Chapter - 2 Is Matter Around Us Pure

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.

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

A homogeneous mixture is a mixture having a uniform composition throughout he 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.

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

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

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.

(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.

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.

 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

Pure substance: Water, salt, sugar etc.

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

(a) Crystallization or Evaporation.

(b) Sublimation.

(c) Centrifugation or Sedimentation.

(d) Chromatography.

(e) Centrifugation.

(f) Separating funnel.

(g) Hand-picking.

(h) Magnetic separation.

(i) Winnowing.

(j) Centrifugation.

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 over tea leaves. Colour of tea leaves goes into solution as filtrate. The remaining tea leaves being insoluble remains as residue. Add requisite sugar which dissolves and the tea is ready.

(a) At 313 K,

Potassium nitrate for saturated solution of 100 grams of water = 62 g

∴ Potassium nitrate for saturated solution of 50 grams of water = 31 g

(b) Some amount of dissolved Potassium Chloride will reappear as undissolved solid as solubility of solute decreases with the decrease of temperature.

(c) Solubility of each salt at 393 K are as follows:

 Potassium nitrate 32

 Sodium chloride 36

 Potassium chloride 35

 Ammonium chloride 37

Ammonium chloride salt has the highest solubility at this temperature.

(d) Solubility of salt increases with the increase in temperature.

(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 50 gm of a solute is the maximum amount that can be dissolved in 100 gm water at 298 K. Then 150 gm of solution so obtained is the saturated solution at 298 K.

(b)Pure Substance: A pure substance consists of a single of matter or particles and cannot be separated into other kind of matter by any physical process. Pure substances always have the same colour, taste and texture at a given temperature and pressure. For example, pure water is always colourless, odorless and tasteless and boils at 373 K at normal atmospheric pressure. 

(c) Colloid: Colloids are heterogeneous mixtures the particle size is too small to be seen with a naked eye, but it is big enough to scatter light. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium. Colloids are useful in industry and daily life. A colloid has the following characteristics:

 It is a heterogeneous mixture.

 The size of particles of a colloid lies between 1 - 100 nm and cannot be seen by naked eyes.

 The particles of colloid can scatter a beam of light passing through it and make the path visible.

 The particles of colloid cannot be separated from the mixture by filtration. The process of separation of colloidal particles is known as ‘centrifugation’.

 They do not settle down when left undisturbed. In other words colloids are quite stable e.g. smoke, milk, fog, cloud etc.

(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. A suspension has the following characteristics:

 It is a heterogeneous mixture.

 The size of particles of a suspension is greater than 100 nm and is visible to naked eyes.

 The particles of suspension can scatter a beam of light passing through it.

 The particles of a suspension settle down when left undisturbed.

 The particles of a suspension can be separated from its mixture by filtration.

Homogeneous mixture - soda water, air, vinegar, filtered tea.

Heterogeneous mixture - wood, soil.

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

(a), (c), (d), (e) and (f) are pure substances.

Solutions among the following mixtures.

(b) Sea water

(c) Air

(e) Soda water.

(b) and (d) are colloids and will show Tyndall Effect.

Elements - sodium, silver, tin, silicon.

Compounds - calcium carbonate, methane, carbon dioxide.

Mixtures - soil, sugar solution, coal, air, soap, blood.

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.