SEBA Class 9 Science: Is Matter Around Us Pure?
INTEXT QUESTION
1. What is meant by a substance?
Answer: A substance is a pure single form of matter. It has the same composition throughout and contains only one kind of pure matter. Example: Hydrogen, oxygen, sulphur, etc.
2. List the points of difference between homogeneous and heterogeneous mixtures.
INTEXT QUESTION:
1. How are sol (colloidal solution), solution and suspension different from each other?
2. 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.
INTEXT QUESTION
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?
Answer: To separate a mixture of kerosene and petrol, which are miscible liquids and have a difference in boiling points of more than 25°C, we use the method of simple distillation.
In this method:
1. The mixture is heated slowly in a distillation flask.
2. Petrol, having the lower boiling point, vaporises first.
3. The vapour is then cooled in a condenser and collected separately.
4. Kerosene remains behind in the flask.
2. Name the technique to separate (i) butter from curd, (ii) salt from sea water, (iii) camphor from salt.
1. Answer: Butter from curd – Centrifugation
2. Salt from seawater – Crystallisation
3. Camphor from salt – Sublimation
3. What type of mixtures are separated by the technique of crystallisation?
Answer: The technique of crystallisation is used to separate solid–liquid mixtures.
EXERCISE QUESTION
1. Which separation techniques will you apply for the separation of the following?
(a) Sodium chloride from its solution in water.
Answer: Crystallisation.
(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride.
Answer: Sublimation. (Ammonium chloride is a sublimable substance)
(C) Small pieces of metal in the engine oil of a car.
Answer: Filtration.
(d) Different pigments from an extract of flower petals.
Answer: Chromatography.
(e) Butter from curd.
Answer: Centrifugation.
(f) Oil from water.
Answer: Separating funnel.
(g) Tea leaves from tea.
Answer: Filtration.
(h) Iron pins from sand.
Answer: Magnetic separation.
(i) Wheat grains from husk.
Answer: Winnowing.
(j) Fine mud particles suspended in water.
Answer: Centrifugation.
2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Answer: Method of preparation of tea
1. Take water as the solvent and heat it.
2. Add sugar and tea leaves. Sugar is a solute and is soluble, so it dissolves in water to form a solution.
3. Tea leaves are insoluble in water, so they do not dissolve.
4. After boiling, pour the mixture through a strainer.
5. The liquid that passes through is the filtrate (tea).
6. The tea leaves left on the strainer are the residue.
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 a change of temperature on the solubility of a salt?
Answer:
(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 and form crystals.
(c) The solubility of the salts at 293 K are:
Potassium nitrate 32
Sodium chloride 36
Potassium chloride 35
Ammonium chloride 37
Thus, Ammonium chloride has the maximum solubility at 293 K.
(d) The solubility of a solid (salt) decreases with a fall in temperature, while it increases with a rise in temperature.
4. Explain the following, giving examples.
(a) saturated solution
Answer: A saturated solution is a solution in which no more solute can be dissolved in the solvent at a given temperature. Example: When you keep adding salt to water and it stops dissolving, the solution becomes saturated.
(b) pure substance
Answer: A pure substance consists of only one type of particle. It has uniform composition and fixed properties throughout. Example: Distilled water or copper.
(c) colloid
Answer: A colloid is a type of heterogeneous mixture where particles are not visible to the naked eye and do not settle down on standing. Example: Milk is a colloid.
(d) Suspension
Answer: A suspension is a heterogeneous mixture in which the particles are visible and they settle down on standing. Example: Muddy water.
5. Classify each of the following as a homogeneous or heterogeneous mixture. soda water, wood, air, soil, vinegar, filtered tea.
Homogeneous Mixtures:
Soda water
Air
Vinegar
Filtered tea
Heterogeneous Mixtures:
Wood
Soil
6. How would you confirm that a colourless liquid given to you is pure water?
Answer: To confirm that a colourless liquid is pure water, we can:
Measure its boiling point.
Pure water boils at exactly 100°C at normal atmospheric pressure.
If the liquid boils exactly at 100°C, then it is pure water.
If it boils above or below 100°C, it is not pure and may contain dissolved impurities.
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
Pure Substances:
Ice
Iron
Hydrochloric acid
Calcium oxide
Mercury
Not Pure Substances (Mixtures):
Milk
Brick
Wood
Air
8. Identify the solutions among the following mixtures.
(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water
Solutions:
(b) Sea water
(c) Air
(e) Soda water
Not Solutions:
(a) Soil
(d) Coal
9. Which of the following will show the “Tyndall effect”?
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution
Tyndall Effect is shown by:
(b) Milk
(d) Starch solution
Tyndall Effect is not shown by:
(a) Salt solution
(c) Copper sulphate solution
10. Classify the following into elements, compounds and mixtures.
(a) Sodium = Elements
(b) Soil = Mixture
(c) Sugar solution = Mixture
(d) Silver = Elements
(e) Calcium carbonate = Compounds
(f) Tin = Elements.
(g) Silicon = Elements.
(h) Coal = Mixture.
(i) Air = Mixture.
(j) Soap = Soap.
(k) Methane = compound.
(l) Carbon dioxide = Compound.
(m) Blood = Mixture.
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.
Chemical Changes:
(a) Growth of a plant
(b) Rusting of iron
(d) Cooking of food
(e) Digestion of food
(g) Burning of a candle
Not Chemical Changes (Physical Changes or Mixtures):
(c) Mixing of iron filings and sand
(f) Freezing of water