Is Matter Around Us Pure

Intext Questions 1

Question 1

What is meant by a substance?

Answer

A substance is a pure single form of matter. It is made up of one kind of atoms or molecules and has definite properties.

Example: Copper, oxygen, hydrogen etc.

Question 2

List the points of difference between homogeneous and heterogeneous mixtures.

Answer

Differences between homogeneous and heterogeneous mixtures are:

Intext Questions 2

Question 1

Differentiate between homogenous and heterogeneous mixtures with examples.

Answer

Question 2

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

Answer

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,

Mass of solute (NaCl) = 36 g

Mass of solvent (H₂O) = 100 g

Mass of solution (NaCl + H₂O) = 100 + 36 = 136 g

Concentration = $\dfrac{\text{Mass of solute}}{\text{Mass of solution}}$ x 100

Concentration = $\dfrac{36}{136}$ x 100 = 26.47%

Hence, the concentration of the solution is 26.47%

Intext Questions 3

Question 1

Classify the following as chemical or physical changes:

1. Cutting of trees
2. Melting of butter in a pan
3. Rusting of almirah
4. Boiling of water to form steam
5. Passing of electric current through water and the water breaking down into hydrogen and oxygen gases.
6. Dissolving common salt in water
7. Making a fruit salad with raw fruits, and
8. Burning of paper and wood

Answer

The following is the classification into physical and chemical change:

1. Cutting of trees — Physical change
2. Melting of butter in a pan — Physical change
3. Rusting of almirah — Chemical change
4. Boiling of water to form steam — Physical change
5. Passing of electric current through water and the water breaking down into hydrogen and oxygen gases. — Chemical change
6. Dissolving common salt in water — Physical change
7. Making a fruit salad with raw fruits — Physical change
8. Burning of paper and wood — Chemical change

Question 2

Try segregating the things around you as pure substances and mixtures.

Answer

Listed below are the classifications based on pure substances and mixtures:

Exercises

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) Sodium chloride from its solution in water — Evaporation

(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 — Filtration

(d) Different pigments from an extract of flower petals — Chromatography

(e) Butter from curd — Centrifugation

(f) Oil from water — By a separating funnel

(g) Tea leaves from tea — By using stainer for filtration

(h) Iron pins from sand — By using a magnet

(i) Wheat grains from husk — Sedimentation/winnowing

(j) Fine mud particles suspended in water — Sedimentation/decantation

Question 2

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

Answer

Steps for making tea are:

1. Take one cup of water as solvent and boil it for few minutes.
2. When solvent boils, add solutes, i.e., milk, tea leaves, sugar. Boil the solution for a few minutes.
3. Milk is soluble in water. Sugar also dissolves in the solution but tea leaves remain as filtrate.
4. The insoluble residue of tea leaves is filtered using a strainer and the tea is ready.

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 a 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 = ?

Required amount = $\dfrac{62}{100}$ x 50 = 31

Hence, 31 g of potassium nitrate is required.

(b) The solubility of potassium chloride in water is decreased when the saturated solution of potassium chloride loses heat at 353 K. So, Pragya would observe crystals of potassium chloride, which would have surpassed its solubility at low temperatures.

(c) As per the given data :

Solubility of potassium nitrate at 293 K = 32 g

Solubility of sodium chloride at 293 K = 36 g

Solubility of potassium chloride at 293 K = 35 g

Solubility of ammonium chloride at 293 K = 37 g

We can observe from this data that ammonium chloride has the highest solubility at 293 K.

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

Question 4

Explain the following, giving examples.

(a) Saturated solution

(b) Pure substance

(c) Colloid

(d) Suspension

Answer

(a) Saturated solution — A saturated solution is one which cannot dissolve more of the solute at a given temperature.

Example: Excess carbon leaves off as bubbles from a carbonated water solution saturated with carbon.

(b) Pure substance — A substance is said to be pure when it comprises of only one kind of molecules or atoms.

Example: Sulphur, diamonds etc.

(c) Colloid — A colloid is a heterogeneous mixture in which the size of solute particles is intermediate between true solution and suspension and it shows tyndall effect.

Examples: Soap solution, starch solution, milk.

(d) Suspension — It is a heterogeneous mixture in which the small particles of a solid are spread throughout a liquid without dissolving in it. Particles of a suspension are visible to the naked eye.

Examples: Chalk water mixture, muddy water etc.

Question 5

Classify each of the following as a homogeneous or heterogeneous mixture.

Soda water, wood, air, soil, vinegar, and filtered tea.

Answer

The given elements are classified below:

1. Soda water — Homogenous mixture
2. Wood — Heterogeneous mixture
3. Air — Heterogeneous mixture
4. Soil — Heterogeneous mixture
5. Vinegar — Homogenous mixture
6. Filtered tea — Homogenous mixture

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 water by boiling it. If it boils at 100°C, then its pure. However, if there is an increase or decrease in the boiling point, then we can say that impurities are added to water.

Question 7

Which of the following materials fall into the category of 'pure substance'?

(a) Ice

(b) Milk

(c) Iron

(d) Hydrochloric acid

(e) Calcium oxide

(f) Mercury

(g) Brick

(e) Wood

(f) Air

Answer

Following substances from the above-mentioned list are pure substances:

• Ice

• Iron

• Hydrochloric acid

• Calcium oxide

• Mercury

Question 8

Identify the solutions among the following mixtures.

(a) Soil

(b) Sea water

(c) Air

(d) Coal

(e) Soda water

Answer

The following are the solutions from the above-mentioned list of mixtures:

• Sea water

• Air

• Soda water

Question 9

Which of the following will show the 'Tyndall effect'?

(a) Salt solution

(b) Milk

(c) Copper sulphate solution

(d) Starch solution

Answer

Tyndall effect is shown by:

(a) milk

(b) 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

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

The following are chemical changes :

(a) Growth of a plant

(b) Rusting of iron

(c) Cooking of food

(d) Digestion of food

(e) Burning of a candle