AP 9th Physical Science Textbook solutions 4. Is Matter Around Us Pure? (2026-27)

4 Is Matter Around Us Pure?

Activity 4.1 (Page 86)

• Report the observations on the uniformity in colour and texture.
  Groups A and B obtained a mixture which has a uniform composition throughout. Groups C and D have obtained mixtures, which contain physically distinct parts and have non-uniform compositions.

Activity 4.2 (Page 88)

• Are the particles in the mixture visible?
  In solutions (Groups A and B), the particles cannot be seen by naked eyes. In suspensions (Group C), the particles are visible to the naked eye. In colloidal solutions (Group D), the particles are too small to be individually seen with naked eyes.
• Direct a beam of light from a torch through the beaker containing the mixture and observe from the front. Was the path of the beam of light visible?
  For solutions (Groups A and B), the path of light is not visible. For suspensions (Group C) and colloidal solutions (Group D), the particles scatter a beam of light passing through it and make its path visible (Tyndall effect).
• Leave the mixtures undisturbed for a few minutes. Is the mixture stable or do the particles begin to settle after some time?
  Solutions (Groups A and B) and colloidal solutions (Group D) are stable, meaning the particles do not settle down when left undisturbed. Suspensions (Group C) are unstable, and the solute particles settle down when left undisturbed.
• Filter the mixture. Is there any residue on the filter paper?
  For solutions (Groups A and B) and colloidal solutions (Group D), there is no residue because they cannot be separated from the mixture by the process of filtration. For suspensions (Group C), the particles can be separated from the mixture by the process of filtration, leaving a residue.

Questions (Page 88)

• 1. What is meant by a substance?
  A substance is a pure single form of matter. It consists of a single type of particle, meaning all constituent particles of that substance are the same in their chemical nature.
• 2. List the points of differences between homogeneous and heterogeneous mixtures.
  Homogeneous Mixtures: These mixtures have a uniform composition throughout. Example: salt dissolved in water.
  
  Heterogeneous Mixtures: These mixtures contain physically distinct parts and have non-uniform compositions. Example: mixtures of sodium chloride and iron filings.

Activity 4.3 (Page 92)

• Is the amount of salt and sugar or barium chloride, that can be dissolved in water at a given temperature, the same?
  No, the amount is not the same. Different substances in a given solvent have different solubilities at the same temperature.

Questions (Page 94)

• 1. Differentiate between homogeneous and heterogeneous mixtures with examples.
  Homogeneous mixtures: They have a uniform composition. For example, sugar in water, salt in water, sulphur in carbon disulphide, water in alcohol, etc.
  
  Heterogeneous mixtures: They have a non-uniform composition. For example, sand and salt, sugar and salt, water in oil, etc.
• 2. How are sol, solution and suspension different from each other?
  Solution: A homogeneous mixture where particles are smaller than 1 nm in diameter. They cannot be seen by naked eyes and do not scatter a beam of light.
  
  Sol (Colloid): A heterogeneous mixture where the size of particles is too small to be individually seen with naked eyes, but they are big enough to scatter a beam of light (Tyndall effect).
  
  Suspension: A heterogeneous mixture in which the solute particles do not dissolve but remain suspended. The particles can be seen by the naked eye and scatter a beam of light.
• 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.
  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
  
  Concentration (Mass percentage) = (Mass of solute / Mass of solution) × 100
  = (36 / 136) × 100 = 26.47%

Questions (Page 96)

• 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.
  Physical Changes: Cutting of 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.
  
  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.
  Pure Substances: Water, sugar, salt, copper, oxygen, iron, hydrogen.
  Mixtures: Sea water, minerals, soil, milk, soft drink.

Activity 4.4 (Page 98)

• Did the material obtained by the two groups look the same?
  No, the products obtained by both the groups show different properties. The material obtained by group I is a mixture showing the properties of its constituents, while the material obtained by group II is a compound with totally different properties.
• Which group has obtained a material with magnetic properties?
  Group I has obtained a material with magnetic properties because the mixture retains the properties of iron.
• Can we separate the components of the material obtained?
  For Group I (mixture), the constituents can be separated fairly easily by physical methods. For Group II (compound), the constituents can be separated only by chemical or electrochemical reactions.
• On adding dilute sulphuric acid or dilute hydrochloric acid, did both the groups obtain a gas? Did the gas in both the cases smell the same or different?
  Both groups obtained a gas, but they smelled different. The gas obtained by Group I is hydrogen, which is colourless and odourless. The gas obtained by Group II is hydrogen sulphide, which is a colourless gas with the smell of rotten eggs.

Exercises (Page 102)

• 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
  (a) Evaporation
  (b) Sublimation
  (c) Filtration
  (d) Chromatography
  (e) Centrifugation
  (f) Separating funnel
  (g) Filtration
  (h) Magnetic separation
  (i) Winnowing
  (j) Filtration / Centrifugation

Exercises (Page 104)

• 2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
  Take water as a solvent in a pan and heat it. Add sugar and milk, which act as solutes that are soluble and will dissolve in the water to form a solution. Then, add tea leaves, which are insoluble in the liquid. Once boiled, filter the mixture. The tea leaves remain on the strainer as residue, and the liquid tea that passes through is the filtrate.
• 3. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below...
  (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?
  (a) At 313 K, the solubility of potassium nitrate is 62 g in 100 g of water. Therefore, the mass required for 50 grams of water is 62 / 2 = 31 g.
  
  (b) She would observe crystals of potassium chloride forming. As the temperature decreases, the solubility of the salt decreases, causing the excess solute to crystallize out.
  
  (c) At 293 K, the solubilities are: Potassium nitrate = 32 g, Sodium chloride = 36 g, Potassium chloride = 35 g, Ammonium chloride = 37 g. Ammonium chloride has the highest solubility at this temperature.
  
  (d) Based on the data table, the solubility of a salt generally increases with an increase in temperature.
• 4. Explain the following giving examples.
  (a) Saturated solution
  (b) Pure substance
  (c) Colloid
  (d) Suspension
  (a) Saturated solution: A solution in which no more solute can be dissolved at a given temperature. Example: dissolving salt in water until it stops dissolving.
  
  (b) Pure substance: A substance consisting of a single type of particle, where all particles are the same in their chemical nature. Example: water, iron, sugar.
  
  (c) Colloid: A heterogeneous mixture in which the particle size is too small to be seen with the naked eye, but big enough to scatter light. Example: milk, fog, shaving cream.
  
  (d) Suspension: A heterogeneous mixture where materials are insoluble in a solvent and have particles that are visible to naked eyes. Example: chalk powder in 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?
  You can confirm its purity by checking its boiling point and freezing point. Pure water has a fixed boiling point of 100°C and a fixed freezing point of 0°C under standard pressure. If it boils or freezes at a different temperature, it is not pure.

Exercises (Page 106 - 108)

• 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
  The pure substances are: (a) Ice, (c) Iron, (d) Hydrochloric acid, (e) Calcium oxide, and (f) Mercury.
• 8. Identify the solutions among the following mixtures.
  (a) Soil (b) Sea water (c) Air (d) Coal (e) Soda water
  The solutions are: (b) Sea water, (c) Air, and (e) Soda water.
• 9. Which of the following will show "Tyndall effect"?
  (a) Salt solution (b) Milk (c) Copper sulphate solution (d) Starch solution
  Colloids show the Tyndall effect. Therefore, (b) Milk and (d) Starch solution will show the Tyndall effect.
• 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
  Elements: (a) Sodium, (d) Silver, (f) Tin, (g) Silicon.
  Compounds: (e) Calcium carbonate, (k) Methane, (l) Carbon dioxide.
  Mixtures: (b) Soil, (c) Sugar solution, (h) Coal, (i) Air, (j) Soap, (m) 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
  The chemical changes are: (a) Growth of a plant, (b) Rusting of iron, (d) Cooking of food, (e) Digestion of food, and (g) Burning of a candle.
• 12. A startup company launches three new products:
  1. A room freshener spray
  2. A medicinal antacid suspension (like milk of magnesia)
  3. A coloured decorative glass panel
  Which of the following correctly identifies the type of colloid in each product?
  A) Spray – Foam; Antacid – Emulsion; Glass - Gel
  B) Spray – Aerosol; Antacid – Sol; Glass - Solid sol
  C) Spray – Emulsion; Antacid – Gel; Glass - Aerosol
  D) Spray – Sol; Antacid – Foam; Glass - Emulsion
  Answer: B) Spray - Aerosol; Antacid - Sol; Glass - Solid sol.

Group Activity (Page 108)

• Take an earthen pot (mutka), some pebbles and sand. Design a small-scale filtration plant that you could use to clean muddy water.
  1. Make a small hole at the bottom of the earthen pot (mutka).
  2. Place a layer of pebbles at the bottom of the pot to allow water to pass while blocking large debris.
  3. Add a thick layer of coarse sand over the pebbles.
  4. Add a top layer of fine sand over the coarse sand.
  5. Pour the muddy water slowly over the fine sand.
  As the muddy water seeps through the fine sand, coarse sand, and pebbles, the suspended impurities will be trapped, and clean filtered water will drain out from the hole at the bottom.

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