CBSE Class 6 Science Chapter 5 Guide

Chapter 5: Separation of Substances - Class 6 Science

Chapter Summary

In our daily life, we often come across many mixtures from which we need to separate useful or unwanted substances. For example, we separate tea leaves from prepared tea, stones from rice, or butter from milk. Different methods are used for different types of separation based on the properties of the components of the mixture.

• Why do we separate substances?
  • To remove undesirable or harmful components.
  • To obtain useful components.
  • To separate two different but useful components.
• Methods of Separation:
• Handpicking: This method is used for separating slightly larger impurities like pieces of dirt, stone, and husk from grains, pulses, and rice. It is suitable when the quantity of the impurity is not very large and the items are distinguishable by sight.
• Threshing: This process is used to separate grain from stalks. In this process, the stalks are beaten to free the grain seeds. Large machines called threshers are also used to thresh large quantities of grain.
• Winnowing: This method is used to separate heavier and lighter components of a mixture by wind or by blowing air. Farmers use this method to separate lighter husk particles from heavier grain seeds. The lighter husk is carried away by the wind, while the heavier grains fall down and form a heap.
• Sieving: This method is used to separate components of a mixture that have different sizes. A sieve is a device with small holes. Smaller particles pass through the holes, while larger particles remain on the sieve. This is used in flour mills to separate bran from flour or at construction sites to separate pebbles and stones from sand.
• Sedimentation: When the heavier component in a mixture settles down after water is added to it, the process is called sedimentation. For example, sand settling at the bottom of a bucket of water.
• Decantation: After sedimentation, the water (or other liquid) is carefully poured off without disturbing the settled sediment. This process is called decantation. For example, pouring off clear water from a mixture of sand and water after the sand has settled.
• Filtration: This method is used to separate insoluble solids from liquids using a filter. A filter paper or a piece of cloth acts as a filter. The liquid that passes through is called the filtrate, and the solid remaining on the filter is called the residue. Examples include separating tea leaves from tea using a strainer or separating fine mud particles from water using filter paper.
• Evaporation: The process of conversion of water into its vapour is called evaporation. This method is used to separate a soluble solid from its liquid. For example, salt is obtained from seawater by the process of evaporation. The water evaporates, leaving the salt behind.
• Condensation: The process of conversion of water vapour into its liquid form is called condensation. This is the reverse of evaporation. This principle is used in distillation to obtain pure water from a salt solution.
• Use of More Than One Method: Sometimes, a single method is not sufficient to separate substances completely. In such cases, a combination of methods like sedimentation, decantation, filtration, and evaporation may be used. For example, to separate sand, salt, and water from a mixture.
• Saturated Solution: A solution in which no more solute can be dissolved at a given temperature is called a saturated solution.
• Solubility: The amount of a substance (solute) that can dissolve in a given amount of liquid (solvent) at a particular temperature. Heating increases the solubility of most solids in liquids.

Questions and Answers

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

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

• To remove undesirable or harmful components.
• To obtain useful components.
• To separate two different but useful components.

Examples:

1. Separating stones from rice before cooking (to remove undesirable components).
2. Churning milk to obtain butter (to obtain useful components).

2. What is winnowing? Where is it used?

Answer: Winnowing is a method of separating components of a mixture where the lighter components are separated from the heavier components by wind or by blowing air. The lighter particles (like husk) are carried away by the wind, while the heavier particles (like grains) fall down.

It is commonly used by farmers to separate lighter husk particles from heavier grain seeds.

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

Answer: Husk or dirt particles from a given sample of pulses before cooking can be separated by:

• Handpicking: If the husk or dirt particles are large enough and in a small quantity, they can be separated by handpicking.
• Winnowing: If the husk is lighter and the pulses are heavier, winnowing can be used.
• Washing (Sedimentation and Decantation): Often, pulses are washed with water before cooking. The dirt particles, if lighter, will float on water and can be decanted (poured off). Heavier dirt or small stones will settle at the bottom (sedimentation) and can be removed.

4. What is sieving? Where can it be used?

Answer: Sieving is a method of separating components of a mixture that have different sizes. A sieve (a mesh or screen with small holes) is used, allowing smaller particles to pass through while retaining larger particles.

Uses of sieving:

• In flour mills, to separate bran from flour.
• At construction sites, to separate pebbles and stones from sand.
• In the kitchen, to separate fine flour from larger impurities or lumps.

5. How will you separate sand and water from their mixture?

Answer: Sand and water from their mixture can be separated using the following methods:

1. Sedimentation: Allow the mixture to stand undisturbed for some time. The sand, being heavier and insoluble, will settle down at the bottom of the container.
2. Decantation: Carefully tilt the container and pour the water into another container, leaving the settled sand behind. This process is called decantation.
3. Filtration: For more complete separation of very fine sand particles, the decanted water can be passed through a filter paper or a fine cloth. The sand particles will be trapped on the filter, and clear water will pass through.

6. Is it possible to separate sugar mixed with wheat flour? If yes, how would you do it?

Answer: Yes, it is possible to separate sugar mixed with wheat flour.

Here's how it can be done:

1. Sieving: If the sugar crystals are significantly larger than the wheat flour particles, the mixture can be sieved. The flour will pass through the sieve, while the sugar crystals will remain on it.
2. Dissolving in water (and then evaporation): If both sugar and flour are very fine, another method would be to add water to the mixture. Sugar will dissolve in water to form a solution, while wheat flour will not dissolve and will either settle down or form a suspension.
   • Allow the flour to settle (sedimentation) and then pour off the sugar solution (decantation).
   • To obtain the sugar back, evaporate the water from the sugar solution. The sugar will be left behind as a solid.

7. How would you obtain clear water from a sample of muddy water?

Answer: To obtain clear water from a sample of muddy water, a combination of sedimentation, decantation, and filtration methods can be used:

1. Sedimentation: Allow the muddy water to stand undisturbed in a container. The mud particles, being heavier, will slowly settle down at the bottom.
2. Decantation: Once the mud has settled, carefully pour the clear water (supernatant liquid) into another container without disturbing the settled mud. This process is decantation. The water obtained might still contain some fine impurities.
3. Filtration: To make the water completely clear, filter the decanted water through a filter paper or a fine piece of cloth. The filter will trap any remaining fine mud particles, allowing clear water to pass through as the filtrate.

8. Fill in the blanks:

• (a) The method of separating seeds of paddy from its stalks is called __________.
• (b) When milk cooled in a refrigerator is churned vigorously, butter is separated out due to __________.
• (c) Salt is obtained from seawater by the process of __________.
• (d) Impurities settled at the bottom when muddy water was kept overnight in a bucket. The clear water was then poured off from the top. The process of separation used in this case is called __________ and __________.

Answer:

• (a) The method of separating seeds of paddy from its stalks is called threshing.
• (b) When milk cooled in a refrigerator is churned vigorously, butter is separated out due to centrifugation (or density difference/lighter particles floating).
• (c) Salt is obtained from seawater by the process of evaporation.
• (d) Impurities settled at the bottom when muddy water was kept overnight in a bucket. The clear water was then poured off from the top. The process of separation used in this case is called sedimentation and decantation.

9. True or False:

• (a) A mixture of milk and water can be separated by filtration.
• (b) A mixture of powdered salt and sugar can be separated by the process of winnowing.
• (c) Separation of sugar from tea can be done by filtration.
• (d) Grain and husk can be separated by decantation.

Answer:

• (a) A mixture of milk and water can be separated by filtration. False (Milk and water form a homogeneous mixture and cannot be separated by simple filtration).
• (b) A mixture of powdered salt and sugar can be separated by the process of winnowing. False (Both have similar densities and cannot be separated by winnowing. Sieving if particle size differs or dissolving in water would work).
• (c) Separation of sugar from tea can be done by filtration. False (Sugar dissolves completely in tea, forming a solution. It cannot be separated by filtration).
• (d) Grain and husk can be separated by decantation. False (Grain and husk are solids; decantation is for separating a liquid from a settled solid. Winnowing is used for grain and husk).

10. Lemonade is prepared by mixing lemon juice and sugar in water. You wish to add ice to cool it. Should you add ice to the lemonade before or after dissolving sugar? In which case would it be possible to dissolve more sugar?

Answer: You should add ice to the lemonade after dissolving the sugar.

Sugar dissolves more readily and in larger quantities in warmer water than in colder water. If you add ice before dissolving sugar, the water will become cold, reducing its capacity to dissolve sugar. Therefore, it would be possible to dissolve more sugar when the water is warmer (before adding ice).