Is Matter Around Us Pure

Question: 1. Which separation techniques will you apply for the separation of the following?

(a) Sodium chloride from its solution in water.

Answer : Evaporation

Explanation : When solution of sodium chloride and water is heated, water gets vaporizes leaving sodium chloride.

Thus, by the method of evaporation, sodium chloride can be separated from its solution in water.

(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride.

Answer : Sublimation.

Explanation : Ammonium chloride goes under sublimation, i.e. changes directly from solid to gas while heating and directly from gas to solid while cooling. When sodium chloride is not a sublime, i.e. does not go under sublimation.

Thus, using the method of sublimation ammonium chloride can be separated from a mixture of sodium chloride and ammonium chloride.

(c) Small pieces of metal in the engine oil of a car.

Answer : Filtration

Explanation : Since small pieces of metal is not dissolved in engine oil, thus it can be separated out using the method of filtration.

(d) Different pigments from an extract of flower petals.

Answer : Chromatography

Explanation : The coloured component that is more soluble in water, rises faster and in this way the colour get separated. This method of separating colours is called chromatography.

(e) Butter from curd.

Answer :Centrifugation

Explanation : Butter is denser than curd, and thus is separated by the method of centrifugation. The principle of centrifugation is that the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.

(f) Oil from water.

Answer : Decantation.

Explantion : Oil is lighter than water and hence in the mixture of oil and water both form separate layers. Oil float above while water remains floating at the bottom level. Thus, oil and water can be separated from their mixture using the method of decantation.

The principle of decantation is that immiscible liquids separate out in layers depending on their densities.

(g) Tea leaves from tea.

Answer : Filtration

Explanation :Tea leaves do not dissolve in tea. Thus tea leaves is separated out using the method of filtration. This is a common process in almost every households.

(h) Iron pins from sand.

Answer : Magnetic separation/ hand picking.

Explanation : Iron attracts toward magnet. Thus using a magnet iron pins can be separated from sand.

However, if the number of pins is very less this can be separated using handpicking, but magnetic separation is more suitable for this separation. Handpicking may be proved very tedious.

(i) Wheat grains from husk

Answer : Winnowing/ sedimentation.

Explanation : Winnowing is a process in which grains are made to fall from a height. Due to gravity grains which are heavier than husk fall down while husk is blown away far.

Winnowing is a common process and can be seen commonly in villages.

(b) Sedimentation: After putting grains having husk in water, grain being heavier settle down in the bottom while husk being lighter than water starts floating which is separated out.

(j) Fine mud particles suspended in water.

Answer :Centrifugation

Question: 2. Write steps you would use for making tea. Use words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.

Answer :

First of all water is taken as solvent.

Tea leaves, sugar and milk are added to it as solutes.

The solution so formed is allowed to boil for some time, so that sugar, milk and juice of tea leaves are get dissolved in solvent and make a solution.

After that solution is filtered using a strainer.

The insoluble tea leaves left on strainer as residue.

Filtrate obtained after filtration in container is the required tea.

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 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 313K?

Answer :

Given,

Mass of KNO₃ at 313 K to produce a saturated solution in 100 g of water = 62 g

Therefore, Mass of KNO₃ to produce a saturated solution in 50 g of water = `(62\ g)/2 =31\ g`

Thus, Answer = 31 g

(b) Prayaga 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.

Answer : On cooling of the saturated solution, crystals of potassium chloride will be obtained.

This happens because saturated solution of potassium chloride is made at high temperature (353 K). At more temperature solubility increases. Thus, on cooling potassium chloride gets crystalise and crystals of potassium chloride are obtained.

(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

Answer : Solubility of each salt given in the talbe at 293 K

Potassium nitrate : 32 g

Sodium chloride : 36 g

Potassium chloride : 35 g

Ammonium chloride : 37 g

(d) What is the effect of change of temperature on the solubility of a salt?

Answer : Solubility of salt increases with increase in temperature.

Question: 4. Explain the following giving examples.

(a) Saturated solution

Answer : At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving, is said to be a SATURATED SOLUTION.

Example: If at 30^oC, in water maximum 50g of sugar can be dissolved, then this solution will be called the saturated solution of sugar in water at 30^oC.

If more sugar is added to it, the it will be settled down in the bottom.

(b) Pure substance

Answer: Substances having same chemical properties to all of their constituent particles are called pure substances. This means when all of the constituent particles of a substance have same chemical properties, then the substance is called pure substance.

For example: Gold, Silver, Copper, etc.

(c) Colloid

Answer: Colloid is a heterogeneous mixture but appears homogeneous because of relative smaller size of particles. The size of particles is too small to be individually seen by naked eyes. Colloids are big enough to scatter a beam of light. Colloids do not settle down when left undisturbed and hence a colloid is stable. Colloids cannot be separated from the mixture by the process of filtration, but can be separated by the process of centrifugation.

Example: milk, fog, cloud, shaving cream, etc.

(d) Suspension

Answer : Suspension is a heterogeneous unstable mixture. Particles size of suspension is bigger than colloids. A suspension scatter beam of visible light. Particles of suspension settled down when left undisturbed, thus suspension is unstable.

Example: mixture of flour in water, mixture of oil and water, mixture of powdered chalk and water, mixture of dust and etc.

Question: 5. Classify each of the following as a homogeneous or heterogeneous mixture.

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

Answer:

Soda water, air, and vinegar: Homogeneous mixture.

Wood, soil, and filtered tea: Heterogeneous mixture.

Question: 6. How would you confirm that a colourless liquid given to you is pure water?

Answer : Boiling point is the characteristic property of every liquid. Boiling point of pure water is 100^oC.

Thus, if given colourless liquid boils at 100^oC, then given liquid is pure water and if it boils even slightly above or below the 100^oC, then given liquid is not pure water.

Thus, by observation of boiling point, the given colourless liquid can be confirmed as pure water.

Question: 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

(d) Coal

(e) Soda water.

Answer : Pure substances:

(a) Ice

Explanation

Ice is the solid form of water. And water is a compound which is formed by the combination of hydrogen and oxygen in 2:1 ratio. Here by considering ice is made up of pure water, it is kept under pure substance. However if ice is considered as made up of tape water or sea water i.e. not of pure water, then ice will not be consider as pure substance.

(b) Iron

(d) Hydrochloric acid

(e) Calcium oxide

(f) Mercury

Question: 8. Identify the solutions among the following mixtures.

(a) Soil

(b) Sea water

(c) Air

(d) Coal

(e) Soda water.

Answer : Following are the mixture

(b) Sea water

Explanation: Sea water is the mixture of water and many salts and other many impurities.

(c) Air

Explanation: Air is the mixture of many gases, such as oxygen, carbon dioxide, nitrogen, etc.

(e) Soda water.

Explanation: Soda water is the mixture of water and carbon dioxide.

Question: 9. Which of the following will show "Tyndall effect"?

(a) Salt solution

(b) Milk

(c) Copper sulphate solution

(d) Starch solution

Answer: Milk and Starch solution will show the "Tyndall Effect" as these are colloidal.

Question: 10. Classify the following into elements, compounds and mixtures.

(a) Sodium

Answer: Element

(b) Soil

Answer: Compound

(c) Sugar solution

Answer: Mixture

(d) Silver

Answer: Element

(e) Calcium carbonate

Answer: Compound

(f) Tin

Answer: Element

(g) Silicon

Answer: Element

(h) Coal

Answer: Element. Explanation: Coal is a form of carbon and hence can be categorized as an element.

(i) Air

Answer: Mixture.

(j) Soap

Answer: Compound

(k) Methane

Answer: Compound

(l) Carbon dioxide

Answer: Compound

(m) Blood

Answer: Mixture

Question: 11. Which of the following are chemical changes?

(a) Growth of plant

Answer: Chemical change

(b) Rusting of iron

Answer: Chemical change

(c) Mixing of iron fillings and sand

Answer: Physical change

(d) Cooking of food

Answer: Chemical change

(e) Digestion of food

Answer: Chemical change

(f) Freezing of water

Answer: Physical change

(g) Burning of candle.

Answer: Chemical change