NCERT Solutions: Is Matter Around Us Pure?
Page No. 15
Q1. What is meant by a substance?
Ans: A substance is a form of matter that has a uniform composition and properties.A pure substance contains only one kind of particle (atoms or molecules) and has a definite composition and distinct properties.
Examples of Pure Substances Examples include are Oxygen and Carbon.
Q2. List the points of differences between homogeneous and heterogeneous mixtures.
Ans:
Page No. 18
Q1. Differentiate between homogeneous and heterogeneous mixtures with examples.
Ans: The following are the differences between heterogeneous and homogenous mixtures.
| Homogeneous Mixture | Heterogeneous Mixture |
|---|---|
| Composition is uniform throughout the mixture. | Composition is not uniform throughout the mixture. |
| Components are completely mixed and cannot be distinguished with the naked eye. | Components are not completely mixed and can be distinguished with the naked eye or microscope. |
| No visible boundaries of separation. | Visible boundaries of separation are present. |
| Physical properties are same in all parts of the mixture. | Physical properties vary from place to place. |
| Usually exists as a single phase. | Usually exists as two or more phases. |
| Examples: Air, salt solution, sugar solution, vinegar. | Examples: Soil, sand in water, oil and water, blood. |
Q2. How are sol, solution and suspension different from each other?
Ans:
| Property | Solution | Sol (Colloid) | Suspension |
|---|---|---|---|
| Nature of mixture | Homogeneous | Heterogeneous | Heterogeneous |
| Size of particles | Less than 1 nm | 1-100 nm | More than 100 nm |
| Visibility of particles | Not visible even under microscope | Not visible to naked eye, visible under ultramicroscope | Visible to naked eye |
| Tyndall effect | Not exhibited | Exhibited | Exhibited |
| Appearance | Transparent | Translucent | Opaque |
| Stability | Highly stable | Fairly stable | Unstable |
| Settling of particles | Do not settle on standing | Do not settle on standing | Settle on standing |
| Separation by filtration | Cannot be separated | Cannot be separated by ordinary filtration | Can be separated by filtration |
| Examples | Salt solution, sugar solution | Milk, blood, smoke | Sand in water, muddy water |
Q3. To make a saturated solution, 36g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.
Ans: Mass of solute (NaCl): 36 g
Mass of solvent (H2O): 100 g
Mass of solution: 136 g (NaCl + H2O)
Concentration: Calculated as follows:
- Concentration = (Mass of solute / Mass of solution) × 100
- Concentration = (36 g / 136 g) × 100
- Concentration = 26.47%
Thus, the concentration of the solution is 26.47%.
Page No. 19
Q1. 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.
Ans: The following is the classification into physical and chemical change:
Q2. Try segregating the things around you as pure substances or mixtures.
Ans: Listed below are the classifications based on pure substances and mixtures:
Page No. 22
Exercises
Q1. 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.
Ans: (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
Page No. 23
Q2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Ans: Steps for Making Tea
- Take water in a pan. Here, water acts as the solvent.
- Heat the water and add tea leaves, which act as the solute.
- The soluble substances present in tea leaves dissolve in water, forming a solution, while tea leaves remain insoluble.
- Add sugar to the solution and stir well. Sugar is a solute and is soluble in water, so it dissolves completely.
- Boil the solution for some time to extract the colour and flavour from the tea leaves.
- Filter the mixture using a strainer. The tea leaves left on the strainer form the residue.
- The liquid tea collected in the cup is the filtrate, which is the final solution.
Q3. 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?
Ans: (a) Given:
Mass of potassium nitrate required to produce a saturated solution in 100 g of water at 313 K = 62g
To find:
Mass of potassium nitrate required to produce a saturated solution in 50 g of water =?
Required amount = 62 x 50/100 = 31
Hence, 31 g of potassium nitrate is required.
(b) The solubility of potassium chloride in water is decreased when a saturated solution of potassium chloride loses heat at 353 K. Consequently, Pragya would observe crystals of potassium chloride, which would have surpassed its solubility at low temperatures.
(c) As per the given data, that is
Solubility of potassium nitrate at 293K = 32 g
Solubility of sodium chloride at 293K = 36 g
Solubility of potassium chloride at 293K = 35 g
Solubility of ammonium chloride at 293K = 37g
We can observe from this data that ammonium chloride has the highest solubility at 293K.
(d) On increasing the temperature, solubility of a salt increases.
Q4. Explain the following, giving examples.
(a) Saturated solution
(b) Pure substance
(c) Colloid
(d) Suspension
Ans: (a) Saturated solution: In a given solvent when no more solute can dissolve further at a given temperature is called saturated solution.
(b) Pure substance: A pure substance consists of a single type of particles. E.g., gold, silver.
(c) Colloid: A colloid is a solution in which the size of solute particles are bigger than that of true solution. These particles cannot be seen with our naked eyes, they are stable, e.g., ink, blood.
(d) Suspension: It is a heterogeneous mixture in which the solute particles are big enough to settle down, e.g., chalk-water, paints, etc.
Q5. Classify each of the following as a homogeneous or heterogeneous mixture.
soda water, wood, air, soil, vinegar, filtered tea.
Ans: The following is the classification of the given substances into homogenous and heterogenous mixtures.
Q6. How would you confirm that a colorless liquid given to you is pure water?
Ans: We can confirm if a colorless liquid is pure by setting it to boil. If it boils at 100°C, it is said to be pure. But if there is a decrease or increase in the boiling point, we infer that water has added impurities and, hence not pure.
Page No. 24
Q7. 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
Ans: Pure substances are: Ice, iron, hydrochloric acid, calcium oxide and mercury.
Q8. Identify the solutions among the following mixtures.
(a) Soil
(b) Seawater
(c) Air
(d) Coal
(e) Soda water
Ans: Solutions are: Sea water, soda water and air.
Q9. Which of the following will show the "Tyndall effect"?
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution
Ans: Milk and starch solution.
Q10. 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
(i) Carbon dioxide
(m) Blood
Ans:
Q11. 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
Ans: Among the options listed, the following are considered chemical changes: Rusting of iron, Cooking of food, Digestion of food, growth of a plant and Burning of a candle.
FAQs on NCERT Solutions: Is Matter Around Us Pure?
| 1. What's the difference between pure substances and mixtures in Science Class 9? |  |
| 2. How do I separate components from a mixture using different methods? | |
| 3. What makes a solution different from a suspension or colloid? | |
| 4. Why can't we see individual components in a pure compound but can in a mixture? | |
| 5. How do I identify whether something is a colloid, solution, or suspension in a Class 9 Science practical? | |
