NCERT Solutions for Class 9 Science Chapter 2 - Is Matter Around us Pure?

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A material that is made up of only one type of particles and has a definite set of properties is called a pure substance. Pure substances include elements and compounds.

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Refer questions 2, NCERT In-Text questions of page 15.

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Mass of sodium chloride (solution) = 36 g
Mass of water (solvent) = 100 g
Total mass of solution = 100 + 36 = 136 g

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By distillation

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(i) Centrifugation
(ii) Evaporation
(iii) Sublimation

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Crytallisation method is suitable for separating the mixtures in which:
(i) The desirable component is soluble in a suitable hot solvent and can be obtained as crystals.
(ii) The other components are insoluble in the solvent.

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(a) Cutting of trees — Physical change
(b) Melting of butter in a pan — Physical change
(c) Rusting of almirah — Chemical change
(d) Boiling of water to form steam — Physical change
(e) Passing of electric current through
water and the water breaking down
into hydrogen and oxygen gases — Chemical change
(f) Dissolving common salt in water — Physical change
(g) Making a fruit salad with raw fruits — Physical change
(h) Burning of paper and wood — Chemical change

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Some examples are:
Water — Pure substance
Milk — Mixture
LPG — Mixture
Bread — Mixture
Kerosene oil — Mixture
Air — Mixture
Paper — Mixture
Ink — Mixture
Curd — Mixture
Wood — Mixture
Ice — Pure substance
Soda water — Mixture
Ice cream — Mixture
Butter — Mixture
Vanaspati ghee — Mixture
Lemon juice — Mixture

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(a) Evaporation
(b) Sublimation
(c) Filtration
(d) Chromatography
(e) Centrifugation
(f) Separating funnel
(g) Filtration
(h) Magnetic separation
(i) Gravity separation/ Winnowing
(j) Sedimentation and Decantation

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Step 1. Boil some solvent (water) in a pan.
Step 2. Add some sugar (or solute) soluble in water to it to make a solution.
Step 3. Add the tea leaves to this solution and boil again.
Step 4. Add milk to the boiling mixture and boil it properly.
Step 5. Now filter the mixture (i.e tea) using a tea stainer and collect the filtrate in cup(s). The insoluble tea leaves are left behind as residue.

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(a) Solubility of KNO₃ in water at 313 K = 62 g/100 g of water
$\therefore$ Mass of KNO₃ present in 100 g of water at 313 K = 62 g
$\therefore$ Mass of KNO₃ present in 50 g of water at 313 K =

$\therefore$ Amount of potassium nitrate needed to produce a saturated solution of potassium nitrate in 50 g of water at 313 K is 31 g.
(b) On cooling the saturated solution of potassium chloride in water at 353 K to room temperature, crystals of potassium chloride will be formed.
(c) Solubility of potassium nitrate at 293 K = 32 g/100 g of water
Solubility of sodium chloride at 293 K = 36 g/100 g of water
Solubility of potassium chloride at 293 K = 35 g/100 g of water
Solubility of ammonium chloride at 293 K = 37 g/100 g of water
Ammonium chloride has the highest solubility at 293 K.
(d) (i) If the process of dissolution of salt in water is exothermic, its solubility decreases with the increase in temperature.
(ii) If the process of dissolution of salt in water is endothermic, its solubility increases with the increase in temperature.

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(a) Saturated solution. A solution in which no more solute can be dissolved at a given temperature is said to be saturated at that temperature. For example, 35.7 g of NaCl can be dissolved in 100 ml of water at 20$°$C.
(b) Pure substance. A pure substance is made of only one type of atoms or molecules. Pure substances have same colour and texture at given temperature and presure. Pure substance has a fixed melting and boiling point at constant pressure. Pure substance includes elements and compounds. For example, hydrogen gas, sodium chloride, water etc.
(c) Colloid. A substance is said to be in the colloidal state if its particle size lies between1and 100 nm. A colloidal solution is heterogeneous in nature and consist of two phases i.e. dispersed phase or colloidal particles and dispersion medium in which colloidal particles are suspended e.g. colloidal solution of starch in water.
(d) Suspension. It is a heterogeneous mixture in which the particles of the solute do not dissolve but remain suspended throughout the bulk of the material and the size of particles is more than 10^–7 m. These particles can be seen with naked eye.

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Soda water — Homogeneous mixture
Wood — Heterogeneous mixture
Air — Homogeneous mixture
Soil — Heterogeneous mixture
Vinegar — Homogeneous mixture
Filtered tea — Homogeneous mixture

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Determine the boiling point of the given liquid. If its boiling point is 100$°$C at 1 atmospheric pressure, it is pure water and if boiling point is above/below 100$°$C, it is impure water.

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(a) Ice (c) Iron
(d) Hydrochloric acid (e) Calcium oxide
(f) Mercury

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(b), (c) and (e)

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(b) and (d)

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(a) Sodium — Element
(b) Soil — Mixture
(c) Sugar solution — Mixture
(d) Silver — Element
(e) Calcium carbonate — Compound
(f) Tin — Element
(g) Silicon — Element
(h) Coal — Mixture
(i) Air — Mixture
(j) Soap — Mixture
(k) Methane — Compound
(l) Carbon dioxide — Compound
(m) Blood — Mixture

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