Ans. Pure substances are: ice, iron, hydrochloric acid, calcium oxide, mercury.

2 Marks Questions

1. What is meant by a substance?

Ans. Substance can be defined as that kind of matter where constituent particles cannot be separated from each other by any physical process since they are all similar in chemical properties.

2. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than), which are miscible with each other?

Ans. We can separate a mixture containing kerosene and petrol by distillation technique since difference in their boiling points is more than . So through distillation we can get them separated.

3. Name the technique to separate

(i) butter from curd,
(ii) salt from sea-water,
(iii) camphor from salt.

Ans. (i) centrifugation method.
(ii) evaporation method.
(iii) sublimation method.

4. What type of mixtures are separated by the technique of crystallisation?

Ans. From impure samples of solids, pure solid crystals can be obtained by the method of crystallization for eg to obtain pure sugar from impure sample of the same.

5. What is a mixture? What are its various types?

Ans. A mixture is constituted by more than one substance (element/or compound) mixed in any proportion. They are of two types:
(a) Homogenous mixture
(b) Heterogeneous mixture

6. Define solute, solvent and solution?

Ans. Solute: - It is the component of the solution which is added to the solvent.
Solvent: - It is the component of the solution to which the solute is added or it dissolves the solute.
Solution: - It is constituted by solute and solvent.
For e.g. solution of NaCl- has NaCl as solute and water as solvent.

7. What is a solution? What are the properties of solution?

Ans. A solution is a homogenous mixture of two or more substance. The various properties of solution are: -
(a) It is a homogenous mixture.
(b) The particles of a solution are smaller than 1nm and hence cannot be seen by naked eyes.
(c) It does not scatter the beam of light passing through it.
(d) The component of solution cannot be separated from each other by the process of filtration.

8. Differentiate between elements and compounds.

Ans.

9. What is tyndall effect? Which kinds of solution show it?

Ans. The scattering of a beam of light by particles of solution when light is passed through it is called tyndall effect. Those solutions where size of the particle is very small for e.g. colloidal solution shows tyndall effect.

10. Differentiate between homogeneous and heterogeneous mixture?

Ans.

11. What is centrifugation? Where it is used?

Ans. Centrifugation is a technique used for separation of constituents of mixture and is based upon the principle that denser particles stay at bottom and lighter particles stays at the top when spun rapidly. It is used separate cream from milk.

12. What is a suspension? What are the properties of suspension?

Ans. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remains suspended throughout the bulk of the medium.

Properties of suspension:
(a) The particles can be seen by naked eyes.
(b) They scatter a beam of light passing through it.
(c) The particles settle down when left undisturbed.

3 Marks Questions

1. List the points of differences between homogeneous and heterogeneous mixtures.

Ans.

2. Differentiate between homo generous and heterogeneous mixtures with examples.

Ans.

3. How are sol, solution and suspension different from each other?

Ans.

4. To make a saturated solution,36 g of sodium chloride is dissolved in 100 g of water at293 K. Find its concentration at this temperature.

Ans. Mass of sodium chloride (solute) = 36 g

Mass of water (solvent) = 100 g

Mass of solution = 36 + 100 = 136 g

Therefore, concentration percentage = mass of solute/mass of solution

=

= 26.47 %

5.Classify the following as chemical or physical changes:

• cutting of trees,
• melting of butter in a pan,
• rusting of almirah,
• boiling of water to form steam,
• passing of electric current, through water and the water breaking down into hydrogen and oxygen gases,
• dissolving common salt in water,
• making a fruit salad with raw fruits, and
• burning of paper and wood.

Ans. cutting of trees = chemical change

melting of butter in a pan = physical change

rusting of almirah = chemical change

boiling of water to form steam = physical change

passing of electric current, through water and the water breaking down into hydrogen and oxygen gases = chemical change

dissolving common salt in water = physical change

making a fruit salad with raw fruits = physical change

burning of paper and wood = chemical change

6. Which separation techniques will you apply for the separation of the following?

(a) Sodium chloride from its solution in water.
(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride.
(c) Small pieces of metal in the engine oil of a car.
(d) Different pigments from an extract of flower petals.
(e) Butter from curd.
(f) Oil from water.
(g) Tea leaves from tea.
(h) Iron pins from sand.
(i) Wheat grains from husk.
(j) Fine mud particles suspended in water.

Ans. (a) Evaporation method

(b) Sublimation method

(c) by heating and then after filtration

(d) by Chromatography

(e) by method of centrifugation

(f) by using separating funnel

(g) by filtration method using strainer

(h) with the help of a magnet

(i) by winnowing

(j) by centrifugation

7. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.

Ans. Take more amount of solvent (water) in a pan and after heating it add little amount of solute (sugar) to the solvent. Solute will dissolve completely in the solvent forming true solution, then add tea leaves that are insoluble along with another soluble liquid milk. After boiling allow filtration with a sieve so the filtrate you obtain is tea while the residue has tea leaves that are thrown away.

8. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below(results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution).

(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in50 grams of water at 313 K?

(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain.

(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

(d) What is the effect of change of temperature on the solubility of a salt?

Ans. (a) At 313 K temperature the amount of potassium nitrate required was 62g in 100ml of water so in 50g water we will need to dissolve potassium nitrate.

(b) At 373K saturated solution preparation needs 54g potassium nitrate and at room temperature (293 K) saturation solution formation occurs with 35g potassium nitrate hence = 54 – 35 = 19g potassium nitrate will precipitate out as undissolved salt.

(c) Solubilities are (in 100 mg of water) 32,36,35,37 respectively for the mentioned salts and the highest solubility is of ammonium chloride at this temperature.

(d) Solubility of salts is directly proportional to the temperature i.e. if temperature increases then solubility will increase and if the temperature decreases solubility will also decrease.

9. Explain the following giving examples.

(a) saturated solution
(b) pure substance
(c) colloid
(d) suspension

Ans. (a) saturated solution: It is a solution in which no more solute particles can be dissolved at a particular temperature.

(b) pure substance: Such substance that has a uniform composition i.e. has particles with identical properties is called pure substance eg sugar, salt, water, nitrogen etc.

(c) colloid: It is a kind of heterogeneous mixture/solution in which particle size is between 1nm and 1000nm. Colloids have dispersion medium and dispersed phase.eg smoke, milk, shaving cream, jelly, cheese etc.

(d) suspension: It is a kind of heterogeneous mixture in which insoluble solid particles remain suspended in the medium and dispersion particles are visible to the unaided eyes.eg muddy river water, chalk powder in water, dust storm, sand in water etc.

10. Write a method to separate different gases from air.

Ans. Air is a homogeneous mixture of various gases.

It can be separated from its various components by fractional distillation.

(a) First compress and cool the air by increasing the pressure and decreasing the temperature.

(b) We obtain the liquid air; now allow the liquid air to warm up slowly in fractional distillation column.

(c) The various gases separate from each other according to their boiling points at various heights of the fractionally column.

11. What is a colloid? What are its various properties?

Ans. Colloids are the heterogeneous mixture of substances in which the particle size is too small and cannot be seen by naked eyes.

(1) It is a heterogeneous mixture, but appears homogenous.

(2) The size of particles is too small to be individually seen by naked eyes.

(3) They scatter beam of light passing through it and makes its path visible.

(4) The particles of colloid do not settle down when left undisturbed.

12. A solution contains 60g of NaCl in 400g of water. Calculate the concentration in term of mass by mass percentage of the solution.

Ans. Mass of solute (NaCl) = 60g

Mass of solvent (water) = 400g

Mass of solution = Mass of solute + Mass of solvent

= 60 + 400 = 460g

Mass percentage of solution =

= = = 13.4%

13. Differentiate between metals and non metal based upon the various properties that they show.

Ans.

14. Differentiate between mixtures and compound by giving appropriate examples?

Ans.

15. Write a method to separate a mixture of salt and ammonium chloride?

Ans. A mixture of salt and ammonium chloride can be separated by the process of sublimation. Since ammonium chloride changes directly from solid into gaseous state on heating and salt does not so this principle is used to the mixture of two.

(1) The mixture of NH₄Cl (ammonium chloride) and salt is taken in a china dish inside an inverted funnel.

(2) The mixture is heated and because NH₄Cl sublimates thus changes into vapours directly.

(3) Salt which is non-sublimable substance settles into the inverted funnel.

Separation of NH₄Cl salt by sublimation

16. What is crystallization? Where is it used? Why is this better than simple evaporation technique?

Ans. Crystallization is a process that separates a pure solid in the form of crystals from its solution. It is used to purify solids. For e.g. salt from sea water is purified using crystallization. It is a better technique than simple evaporation because:

(a) Some solid may decompose or get charred on heating to dryness during evaporation.

(b) On evaporation, some of the impurities still remain dissolved in the solution.

17. What is chromatography? What are its various applications and underline the basic principle involved?

Ans. Chromatography is a technique used for separation of those components whose solubility in the same solvent is different.

Its various applications are:

(a) It is used to separate different colours in dye.

(b) It is used to separate pigments from natural colours.

(c) It is used to separate drugs from blood.

The basic principle in chromatography is the different solutes have different solubility in the same solvent. For e.g. if we take a spot of ink on a paper and dip it in water than that coloured component which is more soluble in water rises faster and the other which is less soluble remains at the bottom and hence the two component can be separated.

18. A solution of acid is labeled is 95%. What is the mass of this that must be diluted with water to get 5L of solution containing 10 g of per litre?

Ans. 1L of the diluted solution must contain 10 g of.

Therefore, 5L of the diluted solution must contain 50 g of.

The concentration of the acid in the bottle is 95%.

This means that

95 g of is present in 100 g of the acid solution

50 g of will be present in

52.64 g of the solution