Class nine Chapter 2 NCERT Solutions Science
Chapter 2: Is Matter Around us Pure
1. What is meant by a substance?
Ans. A pure substance is one that consists of a single type of particles, i.e., all constituent particles of the substance have the same chemical nature. Substances can be classified as elements or compounds.
2. List the points of differences between homogeneous and heterogeneous mixtures.
| Homogeneous Mixture | Heterogeneous Mixture |
| Components do not appear separate from one another. | Components appear to be separate from one another. |
| It has a uniform composition. | It has a non-uniform composition. |
| Examples: salt solution in water, sugar solution in water. | Examples: Mixture of sand and iron fillings. |
Page No. 18
1. Differentiate between homogeneous and heterogeneous mixture with examples.
Ans. A homogeneous mixture is a mixture having a uniform composition throughout the mixture.
For example salt in water, sugar in water, copper sulphate in water. A heterogeneous mixture is a mixture having a non-uniform composition throughout the mixture. For example sodium chloride and iron filings, salt and sulphur, oil and water etc.
2. How are sol, solution and suspension different from each other?
| Property | Sol | Solution | Suspension |
| Nature | Heterogeneous | Homogeneous | Heterogeneous |
| Particle size | 1 nm – 100 nm | Less than 1nm | More than 100nm |
| Stability | Quite stable | Very stable | Unstable |
| Tyndall effect | yes | no | yes/no |
| Appearance | Generally clear | Clear | opaque |
| Visibility | Visible with an ultramicroscope | Not visible | Visible with the naked eye |
| Diffusion | Diffuses very slowly | Not visible | Visible with the naked eye |
| Settling | Get settled in centrifugation | Do not settle | Settle on their own |
| Example | Milk of magnesia | Salt and sugar in water | Sand in water |
3. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 kelvin. Find its concentration at this temperature.
Ans. Mass of solute (sodium chloride) = 36 g (Given)
Mass of solvent (water) = 100 g (Given)
So, the mass of solution = Mass of solute + Mass of solvent
= (36 + 100) g = 136 g
Therefore, concentration (mass by mass percentage) of the solution
Page 24 A
1. How will you separate a mixture containing kerosene and petrol (difference in their boiling
points are more than 25oC) which are miscible with each other?
Ans. The mixture of miscible liquids whose boiling point difference is more than 25oC such as kerosene and petrol can be separated by a technique called simple distillation. The principle of separation is based on the volatility of the substances.
The process of distillation is as follows:
(a) Take the mixture in a distillation flask.
(b) Fit it with a thermometer and heat the mixture.
(c) Petrol has a lower boiling point so evaporates first.
(d) As the vapours rise up and reach the condenser, the temperature is decreased and the vapour
is condensed into liquid and is collected in a flask.
(e) The kerosene that has relatively higher boiling point remains in the flask in the liquid form.
(f) Hence, both liquids are separated out.
2. Name the techniques used to separate the following:
(a) Butter from curd.
(b) Salt from seawater
(c) Camphor from salt
Ans. (a) Centrifugation, (b) Evaporation, (c) Sublimation
3. What type of mixtures are separated by the technique of crystallization?
Ans. Crystallization is a technique of separation of solid from a liquid solution. It relates to precipitation but here, the precipitate obtained is in crystal form and has very high purity.
This technique is thus used in the purification of impure substances. Example: Salt from seawater after the evaporation.
Page No.24 B
1. Classify the following as physical or chemical change:
• 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 water breaking into hydrogen and oxygen gases.
• Dissolving common salt in water
• Making a fruit salad with raw fruits
• Burning of paper and wood
Ans.
| Physical change | Chemical change |
| Cutting the trees Melting of butter in a pan Boiling of water to form steam Dissolving common salt in water Making a fruit salad with raw fruits. | Rusting of almirah Passing of electric current through water, and water breaking into hydrogen and oxygen gases. Burning of paper and wood. |
2. Try segregating the things around you as pure substances and mixtures.
Ans. • Pure substance: water, salt, iron, diamond • Mixture: sand, salad, concrete, air, steel