NCERT Solutions for Class 9 Science Chapter 2 Is Matter Around Us Pure

Topic 2.1 What is a mixture?, Page 15

Question 1. What is meant by a substance?

Answer:

Substance-
A substance is a matter that consists of a single type of particle and has specific properties. For example, tin, sulfur, pure sugar (sucrose), etc.

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

Answer:

The differences between homogeneous and heterogeneous mixtures-

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

Answer:

The difference between sol, solution and suspension are given below:

Question 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.

Answer:

Given that,

Mass of solute (sodium chloride) = 36 g = $w_1$
Mass of water (as a solvent) = 100 g = $w_2$

Therefore, the total mass of solution = 100 + 36 = 136 g = $W$

According to the question,
Concentration = $\frac{w_1}{W}\times 100$
$=\frac{36}{136}\times 100$
$=26.47$ %

Hence, the concentration of the solution at 293 K is 26.47%

Topic 2.3 Separating the components of a mixture

Question 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?

Answer:

The mixture of kerosene and petrol which are miscible with each other can be separated by the distillation method.

Take the mixture in a distillation flask and fit it with the thermometer. Heat the mixture slowly. As the boiling point of petrol is lower than that of kerosene, so, petrol vaporizes first. It condenses in the condenser and is collected from the outlet.

And thus kerosene is left in the flask.

Question 2. Name the technique to separate

(i) butter from curd,
(ii) salt from sea-water
(iii) camphor from salt.

Answer:

The following techniques are used to separates them-

(i) Butter from curd can be seperated from Centrifugation method

(ii) Salt from sae water can be seperated from Evaporation

(iii) Camphor from salt can be seperated from Sublimation.

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

Answer:

The crystallization technique is used to purify solids.

Crystallization is used to separate solid- liquid mixtures, specifically to obtain pure solids from their impure or dissolved form in a solution. It is commonly used for substances that decompose on heating or cannot be purified by evaporation.

Topic 2.4 Physical and Chemical changes, Page 19

Question 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

Answer:

Physical changes are:

• 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 are:

• Rusting of almirah,
• Passing of electric current, through water and the water breaking down into hydrogen and oxygen gases,
• Burning of paper and wood

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

Answer:

Pure substance - Distilled Water, oxygen, iron, sugar, and gold

Mixtures- plastic papers, air, soft drink, salt solution and milk

Question 2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.

Answer:

The steps for making a tea-

1. Use water as a solvent and boil it for a few minutes.
2. Now, add some tea leaves and sugar, and milk (if you want) as a solvent.
3. Again, boil it for a few minutes so that the sugar will dissolve in it.
4. At last, filter the solution. Collect the filtrate in a cup. The insoluble tea leaves left behind as a residue.

Question 3. (c) 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 a substance dissolved in 100 grams of water to form a saturated solution).

Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

Answer:

The solubility of each salt at 293K is

• Potassium nitrate = 32g
• Sodium chloride = 36g
• Potassium chloride = 35 g
• Ammonium chloride = 37g

Question 3. (d) 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 a substance dissolved in 100 grams of water to form a saturated solution).

What is the effect of a change in temperature on the solubility of a salt?

Answer:

Solubility is directly proportional to the temperature.

Therefore, on increasing temperature, the solubility of salt increases.

Question 4. (a) Explain the following giving examples.

saturated solution

Answer:

Saturated solution - In a given solvent, when no more solute can be dissolved in a solution at a given temperature is called a saturated solution.

Question 4. (b) Explain the following giving examples

pure substance

Answer:

Pure substance -

A pure substance is a matter that consists of a single type of particle and has specific properties. For example, tin, sulphur, pure sugar (sucrose), etc.

Question 4. (c) Explain the following giving examples.

Colloid

Answer:

colloid -
A colloid is a solution in which the solute particle is larger in size compared to the true solution. It is a heterogeneous mixture. Because of the small size of colloidal particles, we cannot see them with our naked eyes. For example, milk and blood.

Question 4. (d) Explain the following giving examples.

(d) suspension

Answer:

Suspension-
It is a heterogeneous solution in which the solute particles do not dissolve in the solvent but remain suspended throughout the bulk of the medium. Particles are visible by the naked eye. Chalkwater is an example of this type of solution.

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

Answer:

By boiling the give colourless water, we can check whether it is pure or not. If it is pure then the water boils at 100 $^0C$ at atmospheric pressure.

This is because the melting and boiling points of a pure substance don't change.

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 pure substance is a matter that consists of a single type of particle and has specific properties-
Therefore, the following given substances are '' pure substances" -

• Ice
• Iron
• Hydrochloric acid
• calcium oxide and
• Mercury

Question 8. Identify the solutions among the following mixtures.

(a) Soil

(b) Seawater

(c) Air

(d) Coal

(e) Soda water

Answer:

A solution is a homogeneous mixture of two or more two substances.

So, according to the definition, out of the given seawater, air, and soda water are examples of solutions.

Question 9. Which of the following will show the “Tyndall effect”?

(a) Salt solution

(b) Milk

(c) Copper sulfate solution

(d) Starch solution.

Answer:

Tyndall effect is shown by the colloidal solution and suspension, and it is not shown by a true solution.

Therefore, in the above-given solution, only the milk and starch solution will be able to scatter the light and hence show the Tyndall effect.

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:

Given following are examples of chemical changes-

• Growth of plants
• Rusting of iron
• cooking of food
• Digestion of food
• Burning of candle