Class 9 Science Chapter 2 Question Answers - Is Matter Around Us Pure?

Is Matter Around Us Pure

Exemplar Problems and Ans (NCERT)

Short Ans Type:

Ques 1. Suggest separation technique(s) one would need to employ to separate the following mixtures.
(a) Mercury and water
Ans: The technique of Filtration

(b) Potassium chloride and ammonium chloride
Ans: Sublimation.Fig: Sublimation(c) Common salt, water and sand
Ans: Sedimentation, decantation, filtration and evaporation

(d) Kerosene oil, water and salt
Ans: First filtration using separating funnel then after, vaporization.

Ques  2. Which of the tubes in figure  (a) and (b) will be more effective as a condenser in the distillation apparatus?

Ans: Condenser (a) will be more effective because it provides more surface area for cooling of the vapours passing through it. 

Ques  3. Salt can be recovered from its solution by evaporation. Suggest some other technique for the same?

Ans: Salt can be recovered from its solution by 'crystallisation'. Crystallisation' is a better technique than 'evaporation' because it removes soluble impurities also, which do not get removed in the process of evaporation.  

Ques  4. The 'sea-water' can be classified as a homogeneous as well as a heterogeneous mixture. Comment.
Ans:  'Sea-water' is called homogeneous as it contains dissolved salts in it. It may be called heterogeneous as it contains various insoluble components too as sand, microbes, shells made of calcium carbonate and so many other things. 

Ques  5. 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.
Ans: Acetone is soluble in water, a homogeneous mixture is obtained and hence separation by separating funnel cannot be used. Acetone can be get back by simple distillation because the difference in the boiling points of acetone and water is more than 25oC. Boiling point of acetone 56°C Boiling point of water 100°C In distillation flask, acetone will boil at 56°C and change into vapours and can be collected in flask after condensation.  

Ques  6. What would you observe when
(a) a saturated solution of potassium chloride prepared at 60°C is allowed to cool to room temperature.
Ans: 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.
Ans: When an aqueous 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.
Ans: When a mixture of iron filling and sulphur power is heated strongly ferrous sulphide will be formed.

Ques  7. Explain why particles of a colloidal solution do not settle down when left undisturbed, while in the case of a suspension they do?
Ans: The colloidal particles are smaller and not heavy. They always remain in a state of zig-zag motion, called Brownian movement, which counters the force of gravity acting on colloidal particles and hence, helps in providing stability to colloidal sols by not allowing them to settle down. 

Apart from this, colloidal particles are charged and repel each other. This fact also do not the particles of colloidal solution to settle down. Whereas particles of suspension are larger, heavy and have less movement, thus settle down due to gravity.  

Ques  8. Smoke and fog both are aerosols. In what way are they different?

Ans:  In smoke and fog. dispersion medium is same, i.e., air but they differ in dispersed phase. In smoke, solid carbon particles are dispersed in air while in fog, liquid water particles are dispersed in air.  

Ques  9. 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.
Ans: 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.
Ans: This shows the reaction of zinc with hydrochloric acid, hence it is a chemical property.

Fig:  Reaction of zinc with hydrochloric acid

(c) Metallic sodium is soft enough to be cut with a knife.
Ans: Since, it shows the softness of sodium, thus is a physical property.

(d) Most metal oxides form alkalis on interacting with water.
Ans: This property shows the reaction of metal oxides with water, thus a chemical property.

Ques  10. The teacher instructed three students ‘A’, ‘B’ and ‘C’ respectively to prepare a 50% (mass by volume) solution of sodium hydroxide (NaOH). ‘A’ dissolved 50 g of NaOH in 100 mL of water, ‘B’ dissolved 50 g 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?

Ans: 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%. 

Ques  11. Name the process associated with the following
(a) Dry ice is kept at room temperature and at one atmospheric pressure.
Ans: Since, when dry ice is left at room temperature, it turns into gas, thus this is the process of sublimation.

Fig: Subliming dry ice

(b) A drop of ink placed on the surface of water contained in a glass spreads throughout the water.
Ans: 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.
Ans: 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.
Ans: 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.
Ans: 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.
Ans: When mixture of sand and water is left undisturbed, the sand settle at the bottom of water, thus this is the process of sedimentation.

Fig: Mixture of sand and water

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

Ques  12. 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.
Ans: 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 impAns: 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.

Ques  13. What are the favourable qualities given to gold when it is alloyed with copper or silver for the purpose of making ornaments? ed with copper or silver for making of ornaments.
Ans: Since, Gold is a highly malleable metal so it is difficult to make the ornaments with pure gold, thus to make its rigidity desiring gold is alloyed with copper or silver for making of ornaments.Fig: Gold ring

Ques  14. An element is sonorous and highly ductile. Under which category would you classify this element? What other characteristics do you expect the element to possess?
Ans: Since a metal is sonorous and ductile, thus if an element possesses these qualities this will be kept under the category of metals. The other qualities of metal are good conductor of heat and electricity, lustrous, malleability, etc.

Ques  15. Give an example each for the mixture having the following characteristics. Suggest a suitable method to separate the components of these mixtures.
(a) A volatile and a non-volatile component.
Ans: The mixture of acetone and water. In this acetone is volatile and water is non-volatile. The mixture of water and acetone can be separated by the process of distillation.