NCERT Class 9 Science Chapter 2 Is Matter Around Us Pure?

Chapter 2 Is Matter Around Us Pure?

Intext Questions
Question 1: What is meant by a substance?
Answer: A substance is a pure single form of matter. It cannot be separated into other kinds of matter by any physical process. Example: sodium chloride

Question 2: List the points of differences between homogenous and heterogeneous mixtures.
Answer:
• Homogenous mixture
 → Particles are uniformly distributed throughout the mixture.
→ Has a uniform composition.
→ Components cannot be easily separated.
→ All solutions are homogenous solutions.
→ Example: sugar solution
• Heterogeneous mixture
→ Particles are unevenly distributed throughout the mixture.
→ Doesn’t have a uniform composition.
→ Components can be easily separated.
→ There are no solution that are heterogeneous.
→ Example: mixture of sand & water

Question 3: Differentiate between homogenous and heterogeneous mixtures with examples.
Answer:
• Homogenous mixture
 → Particles are uniformly distributed throughout the mixture.
→ Has a uniform composition.
→ Components cannot be easily separated.
→ All solutions are homogenous solutions.
→ Example: sugar solution
• Heterogeneous mixture
→ Particles are unevenly distributed throughout the mixture.
→ Doesn’t have a uniform composition.
→ Components can be easily separated.
→ There are no solution that are heterogeneous.
→ Example: mixture of sand & water

Question 4: How are sol, solution and suspension different from each other?
Answer:

Question 5: 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:
Given
Mass of sodium chloride (solute) = 36 g
Mass of water = 100 g
Concentration of solution = ?
Mass of solvent = 100 + 36 = 136 g

Mass by Mass percentage of a solution = Mass of solute/Mass of solvent × 100
= 36/136 × 100
= 450/17
= 26.47%
Therefore, the concentration of solution at 293 K is 26.47%.

Question 6: How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25ºC), which are miscible with each other?
Answer: We can separate the mixture containing kerosene and petrol by the process called simple distillation. It 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. Take the mixture in a distillation flask. Fit it with a thermometer. Arrange the apparatus. Heat the mixture slowly keeping a close watch at the thermometer. The petrol vaporises (as it has lower boiling point), condenses in the condenser and can be collected from the condenser outlet. Kerosene is left behind in the distillation flask.

Question 7: Name the technique to separate
i) butter from curd,
ii) salt from sea-water,
iii) camphor from salt.
Answer:
i) Centrifugation; The process is based on the principle of density that the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.
ii) Evaporation; Evaporation is the process by which a liquid turns into a gas, hence leaving salt behind.
c) Sublimation; During the phase change from solid to gas, camphor does not undergo a liquid phase.

Question 8: What type of mixtures are separated by the technique of crystallisation?
Answer: Crystallisation is a process that separates a pure solid in the form of its crystals from a solution.

Question 9: 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.
Answer:
• Cutting of trees: Physical change
• Melting of butter in a pan: Physical change
• Rusting of almirah: Chemical change
• Boiling of water to form steam: Physical change
• Passing of electric current through water and the water breaking down into hydrogen and oxygen gases: Chemical change
• Dissolving common salt in water: Physical change
• Making a fruit salad with raw fruits: Physical change
• Burning of paper and wood: Chemical change

Question 10: Try segregating the things around you as pure substances or mixtures.
Answer:
• Pure substances: Water, Salt, Iron, Glucose, Hydrochloric Acid, Calcium oxide etc.
• Mixture: Sea water, soil, air, coal, soda water, steel etc.

Exercise Questions
Question 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.
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 2: 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 solvent and heat it.
• Add sugar in it which is the solute. Heat it till all sugar dissolves. We get a sugar solution. Sugar is completely soluble in water.
• Add half a tea-spoon of tea leaves, it is insoluble in water.
• Boil the content, add milk and boil it for some time.
• Filter the tea with the help of strainer, the tea leaves collected in strained is residue and the tea collected in glass is the filtrate.

Question 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?
Answer:
a) Given
Mass of potassium nitrate required to produce a saturated solution in 100 g of water at 313 K = 62g
Mass of potassium nitrate required to produce a saturated solution in 50 g of water = ?
= 62 × 50/100 = 31
Therefore, 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. As the solution cools down, Pragya would observe crystals of potassium chloride, which would have surpassed its solubility at low temperatures.

c)
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
So, Ammonium chloride has the highest solubility at 293K.

d) The solubility of the salt is dependent upon the temperature and increases with an increase in temperature. When a salt arrives at its saturation point at a specific temperature, there is a tendency to dissolve more salt through an increase in the temperature of the solution.

Question 4: Explain the following giving examples.
a) saturated solution
b) pure substance
c) colloid
d) suspension
Answer:
a) It is that state in a solution at a specific temperature when solute is no more soluble without an increase in temperature. Example: Adding excess sodium chloride in water at particular temperature settles down.
b) Pure substance comprises of only one kind of molecules, atoms or compounds without any adulteration. Example: Sulphur, Iron
c) A homogeneously appearing heterogeneous consisting of ultra-microscope particles dispersed throughout the system forms colloidal solution. Example: Milk
d) The heterogeneous system in which solids are dispersed in liquids are called suspensions. Example: muddy water

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

Question 6: How would you confirm that a colourless liquid given to you is pure water?
Answer: We can confirm if a colourless 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, hence not pure.

Question 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
Answer:
a) Ice
c) Iron
d) Hydrochloric acid
e) Calcium oxide
f) Mercury

Question 8: Identify the solutions among the following mixtures.
a) Soil
b) Sea water
c) Air
d) Coal
e) Soda water
Answer:
b) Sea water
c) Air
e) Soda water

Question 9: Which of the following will show “Tyndall effect”?
a) Salt solution
b) Milk
c) Copper sulphate solution
d) Starch solution
Answer:
b) Milk
d) Starch solution

Question 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
Answer:
Elements: Sodium, Silver, Tin, Silicon
Compounds: Calcium carbonate, Methane, Carbon dioxide
Mixture: Soil, Sugar solution, Coal, Air, Soap, Blood

Question 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
Answer:
a) Growth of a plant
b) Rusting of iron
d) Cooking of food
e) Digestion of food
g) Burning of a candle