(IN TEXT QUESTIONS) (Page 15)
1: What is meant by a pure substance.
Ans: A pure substance is the one that consists of a single type of particles, i.e., all constituent particles of the substance have the same chemical nature. Pure substances can be classified as elements or compounds.
2: List the points of differences between homogeneous and heterogeneous mixtures.
Ans: Homogeneous mixture is a mixture having a uniform composition throughout the mixture.
e.g. salt in water, sugar in water, copper sulphate in water.
A heterogeneous mixture is a mixture having a non-uniform composition throughout the mixture.
e.g. sodium chloride and iron filings, salt and sulphur, oil and water.
(IN TEXT QUESTIONS) (Page 18)
1: Differentiate between homogeneous and heterogeneous mixtures with examples.
Ans: A homogeneous mixture is a mixture having a uniform composition throughout the mixture.
e.g. mixtures of salt in water, sugar in water, copper sulphate in water, iodine in alcohol, alloy, and air have uniform compositions throughout the mixtures.
On the other hand, a heterogeneous mixture is a mixture having a non-uniform composition throughout the mixture. e.g. composition of mixtures of sodium chloride and iron fillings, salt and sulphur, oil and water, chalk powder in water, wheat flour in water, milk and water are not uniform throughout the mixtures.
2: How are sol, solution and suspension different from each other?
| Sol | Solution | Suspension |
|
(i) Size of particle is between 1nm to 1500 nm |
i) Size of particle is less then 1nm | i) Size of particle is more then 1nm |
| (ii) It is translucent | i) It is transparent | ii) It is opaque |
|
(iii) Particles can be seen with microscope |
ii) Particles cannot be seen | iii) Particles can be seen by naked eye |
3: 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.
Ans: Mass of solute (sodium chloride) = 36 g (Given) Mass of solvent (water) = 100 g (Given)
Then, mass of solution = Mass of solute + Mass of solvent = (36 + 100) g = 136 g
Therefore, concentration (mass by mass percentage) of the solution
= Mass of solute/Mass of solvent×100 %
= 36/136 × 100 %
= 26.4 %
(IN TEXT QUESTIONS) (Page 24)
1: 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.
Ans: Mixture of two miscible liquids having a difference in their boiling points more than 25°C can be separated by the method of distillation. Thus, kerosene and petrol can be separated by distillation
In this method, the mixture of kerosene and petrol is taken in a distillation flask with a thermometer fitted in it. We also need a beaker, a water condenser, and a Bunsen burner. The apparatus is arranged as shown in the above figure. Then, the mixture is heated slowly. The thermometer should be watched simultaneously. Kerosene will vaporize and condense in the water condenser. The condensed kerosene is collected from the condenser outlet, whereas petrol is left behind in the distillation flask.
2: Name the technique to separate
(i) Butter from curd (ii) salt from sea-water (iii) camphor from salt.
Ans: (i) Butter can be separated from curd by centrifugation.
(ii) Salt can be separated from sea-water by evaporation.
(iii) Camphor can be separated from salt by sublimation.
3: What type of mixtures is separated by the technique of crystallization.
Ans: By the technique of crystallization, pure solids are separated from impurities. Also substances soluble in a liquid solvent can be separated by this method
eg: salt obtained from sea is separated from impurities; crystals of alum (Phitkari) are separated from impure samples.
(IN TEXT QUESTIONS) (Page 24)
1: Classify the following as chemical or physical changes:
- Cutting of trees,
- Melting of butter in a pan,
- Rusting of an 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,
- Burning of paper and wood.
Ans:
- Cutting of trees → Physical 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 water breaking down into hydrogen and oxygen gas → 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
2: Try segregating the things around you as pure substances or mixtures.
Ans: (i) Pure substance: Water, salt, sugar etc.
(ii) Mixture: Salt water, soil, wood, air, cold drink, rubber,fog, milk, butter, food,etc.