Chapter 02 Is Matter Around Us Pure? Exercise
Exercises
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
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Answer
(a) Evaporation
(b) Sublimation
(c) Filtration
(d) Chromatography
(e) centrifugal machine or churning the curd by hand
(f) Decantation
(g) Filtration
(h) Magnetic Separation
(i) Winnowing
(j) Coagulation and decantation
2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
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Answer
Method of preparation of tea
(i) Take some water (solvent) in a pan and heat it.
(ii) Add some sugar (solute) and boil to dissolve the sugar completely the obtained homogeneous mixture is called solution.
(iii) Add tea leaves (or tea) in the solution and boil the mixture.
(iv) Now add milk and boil again.
(v) Filter the mixture through the tea stainer and collect the filtrate or soluble substances, i.e., tea in a cup. The insoluble tea leaves left behind as residue in the 8 trainer.
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).
| 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?
(d) What is the effect of change of temperature on the solubility of a salt?
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Answer
(a) Mass of potassium nitrate needed to produce its saturated solution in $100 g$ of water at $313 K=62 g$
Mass of potassium nitrate needed to produce its saturated solution in $50 g$ of water at 313
$$\mathrm{K} = \frac{62}{100}\times 50 \mathrm{g} = 31 \mathrm{g}$$
(b) Crystals of potassium chloride are formed. This happens as solubility of solid decreases with decreasing the temperature.
(c) Solubility of each salt at $293 K$
Potassium nitrate $\quad 32 g$ per $100 g$ water
Sodium chloride $\quad 36 g$ per $100 g$ water
Potassium chloride $\quad 35 g$ per $100 g$ water
Ammonium chloride $\quad 37 g$ per $100 g$ water
Note:Solubility of a solid is that amount in gram which can be dissolved in $100 g$ of water (solvent) to make saturated solution at a particular temperature.
Ammonium chloride has the maximum solubility ( $37 g$ per $100 g$ of water) at $293 K$.
(d) Solubility of a (solid) salt decreases with decrease in temperature while it increases with rise in temperature.
4. Explain the following giving examples.
(a) Saturated solution
(b) Pure substance
(c) Colloid
(d) Suspension
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Answer
(a) Saturated solution: A solution in which no more amount of solute can be dissolved at a particular temperature is called saturated solution. Example: when sugar is dissolved repeatedly in a given amount of water, a condition is reached at which further dissolution of sugar is not possible in that amount of water at room temperature.
(b) Pure substance: A substance made up of single type of particles (atoms and/or molecules) is called pure substance. All elements and compounds are said to be pure, Example: water, sugar etc.
(c) Colloid:A heterogeneous mixture in which the solute particle size is too small to be seen with the naked eye, but is big enough to scatter light is known as Colloid. There are two phases in colloidal solution Dispersed phase: solute particles are said to be dispersed phase Dispersion medium: the medium in which solute particles are spread is called the dispersion medium.
Example: Milk, clouds etc., are the example of colloid.
(d) Suspension: A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of suspension are visible to the naked eye. Example: Mixture of sand, Water and Muddy water etc.
5. Classify each of the following as a homogeneous or heterogeneous mixture.
soda water, wood, air, soil, vinegar, filtered tea.
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Answer
Homogeneous mixtures: Air, soda water, vinegar, filtered tea.
Heterogeneous mixtures: Wood, soil,
6. How would you confirm that a colourless liquid given to you is pure water?
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Answer
If the given colorless liquid boils at $100^{\circ} C$ sharp, it is pure water, otherwise not.
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
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Answer
Ice, iron, calcium oxide, mercury are pure substance as they have definite composition.
Milk is a colloid, so it is a heterogeneous mixture.
Hydrochloric acid is also a mixture of hydrogen chloride gas and water.
8. Identify the solutions among the following mixtures.
(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water
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Answer
Sea water, air and soda water: Homogeneous mixture
Coal, Soil: Heterogeneous solution.
9. Which of the following will show “Tyndall effect”?
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution
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Answer
Milk and starch solution will show “Tyndall effect” as both of these are colloids.
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
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Answer
Elements : Sodium, silver, tin and silicon
Compounds: Calcium carbonate, methane, and carbon dioxide
Mixtures : Soil, sugar solution, coal, air, soap and 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
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Answer
Growth of a plant, rusting of iron, cooking of food, digestion of food, burning of a candle are chemical changes, because here the chemical composition of substance changes.
