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Questions, Page 15

1. What is meant by a pure substance?Solution: A pure substance is one that cannot be separated into different constituents by physical or chemical processes. A pure substance is one that contains particles of only one type of a substance.

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

Homogeneous mixtures:They have uniform compositions. The components of homogeneous mixtures are not physically distinct. Salt in water, sugar in water are examples of homogeneous mixtures.

Heterogeneous mixtures:They contain physically distinct parts and have non-uniform compositions. Mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water are examples of heterogeneous mixtures.

Questions, Page 15

1. Differentiate between homogeneous and heterogeneous mixtures with examples.
Solution: 

Homogeneous mixtures:They have uniform compositions. The components of homogeneous mixtures are not physically distinct. Most solutions are homogeneous mixtures. Salt in water, sugar in water are examples of homogeneous mixtures.

Heterogeneous mixtures:They contain physically distinct parts and have non-uniform compositions. Mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water are examples of heterogeneous mixtures. Suspensions and colloids are also heterogeneous mixtures

2. How are sol, solution and suspension different from each other?
Solution:


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.Solution:



Questions, Page 24

1. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25ÂșC), which are miscible with each other?Solution: The process of distillation is used for the separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points (that is greater than 25°C).

2. Name the technique to separate(i) butter from curd,
(ii) salt from sea-water,
(iii) camphor from salt.
Solution:
(i) Centrifugation
(ii) Evaporation
(iii) Sublimation

3. What type of mixtures are separated by the technique of crystallisation?Solution: The crystallisation method is used to purify solids.

Questions, Page 24

1. 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 the water breaking down into hydrogen and oxygen gases,
• dissolving common salt in water,
• making a fruit salad with raw fruits, and
• burning of paper and wood.

Solution:Physical changes: Cutting of trees, melting of butter in a pan, boiling water to form steam, making a fruit salad with raw fruits
Chemical changes: Rusting of almirah, passing of electric current through water and the water breaking down into hydrogen and oxygen gases, burning of paper and wood

2. Try segregating the things around you as pure substances or mixtures.
Solution:
Pure substances: water, glass, iron nail, pencil lead, etc.
Mixtures: Air, blood, butter, milk, steel, paper, etc.

End of chapter Exercises, page 29 – 30

1. 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.
Solution:
(a) Evaporation
(b) Sublimation
(c) Filtration
(d) Chromatography
(e) Centrifugation
(f) Distillation
(g) Sieving
(h) Magnetic separation
(i) Sieving and winnowing
(j) Sedimentation, decantation and filtration

2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.Solution: Take some water in a pan. Keep the pan over flame. Add sugar and tea leaves, which are solute into the water in the pan, which is the solvent. Heat the water over the pan till the sugar, which is soluble in water dissolves in it. Tea leaves are insoluble in water and will remain suspended. Now add water to the sugar and tea leaves solution and bring the mixture to a boil. Filter the prepared tea through a sieve. Filtrate should be poured in a cup, while the residue can be thrown away.

3. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below (results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution).


(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K?
(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain. (c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
(d) What is the effect of change of temperature on the solubility of a salt?

Solution:(a) Since 62 g of potassium nitrate is dissolved in 100g of water to prepare a saturated solution at 313 K, 31 g of potassium nitrate should be dissolved in 50 g of water to prepare a saturated solution at 313 K.
(b) The amount of potassium chloride that should be dissolved in water to make a saturated solution increases with temperature. Thus, as the solution cools some of the potassium chloride will precipitate out of the solution.
(c) The solubility of the salts at 293 K are:
Potassium nitrate – 32 g
Sodium chloride – 36 g
Potassium chloride – 35 g
Ammonium chloride – 37 g
Ammonium chloride has the highest solubility at 293 K.
(d) The solubility of a salt increases with temperature.

4. Explain the following giving examples.(a) saturated solution
(b) pure substance
(c) colloid
(d) suspension
Solution:
(a) At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving is said to be a saturated solution. Few examples of saturated solutions are soft drinks and nitrogen in Earth’s soil.
(b) A pure substance is one that cannot be separated into different constituents by physical or chemical processes. A pure substance is one that contains particles of only one type of a substance. Pure substances can be elements or compounds. Some examples of pure substances are iron, water, oxygen, etc.
(c) Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Smoke, paint, butter are few examples of colloids.
(d) Materials that are insoluble in a solvent and have particles that are visible to naked eyes form a suspension. A suspension is a heterogeneous mixture. Some examples of suspension are water with chalk particles, sandy water and water with stones

5. Classify each of the following as a homogeneous or heterogeneous mixture.
soda water, wood, air, soil, vinegar, filtered tea.
Solution:
Homogeneous – soda water, air, vinegar, filtered tea
Heterogeneous – wood, soil

6. How would you confirm that a colourless liquid given to you is pure water?Solution: Perform the electrolysis of water, i.e., pass the electricity through it, so that the water molecules break into hydrogen and oxygen. Hydrogen and oxygen are gases at room temperature. Thus, they will be released in the air. If after the complete electrolysis of water, some residue is left, then the water is not pure. If no residue is obtained, the water is pure.

7. 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) Brick
(h) Wood
(i) Air.
Solution: Ice, iron, hydrochloric acid, calcium oxide, and mercury are pure substances.

8. Identify the solutions among the following mixtures.(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water.
Solution: Sea water, air, and soda water are solutions.

9. Which of the following will show “Tyndall effect”?(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution.
Solution: Colloids show Tyndall effect. Milk is a colloid. Thus, it will show Tyndall effect. Therefore, the correct answer is (b).

10. 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
Solution:
Elements: sodium, silver, tin, and silicon.
Compounds: calcium carbonate, soap, methane, and carbon dioxide.
Mixtures: soil, sugar solution, coal, air, and blood.

11. 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.
Solution: Rusting of iron, cooking of food, digestion of food, and burning of candle are chemical changes.

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