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NCERT Class 9 Science Important Chapter 5 Exploring Mixtures and their Separation
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Exploring Mixtures and their Separation
Chapter – 5
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. What is a well-stirred mixture of sugar and water called?
Ans: A well-stirred mixture of sugar and water is called a homogeneous mixture or a solution.
2. Give an example of a heterogeneous mixture.
Ans: A stirred mixture of sand and water is an example of a heterogeneous mixture.
3. What are the two main components of a solution?
Ans: A solution is made up of a solute (the substance that gets dissolved) and a solvent (the substance that dissolves the solute).
4. What is the formula to calculate the mass by mass percentage of a solution?
Ans: Mass by mass percentage is calculated by dividing the mass of the solute by the mass of the solution and multiplying by 100.
5. When is the volume by volume percentage method used to express concentration?
Ans: It is used when two miscible liquids are mixed, for example, in perfumes, cosmetics, and vinegar.
6. What is a saturated solution?
Ans: A solution that cannot dissolve any more solute at a given temperature is called a saturated solution.
7. How does an increase in temperature affect the solubility of a solid solute in a liquid solvent?
Ans: The solubility of a solid solute in a liquid solvent generally increases with temperature.
8. What is a crystal?
Ans: A crystal is a solid that is made up of particles arranged in a regular geometric pattern.
9. Name the separation method used to obtain a pure solid from its saturated solution.
Ans: Crystallization is the technique used to get a pure solid from its saturated solution.
10. What is the minimum boiling point difference required to separate two miscible liquids using distillation?
Ans: A minimum difference of about 25 °C in boiling points is required to separate two miscible liquids using distillation.
11. Which traditional distillation method is used in Kannauj to extract fragrances?
Ans: The Deg-Bhapka method is a traditional distillation method used in Kannauj to make natural perfumes like Mitti ka Ittar.
12. Which separation technique is used to process crude oil in a petroleum refinery?
Ans: Fractional distillation is used to separate crude oil into different fractions like petrol and diesel.
13. Which method is used to separate different pigments present in flower petals?
Ans: Paper chromatography is used to separate coloured pigments from flower petals.
14. What type of liquids are separated using a separating funnel?
Ans: A separating funnel is used to separate two immiscible liquids based on their different densities.
15. Define sublimation.
Ans: The transition of a solid directly into a vapour without passing through the liquid state is called sublimation.
16. What is an alloy?
Ans: A homogeneous mixture of two or more metals, or a metal and a non-metal, is called an alloy.
17. What process involves spinning a mixture in a tube at high speed to separate heavier particles?
Ans: Centrifugation is the process that involves spinning a mixture in a tube at a high speed to separate heavier particles from lighter ones.
18. What is the purpose of adding alum to muddy water?
Ans: Alum acts as a coagulant, causing fine suspended mud particles to clump together and settle down by gravity.
19. What is the Tyndall effect?
Ans: The scattering of light by dispersed particles in a colloid or a suspension is known as the Tyndall effect.
20. What are the two components of a colloid called?
Ans: The components of a colloid are called the dispersed phase and the dispersion medium.
Fill in the Blanks:
1. A well-stirred mixture of sugar and water is called a __________.
Ans: Solution.
2. A solution consists of a __________ and a solvent.
Ans: Solute.
3. The process of obtaining a pure solid from its saturated solution is called __________.
Ans: Crystallization.
4. __________ is used to separate two immiscible liquids.
Ans: Separating funnel.
5. The scattering of light by colloidal particles is known as the __________ effect.
Ans: Tyndall.
True or False:
1. A saturated solution can dissolve more solute at the same temperature.
Ans: False.
2. Fractional distillation is used to separate crude oil into different fractions.
Ans: True.
3. Sublimation is the process in which a solid changes directly into vapour.
Ans: True.
4. Alum is added to muddy water to help suspended particles settle down.
Ans: True.
5. An alloy is a heterogeneous mixture of metals.
Ans: False.
Long Questions & Answers:
1. Describe the process of crystallization in a laboratory. Why is this technique used, and how does the rate of cooling affect the crystals formed?
Ans: (i) Process: To prepare crystals in a laboratory, a solid (like copper sulfate) is gradually added to a solvent (like water) while gently heating and stirring until no more solid can dissolve, creating a hot saturated solution. The hot solution is then filtered to remove any insoluble impurities. Finally, the clear filtrate is covered and allowed to cool slowly without disturbance. As the solution cools, the solubility of the substance decreases, and the excess solid separates out into regular geometric patterns known as crystals.
(ii) Purpose: Crystallization is a crucial separation technique used to obtain a pure solid from a mixture or to purify solids. It takes advantage of the differences in the solubility of a substance at various temperatures.
(iii) Effect of Cooling Rate: The rate at which the solution cools directly affects the size and quality of the crystals. Cooling the solution slowly at room temperature allows enough time for the particles to arrange themselves, resulting in larger, well-formed, and shiny crystals. If the hot solution is cooled rapidly (e.g., in ice-cold water), it will result in smaller and less well-formed crystals.
2. Explain the principle behind distillation. How is a mixture of acetone and water separated using this method, and when is fractional distillation required instead?
Ans: Principle of Distillation: Distillation is a method used to separate a homogeneous mixture of two miscible liquids (liquids that mix completely). The principle is based on heating the mixture until the liquid with the lower boiling point vaporizes first. The vapour is then passed through a condenser where it cools and turns back into a pure liquid (the distillate), which is collected in a separate vessel.
Separating Acetone and Water: Acetone and water can be separated by simple distillation because they have a sufficiently large difference in their boiling points. Acetone boils at approximately 56 °C, while water boils at 100 °C. When the mixture is heated, the large boiling point difference allows the acetone to vaporize completely and condense into a separate flask before the water begins to form significant vapour.
Fractional Distillation: Simple distillation requires a boiling point difference of at least 25 °C. If the miscible liquids have relatively small differences in their boiling points (less than 25 °C), a modified process called fractional distillation is required. This technique is widely used in petroleum refineries to separate crude oil into different useful fractions like petrol, diesel, and kerosene.
3. Physical filtration is not always sufficient to separate fine suspended particles. Explain how the techniques of coagulation and centrifugation solve this problem, providing real-world examples for each.
Ans: When a mixture contains very fine suspended particles that cannot be removed by ordinary filtration, techniques like coagulation and centrifugation are used. In coagulation, a chemical substance called a coagulant is added to the mixture, causing the tiny particles to combine into larger and heavier masses that settle at the bottom due to gravity. For example, alum (fitkari) is added to muddy water to make the impurities settle down, and lemon juice is used in milk to make paneer. In centrifugation, the mixture is spun at very high speed, creating a strong centrifugal force that separates heavier particles from lighter ones. The heavier particles settle at the bottom while the lighter liquid remains at the top. This method is commonly used in medical laboratories and blood banks to separate blood cells from plasma.
4. Describe the principles and methods used to separate a mixture of two immiscible liquids (like oil and water) and a solid mixture containing a sublimable substance (like camphor and sand).
Ans: Immiscible liquids are liquids that do not mix with each other, such as oil and water. They can be separated using a separating funnel. When the mixture is left undisturbed, two separate layers are formed according to their densities. The heavier liquid, water, settles at the bottom, while the lighter oil floats on the top. By opening the stopcock of the separating funnel, the lower water layer is drained out first, leaving the oil behind.
A solid mixture containing a sublimable substance, such as camphor and sand, can be separated by the process of sublimation. In this method, the mixture is heated gently. The camphor changes directly from solid into vapour without becoming liquid, while sand does not change. The vapours of camphor cool down on the surface of an inverted funnel and form solid camphor again. In this way, pure camphor is collected separately and sand remains in the dish.
5. Define solubility and explain the concept of a saturated solution. How does temperature affect the solubility of solid solutes in liquid solvents? Provide examples based on experimental observations.
Ans: Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent at a particular temperature. When no more solute can dissolve in the solvent, the solution is called a saturated solution.
Temperature affects the solubility of solid solutes in liquids. Generally, the solubility of solids increases with an increase in temperature. If a hot saturated solution is cooled, the excess dissolved solute separates out in the form of crystals.
For example, the solubility of potassium nitrate increases greatly from 21 g at 10 °C to 167 g at 80 °C. Potassium chloride and ammonium chloride also show an increase in solubility with rising temperature. However, sodium chloride shows very little change in solubility over the same temperature range. On the other hand, the solubility of gases in liquids usually decreases when temperature increases.
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