Class 9 Science Chapter 2 – Is Matter Around Us Pure
Question 1:
What is meant by a pure substance?
Answer: A pure substance is one which is made up of only one kind of atoms or molecules.
Question 2 :
List the points of differences between homogeneous and heterogeneous mixtures.
Answer:
| Homogeneous mixture | Heterogeneous mixture | |
| (1) | A mixture in which different constituents are mixed uniformly. | A mixture in which different constituents are not mixed uniformly. |
| (2) | It cannot have physically distinct parts. | It has physically distinct parts. |
| (3) | The constituents cannot be easily seen. | The constituents can be easily seen. |
| (4) | The constituents cannot be easily separated. | The constituents can be easily separated. |
| (5) | Examples : Sugar solution, soda water, soft drinks, vinegar, air etc. | Examples : Sugar and sand mixture, milk, ink, paint, wood, blood etc. |
Question 3:
How are sol, solution and suspension different from each other?
Answer:
| Property | Sol | Solution | Suspension | |
| (i) | Nature | Heterogeneous | Homogeneous | Heterogeneous |
| (ii) | Particle size (diameter) | Between 10-7 to 10-5 cm (10-9 to 10-7 m or 1 nm to 100 nm) | Less than 1 nm (less than 10-9 to 10-7 m) | Mordthan 100 nm |
| (iii) | Appearance | Generally clear | Clear | Opaque |
| (iv) | Visibility | Visible with ultra microscope | Not visible | Visible with naked eyes |
| (v) | Diffusion | Diffuses very slowly | Diffuses rapidly | Does not diffuse |
| (Vi) | Tyndall effect | Shows | Does not show | Shows |
| (vii) | Settling of particles | Settle only on centrifugation | Does not settle | Settle of their own |
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 sodium chloride = 36 g Mass of solution = 36 + 100 = 136 g Concentration of solution.
Question 5:
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: The mixture of two miscible liquids such as kerosene and petrol whose boiling points differ by more than 25°C can be easily separated by the technique of simple distillation.
Question 6:
Name the technique to separate
(i) butter from curd
(ii) salt from sea-water
(iii) camphor from salt
Answer: (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.
Question 7:
What type of mixtures is separated by the technique of crystallization?
Answer:
By the technique of crystallization, pure solids are separated from impurities. For example, salt obtained from the sea is separated from impurities
Question 8:
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 water breaking down into hydrogen and oxygen gas
• Dissolving common salt in water
• Making a fruit salad with raw fruits
• Burning of paper and wood
Answer:
• 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
Question 9:
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.
Answer:
(a) Sodium chloride from its solution in water → Evaporation
(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride → Sublimation
(c) Small pieces of metal in the engine oil of a car → Centrifugation or filtration or decantation
(d) Different pigments from an extract of flower petals → Chromatography
(e) Butter from curd → Centrifugation
(f) Oil from water → Using separating funnel
(g) Tea leaves from tea → Filtration
(h) Iron pins from sand → Magnetic separation
(i) Wheat grains from husk → Winnowing
(j) Fine mud particles suspended in water → Centrifugation
