog both are aerosols. In what way are they different?

Answer: Smoke: Smoke is mixture of gases and can goes to higher altitudes because of having more temperature.

Fog: Fog is mixture of liquid and gases. Fog is settled near the earth surface because of less temperature.

Question: 18. Classify the following as physical or chemical properties

(a) The composition of a sample of steel is: 98% iron, 1.5% carbon and 0.5% other elements.

Answer: This is the physical property. Since steel is the alloy and considered as mixture of more than one elements.

(b) Zinc dissolves in hydrochloric acid with the evolution of hydrogen gas.

Answer: This shows the reaction of zinc with hydrochloric acid, hence it is a chemical property.

(c) Metallic sodium is soft enough to be cut with a knife.

Answer: Since, it shows the softness of sodium, thus is a physical property.

(d) Most metal oxides form alkalis on interacting with water.

Answer: This property show the reaction of metal oxides with water, thus a chemical property.

Question: 19. The teacher instructed three students ‘A’, ‘B’ and ‘C’ respectively to prepare a 50% (mass by volume) solution of sodium hydroxide (NaOH). ‘A’ dissolved 50g of NaOH in 100 mL of water, ‘B’ dissolved 50g of NaOH in 100g of water while ‘C’ dissolved 50g of NaOH in water to make 100 mL of solution. Which one of them has made the desired solution and why?

Concentration is the relative percentage of solute compared to the total volume of the solution and it is calculated by dividing mass by volume.

In the case of A, since 50g of NaOH has been dissolved in 100 mL of water, the total volume of solution became about 150 mL, thus concentration of NaOH would be less than 50%.

In the case of B, since 50g of NaOH has been dissolved in 100g of water, therefore, total volume of the solution would become 150 mL, consequently concentration of NaOH would again less than 50%.

In the case of C, 50g of NaOH has been dissolved in water and then volume of the solution made to 100mL, thus concentration of NaOH would become 50%.

Thus, C made the solution of NaOH having concentration equal to 50%.

Question: 20. Name the process associated with the following

(a) Dry ice is kept at room temperature and at one atmospheric pressure.

Answer: Since, when dry ice is left at room temperature, it turns into gas, thus this is the process of sublimation.

(b) A drop of ink placed on the surface of water contained in a glass spreads throughout the water.

Answer: When a drop of ink is placed on the surface of water, it spread over the water and finally mixed with water because of the motion of particles, hence this process is diffusion.

(c) A potassium permanganate crystal is in a beaker and water is poured into the beaker with stirring.

Answer: When potassium permanganate crystals are kept in beaker and water is poured and stirred, the particles of potassium permanganate would mix with water, because of motion of particles, stirring speed up the mixing process, hence this is the process of diffusion.

(d) A acetone bottle is left open and the bottle becomes empty.

Answer: Since acetone vaporizes at room temperature, when a acetone bottle is left open the acetone would vaporize and mix with air, thus this is the process of vaporization.

(e) Milk is churned to separate cream from it.

Answer: While milk is churned, the cream and milk is separated because of centrifugal force, thus this is the process of centrifugation.

(f) Settling of sand when a mixture of sand and water is left undisturbed for some time.

Answer: When mixture of sand and water is left undisturbed, the sand settle at the bottom of water, thus this is the process of sedimentation.

(g) Fine beam of light entering through a small hole in a dark room, illuminates the particles in its paths.

Answer: When fine beam of light entered through a small hole in dark room, because of collision of particles of air and dust, sunbeam illuminates the particles in its path and dust particles are appeared dancing, this happens because of the Tyndall effect.

Question: 21. You are given two samples of water labelled as ‘A’ and ‘B’. Sample ‘A’ boils at 100°C and sample ‘B’ boils at 102°C. Which sample of water will not freeze at 0°C? Comment.

Answer: Since impurities in water raise its boiling point, thus water in sample B is impure. Hence it will not freeze at 00C because of impurities since impAnswer: Since impurities in water raise its boiling point, thus water in sample B is impure. Hence it will not freeze at 00C because of impurities since impurities decreases the freezing point below the 00C, this is the cause that’s why sea water remain liquid below the 00C.