Is Matter Around Us Pure

INTEXT QUESTIONS

Q1. What is meant by a pure substance?

Ans.

A pure substance is the one that consists of a single type of particles, i.e., all constituent particles of the substance have the same chemical nature. Pure substances can be classified as elements or compounds.

Q2. List the points of differences between homogeneous and heterogeneous mixtures.

Ans.

A homogeneous mixture is a mixture having a uniform composition throughout the mixture. For example: salt in water, sugar in water, copper sulphate in water

A heterogeneous mixture is a mixture having a non-uniform composition throughout the mixture. For example: sodium chloride and iron fillings, salt and sulphur, oil and water.

INTEXT QUESTIONS

Q1. Differentiate between homogeneous and heterogeneous mixtures with examples.

Ans.

A homogeneous mixture is a mixture having a uniform composition throughout the mixture. For example, mixtures of salt in water, sugar in water, copper sulphate in water, iodine in alcohol, alloy, and air have uniform compositions throughout the mixtures.

On the other hand, a heterogeneous mixture is a mixture having a non-uniform composition throughout the mixture. For example, composition of mixtures of sodium chloride and iron fillings, salt and sulphur, oil and water, chalk powder in water, wheat flour in water, milk and water are not uniform throughout the mixtures.

Heterogeneous mixture is a mixture having a non-uniform composition throughout the mixture. For example: sodium chloride and iron fillings, salt and sulphur, oil and water

Q2. How are sol, solution and suspension different from each other?

Ans.

Sol is a heterogeneous mixture. In this mixture, the solute particles are so small that they cannot be seen with the naked eye. Also, they seem to be spread uniformly throughout the mixture. The Tyndall effect is observed in this mixture. For example: milk of magnesia, mud

Solution is a homogeneous mixture. In this mixture, the solute particles dissolve and spread uniformly throughout the mixture. The Tyndall effect is not observed in this mixture. For example: salt in water, sugar in water, iodine in alcohol, alloy.

Suspensions are heterogeneous mixtures. In this mixture, the solute particles are visible to the naked eye, and remain suspended throughout the bulk of the medium. The Tyndall effect is observed in this mixture. For example: chalk powder and water, wheat flour and water.

Q3. 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.

Ans.

Mass of solute (sodium chloride) = 36 g (Given)

Mass of solvent (water) = 100 g (Given)

Then, mass of solution = Mass of solute + Mass of solvent

= (36 + 100) g

= 136 g

Therefore, concentration (mass by mass percentage) of the solution

= mass of solute  mass of solvent ×100=36136×100=26.47%

INTEXT QUESTIONS

Q1. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25°C), which are miscible with each other?

Ans.

A mixture of two miscible liquids having a difference in their boiling points more than 25°C can be separated by the method of distillation. Thus, kerosene and petrol can be separated by distillation.

In this method, the mixture of kerosene and petrol is taken in a distillation flask with a thermometer fitted in it. We also need a beaker, a water condenser, and a Bunsen burner. The apparatus is arranged as shown in the above figure. Then, the mixture is heated slowly. The thermometer should be watched simultaneously. Kerosene will vaporize and condense in the water condenser. The condensed kerosene is collected from the condenser outlet, whereas petrol is left behind in the distillation flask.

Q2. Name the technique to separate

(i) butter from curd

(ii) salt from sea-water

(iii) camphor from salt

Ans. 

(i) Butter can be separated from curd by centrifugation.

(ii) Salt can be separated from sea-water by evaporation.

(iii) Camphor can be separated from salt by sublimation.

Q3. What type of mixtures is separated by the technique of crystallization?

Ans.

By the technique of crystallization, pure solids are separated from impurities. For example, salt obtained from sea is separated from impurities; crystals of alum (Phitkari) are separated from impure samples.

INTEXT QUESTIONS

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 water breaking down into hydrogen and oxygen gas
  • Dissolving common salt in water
  • Making a fruit salad with raw fruits
  • Burning of paper and wood

Ans. 

  • 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 water breaking down into hydrogen and oxygen gas → 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

Q2. Try segregating the things around you as pure substances or mixtures.

Ans.

Pure substance: Water, salt, sugar

Mixture: Salt water, soil, wood, air, cold drink, rubber, sponge, fog, milk, butter, clothes, food.

NCERT TEXT BOOK 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) Crystallization or evaporation

(b) Sublimation

(c) Centrifugation or sedimentation

(d) Chromatography

(e) Centrifugation

(f) Separating funnel

(g) Filtration

(h) Magnetic separation

(i) Winnowing

(j) Centrifugation

Q2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.

Ans.

Take the solvent, water in a kettle. Heat it when the solvent boils, add the solute, milk. Milk and water forms a solution. Then pour some tea leaves over a sieve. Pour slowly hot solution of milk solution over tea leaves. Colour of tea leaves goes into solution as filtrate. The remaining tea leaves being insoluble remains as residue. Add required amount of sugar, a solute to the tea solution which dissolves and the tea is ready

Q3. Pragya tested the solubility of three different substances at different temperatures and collected 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)

Substance dissolved Temperature in K
283 293 313 333 353
Potassium

nitrate

21 32 62 106 167
Sodium chloride 36 36 36 37 37
Potassium chloride 35 35 40 46 54
Ammonium chloride 24 37 41 55 66

(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?

Ans.

(a) At 313 K, 62 gm of potassium is added in 100 gm of water to form a saturated solution.
So 50 gm of water requires 31 gm potassium nitrate.

(b) As solution cools, potassium chloride gets crystallized or precipitated. Because at 353K, solubility of    potassium chloride is 54 g per 100g of water. Whereas at room temperature (20°C) it is 35g. So excess of potassium chloride dissolved gets precipitated.

(c) At 20°C or 293 K, ammonium chloride has the highest solubility.

(d) As the temperature increases, solubility increases.

Q4. Explain the following giving examples.

(a) Saturated solution

(b) Pure substance

(c) Colloid

(d) Suspension

Ans.

(a) Saturated solution: A solution in which no more of the solid (solute) can be dissolved at a given temperature is called a saturated solution. Suppose 40 g of a solute is the maximum amount that can be dissolved in 100 g water at 298 K. Then (100 + 40)g. of solution so obtained is the saturated solution at 298 K.

(b) Pure substance: The substances, which always have the same colour, taste or texture at a given temperature and pressure are pure substances. A pure substance has a characteristic melting point or boiling point at a given pressure. For example, pure water is always colourless, odourless and tasteless and boils at 373 K at normal atmospheric pressure.

(c) Colloid: The particles of a colloid are uniformly spread throughout the solution. A colloid has the following characteristics:

(i) It is a heterogeneous mixture

(ii) The size of particles of a colloid lies between 1 – 100 nm and are not visible to naked eye.

(iii) The particles of a colloid can scatter a beam of light

(iv) The particles of a colloid cannot be separated from the mixture by filtration

(v) The particles of a colloid do not settle when left undisturbed.

(d) Suspension: A suspension has the following characteristics:

(i) It is a heterogeneous mixture.

(ii) The particles of a suspension are greater than 100 nm and are visible to naked eye.

(iii) The particles of suspension can scatter a beam of light

(iv) The particles of a suspension settle down when left undisturbed for some time.

(v) The particles of a suspension can be separated from its mixture by filtration.

Examples: Fine clay particles in water.

Q5. Classify each of the following as a homogeneous or heterogeneous mixture: soda water, wood, air, soil, vinegar, filtered tea.

Ans.

Homogeneous mixture: Soda water, air, vinegar, filtered tea

Heterogeneous mixture: Wood, soil

Q6. How would you confirm that a colourless liquid given to you is pure water?

Ans.

Every liquid has a characteristic boiling point at 1 atmospheric pressure. If the given colourless liquid boils at exactly 373 K at 1 atmospheric pressure, then it is pure water. If the boiling point is even slightly above 373K, then it is contaminated.

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

(a) Ice     –          a compound

(c) Iron   –          an element

(d) Hydrochloric acid – a compound

(e) Calcium oxide – a compound

(f) Mercury – an element

Q8. Identify the solutions among the following mixtures.

(a) Soil

(b) Sea water

(c) Air

(d) Coal

(e) Soda water

Ans.

Solutions are sea water, air and soda water

Q9. Which of the following will show Tyndall effect?

(a) Salt solution

(b) Milk

(c) Copper sulphate solution

(d) Starch solution

Ans.

(b) Milk and

(d) Starch solution are colloids and will show Tyndall effect.

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                        (l)Carbon dioxide

(m) Blood

Ans.

Elements: Sodium, silver, tin, silicon

Compounds: Calcium carbonate, methane, carbon dioxide

Mixtures: Soil, sugar solution, coal, air, soap, blood

Q11. Which of the following are chemical changes?

(a) Growth of a plant

(b) Rusting of iron

(c) Mixing of iron fillings and sand

(d) Cooking of food

(e) Digestion of food

(f) Freezing of water

(g) Burning of candle

Ans.

Chemical changes are: Growth of a plant, Rusting of iron, Cooking of food, Digestion of food, burning of candle.

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