Question 1. What is meant by a substance?
Answer: A substance made up of a single type of particle is known as a pure substance.
Question 2. List the points of differences between homogeneous and heterogeneous mixtures.
Answer:
| S. No. | Homogeneous | Heterogeneous |
| 2 | It has a uniform composition. | Its composition is non- uniform. |
| 3 | It has no visible boundaries of separation. | It has visible boundaries of separation. |
| 4 | They are composed of only one phase. | They are composed of more than one phase. |
Question 3. How are sol, solution and suspension different from each other?
Answer:
| S.No. | Sol (colloid) | Solution
|
Suspension |
| 1
|
The particle size of the solute is between 1 nm to 100 nm | The particle size of the solute is less than 1 nm | The particle size of the solute is more than 100 nm |
| 2 | It is stable. | It is stable. | It is unstable. |
| 3
|
The particles scatter a beam of light. | The particles does not scatter a beam of light. | The particles scatter a beam of light. |
| 4
|
Particles are not visible to the naked eye. | Particles are not visible to the naked eye. | Particles are visible to the naked eye. |
Question 4. 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 solute (sodium chloride) = 36 g
Mass of solvent (water) = 100 g
Mass of solution = Mass of solute + Mass of solvent
= 36 g + 100 g = 136 g
Question 5. How will you separate a mixture containing kerosene and petrol (the difference in their boiling points is more than 25°C), which are miscible with each other?
Answer: A mixture of kerosene and petrol which are miscible with each other can be separated by distillation.
Method:
Take the mixture of petrol and kerosene in a distillation flask.
Fit it with a thermometer.
Arrange the apparatus as shown in the figure.
Heat the mixture slowly.
Petrol vaporises first as it has a lower boiling point. It condenses in the condenser and is collected from the condenser outlet.
Kerosene is left behind in the distillation flask.
Question 6. Name the technique to separate
(i) butter from curd,
(ii) salt from sea-water,
(iii) camphor from salt.
Answer: (i) Centrifugation,
(ii) Evaporation,
(iii) Sublimation.
Question 7. What types of mixtures are separated by the technique of crystallisation?
Answer: The crystallisation technique is used to obtain pure crystals from an impure mixture. Example: Salt from seawater.
Question 8. Classify the following as chemical or physical changes:
cutting off trees,
melting 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 gas,
dissolving common salt in water,
making a fruit salad with raw fruits and
burning of paper and wood.
Answer:
| Physical change | Chemical change |
| Cutting of trees | rusting of almirah, |
| Melting of butter in a pan | passing of electric current, through water and the water breaking down into hydrogen and oxygen gas |
| dissolving common salt in water | burning of paper and wood. |
| making a fruit salad with raw fruits | |
| boiling of water to form steam |
Question 9. Try segregating the things around you as pure substances or mixtures
Answer: Pure substances—Water, bread, sugar and gold.
Mixtures—Steel, plastic, paper, talc, milk and air.
Question 10. 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) Evaporation
(b) Sublimation
(c) Filtration
(d) Chromatography
(e) Centrifugation
(f) Separating funnel
(g) Filtration
(h) Magnetic separation
(i) Winnowing/ sedimentation
(j) Decantation and filtration
Question 11. Write the steps you would use for making tea. Use the words, solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Answer:
- Take a cup of water in a container as a solvent and heat it.
- Add sugar in it which is solute. Heat it till all the sugar dissolves.
- After heating, you get a solution of sugar and water.
- Sugar dissolves in water completely.
- Add one teaspoon of tea leaves, it is insoluble in water.
- Boil the content, add some milk which is again soluble in water and boil the content again.
- Filter the tea with the help of a strainer, the tea collected in the cup is filtrate and the tea leaves collected on the strainer are the residues.
Question 12. Explain the following giving examples:
(a) Saturated solution
(b) Pure substance
(c) Colloid
(d) Suspension
Answer: (a) Saturated solution: In a given solvent when no more solute can be dissolved further at a given temperature it is called a saturated solution.
(b) Pure substance: A substance that is composed of a single type of particle is known as a pure substance. E.g., gold, silver.
(c) Colloid: A colloid is a solution in which the size of solute particles is bigger than that of a true solution. It is a heterogeneous mixture. These particles are not visible to our naked eyes, they are quite stable, e.g., ink, and blood.
(d) Suspension: It is a heterogeneous mixture in which the solute particles are big enough to settle down. The particles are visible to our naked eye. eg., chalk water, paints, etc.
Question 13. Classify each of the following as a homogeneous or heterogeneous mixture: soda water, wood, and air. soil, vinegar, and filtered tea.
Answer: Homogeneous: Soda water, vinegar, filtered tea.
Heterogeneous: Wood, air, soil.
Question 14. How would, you confirm that a colourless liquid given to you is pure water?
Answer: By finding the boiling point of a given colourless liquid. If the liquid boils at 100°C at atmospheric pressure, then it is pure water. This is because pure substances have fixed melting and boiling point.
Question 15. Which of the following materials fall in the category of a “pure substance”?
(a) Ice (b) Milk (c) Iron
(d) Hydrochloric acid (e) Calcium oxide (f) Mercury
(g) Back (h) Wood (i) Air.
Answer: Pure substances are: Ice, iron, hydrochloric acid, calcium oxide and mercury.
Question 16. Identify the solutions among the following mixtures.
(a) Soil (b) Seawater
(c) Air (d) Coal
(e) Soda water.
Answer: Solutions are: Sea water, soda water and air.
Question 17. Which of the following will show the “Tyndall effect”?
(a) Salt solution (b) Milk
(c) Copper sulphate solution (d) Starch solution.
Answer: Milk and starch solution.
Question 18. Classify the following into elements, compounds and mixtures.
(a) Sodium (b) Soil (c) Sugar solution
(d) Silver (e) Calcium carbonate (f) Tin
(g) Silicon (h) Coal (i) Air
(j) Soap (k) Methane (l) Carbon dioxide
(m) Blood
Answer:
Elements – Compounds – Mixtures
Sodium – Calcium carbonate – Sugar solution
Silver – Methane – Soil
Tin – Carbon dioxide – Coal
Silicon – Soap – Air, Blood
Question 19. Which of the following are chemical changes?
(a) Growth of a plant (b) Rusting of iron
(c) Mixing of iron filings and sand (d) Cooking of food
(e) Digestion of food (f) Freezing of water
(g) Burning of a candle.
Answer: Chemical changes are:
(a) Growth of a plant (b) Rusting of iron
(c) Cooking of food (d) Digestion of food
(e) Burning of a candle