.DAV Class 6 Science Chapter 2 Food Solutions – Detailed Answers to Questions 1 to 6
A. Fill in the blanks
- Housewives sometimes use the method of hand picking to remove unwanted substances from eatables like rice and pulses.
- Common salt is prepared on a large scale through the process of evaporation.
- Separation of tea leaves from tea is done by a process called filtration.
- The process of loading can be used to increase the rate of sedimentation of the suspended particles in a solid-liquid mixture.
- The method, used to separate the fine particles, suspended in a liquid, by rotating the mixture at high speed, is known as centrifugation.
B. Match the following:
- Wind — (b) winnowing
- Chalk powder and water — (c) sedimentation
- Oil and water — (e) separating funnel
- Separating peas from cooked pulao — (a) hand picking
- Muslin cloth — (d) filtration
- Camphor and salt — (f) sublimation
C. Tick (✓) the correct option.
1. Which of the following is an example of a heterogeneous mixture?
✓ pebbles in rice
2. Which one of the following is not an example of a pure substance?
✓ air
3. The process of loading helps to—
✓ speed up the rate of sedimentation
4. Transferring the clear liquid, into another container, leaving behind the residue, is called—
✓ decantation
5. A substance, that sublimes easily, is—
✓ naphthalene
D. Answer the following questions in brief.
1. Name any three methods used for separating a solid-solid mixture.
Three methods used for separating a solid-solid mixture are:
- Hand picking
- Winnowing
- Sieving
These methods help us separate useful substances from unwanted materials in our daily life.
2. What is winnowing? Give an example of a mixture whose components can be separated by this method.
Winnowing is the method of separating lighter particles from heavier particles with the help of wind or blowing air. Farmers use this method to separate husk from grains. When the mixture is dropped from a height, the lighter husk is carried away by the wind while the heavier grains fall down.Example: A mixture of wheat grains and husk can be separated by winnowing.
3. Some iron pins get scattered on the carpet when you are expecting guests. How can one collect all the pins in the shortest possible time?
The iron pins can be collected quickly by using a magnet.
When the magnet is moved over the carpet, the iron pins get attracted towards it and can be gathered easily.
This method is called magnetic separation and saves both time and effort.
4. Name any three substances which sublime on heating.
Three substances that sublime on heating are:
- Camphor
- Naphthalene
- Iodine
These substances directly change from solid state to vapour on heating without becoming liquid.
5. What is crystallisation? Name any three substances which can be purified using this technique.
Crystallisation is the process of obtaining pure solid crystals from a solution by evaporation and cooling. It is used to purify substances dissolved in water.
Substances that can be purified by crystallisation are:
- Common salt
- Sugar
- Copper sulphate
This method helps in obtaining clean and pure substances for daily and scientific use.
E. Answer the following questions.
1. Differentiate between a pure substance and a mixture. Give two examples of each.
| Pure Substance | Mixture |
|---|---|
| A pure substance contains only one kind of particles. | A mixture contains two or more substances mixed together. |
| It has fixed properties throughout. | Its composition may vary from place to place. |
| The components cannot be separated by simple physical methods. | The components can usually be separated by physical methods. |
Examples of pure substances:
- Oxygen
- Copper
Examples of mixtures:
- Air
- Salt in water
Pure substances are important because they give uniform results, while mixtures are useful in our daily life for different purposes.
2. State the difference between heterogeneous and homogeneous mixtures. Give two examples of each type.
| Homogeneous Mixture | Heterogeneous Mixture |
|---|---|
| The components are mixed uniformly throughout. | The components are not mixed uniformly. |
| Different parts of the mixture look the same. | Different parts can be seen separately. |
| The substances cannot be easily distinguished. | The substances can be easily distinguished. |
Examples of homogeneous mixtures:
- Salt in water
- Sugar in water
Examples of heterogeneous mixtures:
- Pebbles in rice
- Oil and water
Homogeneous mixtures appear as a single substance, whereas heterogeneous mixtures show different substances clearly. Understanding this difference helps us choose the correct method of separation.
3. Can a mixture of iodine and camphor be separated by the sublimation method? Give reason for your answer.
No, a mixture of iodine and camphor cannot be separated by the sublimation method. This is because both iodine and camphor are sublimable substances. On heating, both change directly from solid state into vapours. Since both substances sublime together, they cannot be separated properly by this method. For sublimation to work effectively, only one component of the mixture should sublime while the other should not.
4. Why does the size of naphthalene balls, put in open, keep on reducing with time?
Naphthalene balls slowly change directly from solid state into vapours when kept in the open air. This process is called sublimation.
As the vapours mix with air continuously, the size of the balls keeps reducing gradually with time. That is why naphthalene balls become smaller after a few days. They are commonly used in cupboards to protect clothes from insects because their vapours repel insects.
5. After a dust storm, how does rain help in making air clear?
During a dust storm, a large amount of dust particles remains suspended in the air. When rain falls, the raindrops trap these dust particles and bring them down to the ground. As a result, the air becomes cleaner and fresher. This natural process is similar to sedimentation, where heavier particles settle down. Rain therefore helps nature in purifying the atmosphere and making breathing easier for living beings.
6. Sawdust, mixed in water, cannot be separated by the sedimentation method. Why?
Sawdust is lighter than water and floats on its surface instead of settling down at the bottom. Sedimentation works only when heavier insoluble particles settle down under gravity. Since sawdust does not settle down, it cannot be separated from water by sedimentation. It can instead be removed by filtration or decantation.
7. The filtration method cannot be used to separate sugar and salt mixed in water. Why?
Filtration cannot separate sugar and salt dissolved in water because both substances completely dissolve in water and form a homogeneous solution. The dissolved particles are so small that they pass easily through the filter paper along with water. Therefore, no residue remains on the filter paper. Such mixtures are usually separated by evaporation or crystallisation methods.
8. Describe the method of centrifugation using an appropriate example.
Centrifugation is the method of separating very fine suspended particles from a liquid by rotating the mixture at high speed. The heavier particles settle down due to rapid spinning, while the lighter liquid remains above. For example, cream is separated from milk by centrifugation. In dairies, milk is rotated rapidly in a machine called a centrifuge. The heavier milk remains below, while the lighter cream separates out at the top. This method is also used in laboratories and washing machines for separating substances efficiently.