CBSE Class 6 Science Chapter 5: Separation of Substances NCERT Solutions

Solution: We need to separate different components of a mixture for several reasons. Firstly, to remove harmful or non-useful substances that might be mixed with a desired substance, making it safe and suitable for use. Secondly, sometimes we need to separate even useful components if we intend to use them for different purposes individually.

For example:

1. We often separate stones, dirt, and husk from grains like wheat or rice using the handpicking method before cooking. This ensures we consume clean food.
2. When cooking rice or pulses, we usually wash them. This washing process helps to separate impurities like dust and soil particles, which are lighter and get washed away with water.

Solution: Winnowing is a method used to separate components of a mixture that contain particles of different weights, typically heavier grains and lighter husk or chaff. This separation is achieved by using wind or by blowing air. The lighter components are carried away by the wind, while the heavier components fall down.

This method is commonly used by farmers to separate the husk particles from seeds of grains like paddy or wheat after harvesting.

Solution: To separate husk or larger dirt particles from a sample of pulses before cooking, the most suitable method is handpicking. This involves manually picking out the bigger pieces of husk, stones, or dirt from the pulses. This method is effective when the impurities are different in size, shape, or color from the pulses and are large enough to be easily distinguished and removed by hand.

Solution: Sieving is a separation process used to separate components of a mixture based on their particle size. It involves passing the mixture through a sieve, which has small holes. The finer particles pass through the holes of the sieve, while the larger particles or impurities are retained on the sieve.

Sieving is widely used in various situations:

• In flour mills, it is used to separate fine flour from any broken particles of grains or bran.
• At construction sites, sieving is employed to separate lumps and smaller stones from sand or cement, ensuring a uniform mixture.

Solution: To separate sand and water from their mixture, we can use a combination of sedimentation and decantation, and optionally filtration.

1. Sedimentation: First, leave the mixture undisturbed in a container for some time. Since sand is heavier than water, it will settle down at the bottom of the container.
2. Decantation: After the sand has settled, gently pour the water from the top into another clean container, leaving the settled sand behind. This process is called decantation.
3. Filtration (Optional): If there are still very fine particles of sand suspended in the water, we can further purify the water by filtering it through a filter paper. This process, called filtration, will remove even the smallest suspended particles of sand.

Solution: Yes, it is possible to separate sugar mixed with wheat flour. The most effective method for this separation is sieving. Since sugar crystals and wheat flour particles are generally of different sizes (flour particles are usually finer than sugar crystals, or vice versa depending on the type of sugar), sieving can be used. By passing the mixture through a sieve with appropriate hole size, either the sugar or the flour can be separated from the other.

Solution: To obtain clear water from muddy water, we can follow these steps involving sedimentation and decantation, and potentially filtration:

1. Sedimentation: Allow the muddy water to stand undisturbed in a container, preferably overnight. The heavier mud particles will settle down at the bottom of the container.
2. Decantation: After the mud has settled, carefully pour the clear water from the top into another container. This process is called decantation.
3. Filtration: To ensure the water is completely clear and free from any finer suspended impurities that might have been poured off, it can be further filtered using filter paper. This process, filtration, will remove any remaining fine particles, yielding very clear water.

Solution:

1. The method of separating seeds of paddy from its stalks is called threshing.
2. When milk, cooled after boiling, is poured onto a piece of cloth the cream (malai) is left behind on it. This process of separating cream from milk is an example of filtration (or straining).
3. Salt is obtained from seawater by the process of evaporation.
4. 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 example is called decantation (following sedimentation).