Class 9 Science


In Text Question Answer

Question 1. What is meant by a pure substance?

Answer: When components of a substance cannot be separated by any physical method, the substance is called a pure substance. Elements and compounds are pure substances.

Question 2. List the points of differences between homogeneous and heterogeneous mixtures.


Homogeneous MixtureHeterogeneous Mixture
Components do not appear distinct from one another.Components appear to be distinct from one another.
It has uniform composition.It has non-uniform composition.
Examples: salt solution in water, sugar solution in water.Examples: Mixture of sand and iron fillings.

Question 3. Differentiate between homogeneous and heterogeneous mixtures with examples.

Answer: Same as in previous question

Question 4. How are sol, solution and suspension different from each other?


It is a heterogenous mixture.It is a homogeneous mixture.It is a heterogenous mixture.
Particles are too small to be seen by naked eyes, but large enough to produce Tyndall effect.Particles are large.Particles are very small, i.e. less than 1 nm.
Particles do not settle when sol is left undisturbed.Particles do not settle down when left undisturbed.Particles settle down when left undisturbed.
Components cannot be separated by process of filtration.Components cannot be separated by process of filtration.Components can be separated by process of filtration.

Question 5. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.

Answer: Mass of water = 100 g, mass of sodium chloride = 36 g

Total mass of solution = 100 + 36 = 136 g

Concentration `= (mass of solute) / (mass of solution) xx 100`

`= 36/136 xx 100 = 26.47%`

Question 6. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25°C), which are miscible with each other?

Answer: Kerosene and petrol can be separated from the mixture by the process of fractional distillation.

Question 7. Name the technique to separate

(a) butter from curd,
(b) salt from sea-water,
(c) camphor from salt.

Answer: (a) Centrifugation, (b) Evaporation, (c) Sublimation

Question 8. What type of mixtures are separated by the technique of crystallisation?

Answer: Crystallisation is used obtain pure crystals of solid from a solution.

Question 9. Classify the following as chemical or physical changes:

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

Answer: Chemical changes (c, e, h) Physical changes (a, b, d, f, g)

Question 10. Try segregating the things around you as pure substances or mixtures.

Answer: Pure substances (iron, copper, gold, etc.) Mixtures (milk, cooking oil, butter, drinking water, etc.)