Class 9 Science Chapter 2 Notes

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Class 9 Science Chapter 2 Notes

Chapter Name: - Is matter Around us pure?

Class 9 Science Chapter 2 Notes

Section - A
(Questions - Page 15 )

1. What is meant by a substance?

A substance is a pure single form of matter. A pure substance can be an element or compound.

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

Homogeneous mixture: –

1. A homogeneous mixture has a uniform composition throughout.

2. This type of mixture can have a variable composition. 

3. The examples of such mixture are: –

salt dissolved in water, sugar dissolved in water.

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Heterogeneous mixture: –

1. A mixture that contains physically distinct parts (We can recognize both solute and solvent distinctly).

2. It has a non-uniform composition. 

3. The examples of such mixture are: –

mixtures of sodium chloride and iron filings, salt and sulfur, and oil and water.

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Class 9 Science Chapter 2 Notes

Section - B
(Questions - Page 18)

1. Differentiate between homogeneous and heterogeneous mixtures with examples.

Homogeneous mixtureHeterogeneous mixture
Uniform composition throughout.Non-uniform composition. 
The solute can not be seen by naked eyes.The solute can be seen by the naked eyes.
The path of light is not visible through these types of mixtures.It scatters a beam of light passing through it and makes its path visible.
The homogeneous mixtures are stable.The heterogeneous mixtures are unstable. 
The solute particles can not be separated by the process of filtration.The solute can be separated by the process of filtration.
The examples of such mixture are i) salt dissolved in water, ii) sugar dissolved in water.The examples of such are i) mixtures of sodium chloride and iron filings, salt and sulfur, and oil and water.

 

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

Colloid solution or sol:

1. It is a heterogeneous mixture. 

2. The size of the particles is too small to be seen by naked eyes.

3. They scatter a beam of light passing through it and make its path visible.

4. They do not settle down when left undisturbed.

5. They can not be separated from the mixture by the process of filtration. A special technique called centrifugation is used the separate the colloidal particles.

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Solution

1. The solution is a homogeneous mixture.

2. The particles of a solution are smaller than 1nm (10-9 meters) in diameter. So, they can not be seen by naked eyes.

3. They do not scatter a beam of light passing through the solutions. So, the path of light is not visible.

4. The solute particles can not be separated from the mixture by the process of filtration.

5. The particles do not settle down. so, we can say that a solution is stable.

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Suspension

1. The suspension is a heterogeneous mixture.

2. Its particles can be seen by naked eyes.

3. Its particles scatter a beam of light passing through it and make its path visible.

4. The suspension is unstable as its particles settle own when left undisturbed.

5. The particles can be separated from the mixture by the process of filtration.

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

We know,
Mass of solution = Mass of solute + Mass of solvent
= 36 g + 100 g
= 136 g

Therefore, Mass percentage of the required solution

⇒ Mass of solute / Mass of solution x 100

⇒ 36/136 x 100

⇒ 26.47%

Aiye Samjhein (आइए समझें)

Class 9 Science Chapter 2 Notes

Section - C
(Questions - Page 24)

1. 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?

Distillation is the process used to separate a mixture containing two miscible liquids with a difference in their boiling point of more than 25°C.

The boiling points of the given two miscible liquids are: –

Petrol = 80 – 100°C

Kerosene = 200 – 250°C

The principal of the distillation process: -The process of the distillation depends upon the boiling points of the miscible liquids. The liquid with a lesser boiling point will vaporise and condense in a condenser. And, the liquid with a higher boiling point will be left behind in the distillation flask.

Steps: –

1. Take the mixture of kerosene and petrol in a distillation flask. Fit it with a thermometer.

2. Heat the mixture slowly keeping a close watch at the thermometer.

3. The petrol vaporises at a temperature nearly equal to 80 – 100°C.  It gets condenses in the condenser and can be collected from the condenser outlet.

4. Kerosene is left behind in the distillation flask.

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

i) Butter is separated from curd by using centrifugation.

ii) Salt is separate from seawater using evaporation (The purification of the salt is carried out by using crystallisation).

iii) Camphor is separated from salt by sublimation.

3. What type of mixtures are separated by the technique of crystallisation?

Crystallisation is a process that separates a pure solid in the form of its crystal from a solution. The type of mixtures used to separate by this technique are impure salt and crystals.

For examples: –

i) Impure salt that we get from sea water.

ii) separation of crystals of alum (phitkari) from impure samples.

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Class 9 Science Chapter 2 Notes

Section - D
(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.

• 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

2. Try segregating the things around you as pure substances or mixtures.

Pure substances → salt (sodium chloride), sugar, copper sulphate

Mixtureseawater, minerals, soil, air, ink, cold drink, milk

Aiye Samjhein (आइए समझें)

Class 9 Science Chapter 2 Notes

Section - E
(Questions - Exercise)

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) Sodium chloride from its solution in water → Vaporisation and crystallisation
(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride → Sublimation
(c) Small pieces of metal in the engine oil of a car → Filteration (Oil filters are used for this purpose)
(d) Different pigments from an extract of flower petals → Chromatography
(e) Butter from curd → Centrifugation or Churning
(f) Oil from water → Using separating funnel
(g) Tea leaves from tea →  Filteration
(h) Iron pins from sand → Using a magnet
(i) Wheat grains from husk → Winnowing
(j) Fine mud particles suspended in water → 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.

Steps for making tea: –

1. Take some water as a solvent in a tea pan.

2. Put it on a gas stove and heat it until the solvent boils.

3. Make it a solution by adding sugar as a solute in it. And stir it until it dissolves.

4. Add some tea leaves, and milk. The sugar and milk are soluble in the solvent but the tea leaves are insoluble. But the flavor of the tea leaves get dissolve in the mixture as the temperature of the mixture rises.

5. After some time, the tea gets prepared. Use a strainer as filtrate. The tea solution is filtered into the cup and the tea leaves deposits in the strainer as 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).

Pragya tested the solubility of three different substances at different temperatures and collected the data as given below-min

(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) For a saturated solution at 313 K 

The mass of potassium nitrate used for 100 gms of water = 62 gms

The mass of potassium nitrate used for 1 gms of water = 62/100 gms

∴ The mass of potassium nitrate required for 50 gms of water = 62/100 x 50 gms ⇒ 31 gms

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b) Potassium chloride is highly soluble in water. If we add up to 36 gms of potassium chloride (KCl) in 100 gms of water, its solution will remain stable on heating up to 353 K as well as after allowing it to cool down.

But, here we are using its saturated solution (54 gms in 100 gms of water) at 353 K and allowing it to cool down. In this case, some amount of potassium chloride will accumulate back as crystal.

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c) The solubility of each salt at 293 K

Potassium nitrate → 32 gms

Sodium chloride → 36 gms

Potassium chloride → 35 gms

Ammonium chloride → 37 gms

Ammonium chloride has the highest solubility at 293 K.

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d) The solubility of salt increases with an increase in temperature.

4. Explain the following giving examples. (a) saturated solution (b) pure substance (c) colloid (d) suspension

Saturated solution: –

When no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution.

For example, let 100 gms of water can dissolve 15 gms of a substance ‘X’ at room temperature (298 K). If we keep on adding more amount of substance ‘X’ to the water, at the same temperature, the solution will no longer remain stable and it will become saturated.

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Pure substance: –

A pure substance consists of a single type of particles. It means that all the constituent particles of that substance are the same in their chemical nature. Whatever the source of a pure substance may be, it will always have the same characteristic properties.

For example, it does not matter how small the particle of sugar we take, its composition remains the same throughout.

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Colloid: –

The colloid is a heterogeneous solution but its particles are uniformly spread throughout the solution. This is because the particles of solute are of very small size. The following are the properties of ta colloid.

1. It is a heterogeneous mixture. 

2. The size of the particles is too small to be seen by naked eyes.

3. They scatter a beam of light passing through it and make its path visible.

4. They do not settle down when left undisturbed.

5. They can not be separated from the mixture by the process of filtration. A special technique called centrifugation is used the separate the colloidal particles.

————————–

Suspension: –

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remains suspended throughout the bulk of the medium. The following are the properties of a suspension.

1. The suspension is a heterogeneous mixture.

2. Its particles can be seen by naked eyes.

3. Its particles scatter a beam of light passing through it and make its path visible.

4. The suspension is unstable as its particles settle own when left undisturbed.

5. The particles can be separated from the mixture by the process of filtration.

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

Homogeneous mixture: –

soda water, air, vinegar, filtered tea

Heterogeneous solution: –

wood, soil

6. How would you confirm that a colourless liquid given to you is pure water?

To confirm that a colourless liquid given to us is pure water, we will either cool it at 0°C or boil it in a pan at 100°C. The water converts into ice at 0°C and boils at 100°C. 

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 materials that are pure substance:

a) ice

c) iron

d) hydrochloric acid

e) calcium oxide

f) mercury

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

The following are the solution: –

b) sea water

c) air

e) soda water

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

A homogeneous solution does not show Tyndall effect. So, 

b) Milk 

d) Starch solutions are colloids and 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: –

sodium, silver, tin, silicon

Compounds: –

calcium carbonate, methane, carbon dioxide

Mixtures: –

soil, sugar solution, coal, air, soap, 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.

Chemical change: – A process in which one reacts with another to undergo a change in chemical composition. It brings a change in the chemical properties of matter and a new substance is formed.

The following are the chemical changes: –

(a) Growth of a plant
(b) Rusting of iron
(d) Cooking of food
(e) Digestion of food
(g) Burning of a candle.

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Class 9 Science Chapter 2 Notes

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