# Is Matter Around Us Pure – Important Questions

Get the most probable question and answers of chapter 2 of Chemistry Class 9. Is matter around us pure; the chapter has high importance in the exam and is very scoring one too. A student can score full marks in this chapter. NTSEGuru experts have designed the important questions of chapter 2 chemistry class 9. Practice them now.

## Chemistry Class 9 Chapter 2 – Is Matter Around Us Pure

1. Write two reasons for considering sodium chloride or sugar a pure substance.

Ans.  a)  They are made up of only one kind of particles

b) They have a definite melting point.

2. What is a solution? What are its two components, define them. Give one example for each of the following:

a) Solid in liquid solution           b) Gas in liquid solution

c) Gas in gas solution                   d) Liquid in liquid solution

e) Solid in solid solution

Homogeneous mixture of two or more non reacting chemical  substances is called as solution.

Solute: Component present in lesser amount.

Solvent: Component present in larger amount

a) Sugar/salt in water              b) O2 in H2O/CO2 in H2O

c) Air                                            d) Water and alcohol

e) Alloys

3. A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by mass percentage of the solution.

Mass of solute (salt) = 40 g

Mass of solvent (water) = 320 g

We know,

Mass of solution    = Mass of solute + Mass of solvent

= 40 g + 320 g

= 360 g

Mass percentage of solution  $=\frac{mass\,of\,solute}{mass\,of\,solution}\times&space;100$

$=\frac{40}{360}\times&space;100=11.1$%

4. Mention few characteristics of compounds?

Ans: (i) A compound contains atom of two or more elements combined by chemical bonds.

(ii) It has a homogenous composition.

(iii) The elements in a compound are present in fixed proportion by mass.

(iv) The properties of compound are different from those of elements of which they are made.

(v) The separation of compound into its constituent element is possible mean by chemical mean.

(vi) During the formation of a compound by the combination of elements, energy is either liberated or absorbed.

(vii) A compound has its characteristic melting and boiling point.

5. What do you mean by Tyndall effect? Why do colloidal solutions show this effect? Give some examples of Tyndall effect observed in your surroundings.

Ans:  When a beam of light passes through a colloidal solution its path becomes visible due to scattering of light by colloidal particles. This is known as Tyndall effect.

Colloidal solutions show this effect due to larger particle size (1-100nm) than true solutions. Examples:

i) When a fine beam of light enters in room through a small hole its path becomes visible due to scattering of light by particles of dust and smoke.

ii) Sunlight entering into a thick canopy of trees gets scattered & its path becomes visible.

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

Ans: In both smoke & fog dispersion medium is gas (air), but dispersed phase is solid (unburnt carbon particles) in smoke, whereas liquid (water droplets) in fog.

7. What is the difference between simple distillation and fractional distillation? How can we obtain different gases from air?

Ans: Simple distillation:

i) Done for liquids having sufficient difference in their boiling points (more than 25oC)

ii) Condenser is directly attached to distillation flask.

Fractional distillation:

i) Done for liquids having small difference in their boiling point (less than 25oC)

ii) A fractionating column is attached to the distillation flask.

Gases can be liquefied by cooling and compressing the air and then liquefied air is subjected to fractional distillation to obtain different gases.

8. How will you confirm that a colourless liquid given to you is pure water?

If the boiling point and freezing point of the given liquid comes out to be  and  respectively under one atmosphere pressure, it confirms that the given liquid is pure water.

9. Explain why crystallization is a better technique than evaporation by giving two reasons?

Ans: Crystallization is a better technique than evaporation because:-

i) Some substances decompose on heating up to dryness during evaporation, crystallization does not require heating till dryness.

ii) Some soluble impurities may also be present in the solution which cannot be filtered & can contaminate the residue obtained after evaporation.

10. Differentiate between homogeneous and heterogeneous mixtures.

 Ans. Homogeneous  mixture Heterogeneous mixture a) Has uniform composition throughout the mixture. a) Composition throughout the mixture is not uniform. b) There is no visible boundary between the components/constituents. b) Components are visible with naked eyes or under microscope. c) eg. True solution c) eg. Suspension, colloids

11. Write the principle involved and give two applications of following techniques:

a) Centrifugation

b) Chromatography

Ans.  a) Centrifugation:

When a colloidal solution is spin rapidly the denser particles are forced to the bottom and lighter particles stay on the top and thus can be separated.

Applications

i) To separate butter from milk

ii) In diagnostic laboratories for blood and urine tests

b) Chromatography:

It is based on the difference in solubilities of two components of a mixture in a solvent due to which they rise to different heights on chromatography paper.

Applications:

i) To separate dyes present in black ink

ii) To separate natural pigments present in leaves or flowers

12. (a) Under which category of mixtures will you classify alloys and why?

(b) A solution is always a liquid. Comment.

(c) Can a solution be heterogeneous?

Ans: (a) Homogenous mixture, because they have a uniform composition throughout

(b) No, solid solutions and gaseous solutions are also possible. Examples brass and air

(c) No, solution is a homogenous mixture of two or more substances

13. Draw labelled diagram of the apparatus used for fractional distillation.

Ans:

14. 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?

Ans.

Step-1 Separate iron filings with the help of a magnet

Step-2 Sublimation of the remaining mixture separates ammonium chloride

Step-3 Add water to the remaining mixture, stir and filter

Step-4 The filtrate can be evaporated to get back sodium chloride

15. Boiling points in oC of three different gases were observed and recorded:

Oxygen           Argon              Nitrogen

Boiling point (oC)        –183                –186                –196

a) Arrange the gases in increasing order of their boiling points.

b) Which gas forms the liquid first as the air is cooled?

Ans:

a) Nitrogen < Argon < Oxygen

b) Oxygen, as it has highest boiling point.

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