CBSE Notes Class 6 Science Chapter 3 Separation Of Substances

Different methods are used for separation based on the type of mixture and the properties of its components. Some standard separation techniques include handpicking, sieving, filtration, evaporation, and decantation. Understanding these methods allows us to choose the most suitable process for different mixtures.

1.0Introduction 

In our daily lives, we often encounter mixtures, which are combinations of two or more substances. However, not all mixtures are helpful in their combined form, and sometimes, we need to separate the individual components for specific purposes, such as separating stones from rice or filtering tea leaves from brewed tea.

In Chapter- Separation of Substances, we'll explore the basics of separating substances.

Substances are separated to remove non-useful or harmful parts, extract valuable components, or clean materials to get a pure substance. Different materials have different properties that help in separating mixtures. 

2.0Need To Separate Substances

We usually separate substances for one or more of the following reasons:

• Separating two or more useful components, as in the case of butter and milk, is important. Milk is churned to obtain butter.
• Remove useless components from useful ones, like separating tea leaves from brewed tea.
• To eliminate impurities or harmful substances, such as picking out small stones or other impurities from rice and wheat before cooking.

3.0Methods Of Separation

Solid - Solid Mixture 

1. Hand-picking: 

• This method manually removes unwanted substances from a mixture when the components are large enough to be seen and easily handled. For example, stones or other impurities are removed from grains like rice or wheat. Hand-picking is a simple, effective method used when the quantity of impurities is small.

2. Threshing

• After harvesting, bundles of wheat or paddy stalks lie in fields. These stalks are dried in the sun before the grain is separated from them. Each stalk holds numerous grain seeds; manually plucking them would be impractical. So, how is the grain separated from the stalks?
• The process used for this is called threshing. In threshing, the stalks are beaten to free the grain seeds. Traditionally, bullocks were used for this purpose, but nowadays, machines are often employed to thresh large quantities of grain efficiently.

3. Winnowing

• Winnowing separates heavier components from lighter ones by utilising wind or blowing air, allowing the lighter particles to be carried away while the heavier ones remain behind.
• In this method, the mixture is allowed to fall from a height. The lighter components, such as husk, are carried away by the wind, while the heavier components, like grains, fall straight down. This is commonly used in agriculture to separate grain from husk after harvesting.

4. Sieving 

• Sieving is used to separate components of a mixture based on their size. Sieving is a straightforward, efficient, and time-saving method for separating particles of different sizes. It involves using a sieve, a simple device with small holes that allows finer materials, like flour, to pass through while trapping larger impurities.
• It allows smaller particles, like fine flour, to pass through the holes of the sieve while trapping larger impurities.

5. Magnetic Separation

• Magnetic separation is used when one of the components in a mixture is magnetic. This technique uses a magnet to attract magnetic substances such as iron, cobalt, nickel, or steel from a mixture.
• A magnet can attract and remove iron filings from a mixture of iron and sulfur powder, leaving behind the sulfur powder.

Solid- Liquid Mixture (solid -insoluble)

1. Sedimentation 

• Sedimentation separates heavier insoluble substances from a liquid by letting them settle at the bottom due to gravity. For example, the dirt settles at the bottom in muddy water, and the clear water is poured off. This method also works for separating immiscible liquids like oil and water.
• Loading speeds up sedimentation by adding a chemical like alum. This chemical makes small impurities heavier, allowing them to settle faster. For example, alum helps mud particles settle in muddy water, leaving clear water on top.

2. Decantation

• After sedimentation, decantation involves carefully pouring the clear upper liquid (supernatant) without disturbing the sediments. For example, sand and soil settle in muddy water, and the clear water can be poured off.

3. Filtration:

• Filtration separates insoluble solids from liquids by passing the mixture through filter paper. The paper traps the solid particles (residue) while allowing the liquid (filtrate) to pass through.

Solid- Liquid Mixture (solid-soluble)

1. Evaporation

• Evaporation is a process used to separate a dissolved substance from its solvent or to obtain a concentrated solution. The solvent (often water) evaporates by heating the solution, leaving behind the dissolved substance. This method is used to separate salt from seawater or to concentrate solutions in laboratories and industries.

2. Condensation

• Condensation is the process where water vapour turns back into liquid form. For example, water vapour in the air condenses and falls as rain. 

3. Distillation

• Distillation is a process used to separate components of a liquid mixture based on their different boiling points. It involves selectively boiling the mixture and then condensing the vapours. The substance with the lower boiling point evaporates first and is collected after condensation.
• Distillation can purify liquids or concentrate a specific component from a mixture. This method is commonly used in industries and laboratories to obtain pure substances, such as in the distillation of water or alcohol.

4.0How Much of a Substance Water can Dissolve?

Water can dissolve a certain amount of substances, but there is a limit. Once this limit is reached, the solution becomes saturated, meaning it can't dissolve more of the substance, and the excess collects at the bottom. This limit depends on factors such as temperature and the nature of the substance being dissolved.

For example, if you keep adding salt to water, it will eventually stop dissolving and settle at the bottom, indicating saturation.