NCERT Solutions for Class 6 Science Chapter 5 — Separation of Substances

Key Concepts

Intext Questions (Page-by-Page)

Intext Question 1 (Page 36)

Q: Why do we need to separate different components of a mixture? Give two examples.

We need to separate different components of a mixture for the following reasons:

1. To remove unwanted or harmful substances — For example, we remove stones and husk from rice before cooking. If these are not removed, they can cause harm or spoil the taste. 2. To obtain useful substances — For example, we separate butter from milk by churning. Both cream (butter) and buttermilk are useful products. 3. To remove impurities — For example, tea leaves are separated from the tea using a strainer. The liquid tea is consumed while the leaves are discarded. 4. To obtain pure substances — For example, salt is obtained from seawater by evaporation. The water evaporates leaving behind pure salt.

Intext Question 2 (Page 38)

Q: What is winnowing? Where is it used?

Winnowing is a method of separating heavier and lighter components of a mixture by using wind or blowing air.

How it works:

• The mixture is allowed to fall from a height.
• The wind blows away the lighter component (husk, dust).
• The heavier component (grains) falls straight down and gets collected separately.

Where it is used:

• Farmers use winnowing to separate husk from grains after threshing.
• It is used to separate lighter chaff from heavier seeds.

Intext Question 3 (Page 39)

Q: How will you separate husk or dirt particles from a given sample of pulses before cooking?

Husk or dirt particles can be separated from pulses using the following methods:

1. Handpicking — If the husk or dirt particles are large and few in number, they can be picked out by hand. 2. Winnowing — If the husk is lighter, winnowing can be used. The pulses are dropped from a height and the wind blows away the lighter husk and dirt. 3. Sieving — A sieve with appropriate mesh size can separate larger pulses from finer dirt particles.

Intext Question 4 (Page 41)

Q: What is sieving? Where is it used?

Sieving is a method of separating particles of different sizes using a sieve (a device with fine pores or mesh).

How it works:

• A sieve has small holes of a specific size.
• When the mixture is shaken on the sieve, smaller particles pass through the holes.
• Larger particles remain on the sieve.
• Thus, particles of different sizes get separated.

Where it is used:

• At construction sites, sieving is used to separate pebbles and stones from sand.
• In the kitchen, sieving is used to separate bran from flour.
• In flour mills, sieving separates different grades of flour.

Intext Question 5 (Page 42)

Q: How will you separate sand and water from their mixture?

Sand and water can be separated using a combination of sedimentation, decantation, and filtration:

Step 1 — Sedimentation:

• Allow the mixture of sand and water to stand undisturbed in a beaker.
• After some time, the heavier sand particles settle at the bottom forming sediment.

Step 2 — Decantation:

• Carefully tilt the beaker and pour the clear water into another container.
• Take care not to disturb the sediment (sand) at the bottom.

Step 3 — Filtration (if needed):

• If some sand particles are still present in water, filter it using filter paper.
• The sand gets trapped on the filter paper and clear water passes through.

Intext Question 6 (Page 44)

Q: Is it possible to separate sugar mixed with water by filtration? Explain.

No, it is not possible to separate sugar mixed with water by filtration.

Reason: Sugar dissolves in water to form a homogeneous solution. In a solution, the solute (sugar) particles are so small that they cannot be seen and cannot be trapped by filter paper. Filtration can only separate insoluble solids from a liquid. Since sugar is completely dissolved in water, filtration cannot separate it.

Correct method: Sugar can be separated from water by evaporation. When the sugar solution is heated, water evaporates and sugar crystals are left behind.

Intext Question 7 (Page 45)

Q: What is a saturated solution?

A saturated solution is a solution in which no more solute can be dissolved at a given temperature.

• When we keep adding solute (like salt) to a solvent (like water) and stir, the solute keeps dissolving.
• At a certain point, no more solute dissolves even with continued stirring.
• The solution is now saturated — it cannot hold any more solute at that temperature.
• Any additional solute added will remain undissolved at the bottom.

Example: If we keep adding salt to a glass of water and stir, after some time no more salt dissolves. The solution has become saturated.

Intext Question 8 (Page 46)

Q: How would you obtain clear water from a sample of muddy water?

Clear water can be obtained from muddy water through the following steps:

Step 1 — Sedimentation:

• Leave the muddy water undisturbed for some time.
• The heavy mud particles settle down at the bottom.

Step 2 — Decantation:

• Carefully pour the upper clear layer of water into another container without disturbing the sediment.

Step 3 — Filtration:

• Pass the decanted water through filter paper.
• Any remaining fine mud particles are trapped on the filter paper.
• Clear water collects in the container below the funnel.

Intext Question 9 (Page 47)

Q: Explain the process of evaporation with an example.

Evaporation is the process in which a liquid changes into vapour on heating.

Process:

• When a liquid is heated, its particles gain energy and start moving faster.
• Some particles at the surface gain enough energy to escape into the air as vapour.
• The dissolved solid remains behind.

Example — Obtaining salt from seawater: 1. Seawater is collected in shallow pits called salt beds. 2. Water evaporates slowly under the sun. 3. After all the water evaporates, salt crystals are left behind. 4. These salt crystals are collected and purified.

Intext Question 10 (Page 48)

Q: Explain how salt is obtained from seawater.

Salt is obtained from seawater through the following process:

1. Collection: Seawater is collected in large, shallow pits called salt pans or salt beds near the sea coast. 2. Evaporation: The water is left in the sun. The heat from the sun causes water to evaporate slowly. 3. Crystallisation: As more and more water evaporates, the concentration of salt increases. Eventually, salt crystals begin to form. 4. Collection of salt: After all the water has evaporated, white crystals of salt are left behind. These are collected by workers.

This is a natural method using solar energy and is widely used in coastal areas of India (Tamil Nadu, Gujarat, Maharashtra).

Exercise Questions and Answers

Q1. Why do we need to separate different components of a mixture? Give two examples.

We need to separate different components of a mixture for the following reasons:

1. To remove unwanted or harmful substances: We remove stones from rice before cooking. We separate tea leaves from tea before drinking. 2. To obtain pure substances: We separate salt from seawater. We obtain pure water from dirty water. 3. To obtain useful components: We separate cream from milk. We separate petrol, diesel, and kerosene from crude oil. 4. To remove non-useful substances: We remove husk from wheat. We remove dirt from food grains.

Examples:

• Stones are separated from rice by handpicking before cooking.
• Butter is separated from milk by churning.

Q2. What is winnowing? Where is it used?

Winnowing is the process of separating lighter and heavier components of a mixture by using wind or blowing air.

How it works:

• The mixture is dropped from a height.
• The lighter particles (husk, chaff) are blown away by the wind.
• The heavier particles (grains) fall directly down.

Where it is used:

• Farmers use winnowing to separate husk from grains after threshing.
• It is commonly used in villages to separate lighter chaff from heavier food grains.

Q3. You must have seen your mother separating tea leaves from prepared tea. Which method of separation is used here?**

The method used is filtration.

• A strainer (sieve) is used to separate the tea leaves from the liquid tea.
• The strainer has small holes that allow the liquid (tea) to pass through.
• The solid tea leaves are trapped on the strainer and do not pass through.
• The liquid tea is collected in the cup below.

Filtration is used because tea leaves are insoluble solids that can be trapped by a strainer or filter paper.

Q4. While making tea, what method of separation is used to separate tea leaves from the prepared tea?

The method of separation used is filtration.

• When tea is prepared, tea leaves are mixed with boiling water along with milk and sugar.
• After the tea is ready, it is poured through a strainer into a cup.
• The strainer has fine holes that allow the liquid tea to pass through.
• The solid tea leaves remain on the strainer and are separated from the tea.

Filtration is the most suitable method here because tea leaves are solid particles that do not dissolve in water and can be easily separated by passing through a filter or strainer.

Q5. How will you separate husk particles from a given sample of pulses before cooking?

Husk particles can be separated from pulses using the following methods:

1. Winnowing: The mixture of pulses and husk is dropped from a height. The wind blows away the lighter husk while the heavier pulses fall down and get collected. 2. Handpicking: If the number of husk particles is small, they can be picked out by hand. 3. Sieving: A sieve with appropriate mesh size can be used. The husk particles, being smaller, pass through the sieve while the pulses remain on top.

Q6. How will you separate sand from water?

Sand can be separated from water using the following methods:

Method 1 — Sedimentation and Decantation: 1. Allow the sand-water mixture to stand undisturbed in a beaker. 2. Sand particles, being heavy, settle at the bottom (sedimentation). 3. Carefully pour the clear water into another container without disturbing the sand (decantation).

Method 2 — Filtration: 1. Pass the sand-water mixture through filter paper placed in a funnel. 2. Sand particles get trapped on the filter paper as residue. 3. Clear water passes through as filtrate.

Q7. How would you obtain a sample of pure salt from a mixture of salt and water?

Pure salt can be obtained from a mixture of salt and water by the method of evaporation:

1. Pour the salt solution into a beaker or evaporating dish. 2. Heat the solution on a burner or leave it in the sun. 3. Water evaporates gradually, turning into vapour. 4. As water evaporates, the salt concentration increases. 5. When all the water has evaporated, white crystals of salt are left behind in the container. 6. Collect the salt crystals and let them dry completely.

Note: Since salt is dissolved in water (it is soluble), it cannot be separated by filtration or sedimentation. Evaporation is the correct method because it removes the liquid component and leaves the solid solute behind.

Q8. Which method would you use to separate:

(a) Salt from a salt solution?

Method: Evaporation

Heat the salt solution. Water evaporates and salt is left behind as solid crystals.

(b) Tea leaves from prepared tea?

Method: Filtration

Pour the tea through a strainer. The liquid passes through and tea leaves are trapped on the strainer.

(c) Sand from sand-water mixture?

Method: Sedimentation followed by Decantation and Filtration

Allow sand to settle, pour off the clear water, then filter any remaining sand particles.

(d) Husk from wheat grains?

Method: Winnowing

Drop the mixture from a height. Wind blows away the lighter husk while heavier wheat grains fall down.

(e) Pebbles from sand?

Method: Sieving

Pass the mixture through a sieve. Sand passes through the small holes while pebbles remain on the sieve.

Q9. Fill in the blanks:

(a) The method of separating seeds of paddy from its stalks is called \_\_\_\_\_.

Threshing is the process of beating the stalks to separate the grain seeds. It can be done manually or using a machine called a thresher.

(b) When milk is cooled after boiling, cream settles at the top because of \_\_\_\_\_.

Cream, being lighter, rises to the top and forms a separate layer. This process of settling is similar to sedimentation where particles separate based on density.

(c) The process of converting water into its vapour is called \_\_\_\_\_.

Evaporation is the process by which water changes from liquid to vapour state on heating.

(d) The process of converting water vapour into ice directly is called \_\_\_\_\_.

When water vapour directly changes into ice without passing through the liquid state, the process is called deposition. (Note: In the NCERT context, this may also be referred to as freezing when it occurs through the liquid state.)

Q10. State whether the following statements are true or false:

(a) A mixture of milk and water can be separated by filtration.

Milk and water form a homogeneous mixture (solution). Both are liquids and cannot be separated by filtration since there are no insoluble solid particles to trap.

(b) A mixture of powdered salt and sugar can be separated by the process of winnowing.

Both salt and sugar are powdered solids of similar weight. Winnowing separates lighter from heavier particles using wind. Since both are similarly fine and dense, winnowing will not work. They can be separated by dissolving in water (sugar and salt dissolve differently) followed by crystallisation.

(c) Separation of sugar from tea can be done by filtration.

Sugar dissolves completely in tea to form a solution. Filtration can only separate insoluble solids from liquids. Since sugar is dissolved, it cannot be filtered out.

(d) Grain and husk can be separated by the process of winnowing.

Grains are heavier and husk is lighter. When the mixture is dropped from a height, the wind blows away the lighter husk while the heavier grains fall directly down. Winnowing is the appropriate method.

Multiple Choice Questions (MCQs)

Common Mistakes to Avoid

Exam Checklist

• [ ] Can you explain why separating mixtures is necessary and give at least 3 reasons?
• [ ] Can you define handpicking, threshing, and winnowing with examples?
• [ ] Can you describe sieving and state where it is used?
• [ ] Can you explain filtration and give examples from daily life?
• [ ] Can you distinguish between sedimentation and decantation?
• [ ] Can you explain how to obtain salt from seawater (evaporation)?
• [ ] Do you know what a saturated solution is and can you give an example?
• [ ] Can you describe the complete process of obtaining clear water from muddy water?
• [ ] Can you match each method of separation with the correct type of mixture?
• [ ] Can you explain condensation and distillation as separation methods?

Summary of Separation Methods