Separation Of Substances Class 6 Chapter Notes

Key Terms and Definitions

Mixture

A substance containing two or more components that are physically mixed but not chemically combined. For example, sandy water or salt dissolved in water.

Pure Substance

A substance consisting of only one type of particle. For example, sugar, salt, or water.

Handpicking

A method of separation used to remove slightly larger impurities like dirt, stones, and husks from grains by hand.

Threshing

The process of beating stalks to separate the grains from the harvested crop. It can be done manually or with machines.

Winnowing

A method to separate heavier components (like grain) from lighter components (like husk) by using wind or blowing air.

Sieving

A process to separate components of a mixture that have different sizes, using a sieve (a mesh or net).

Sedimentation

The process where heavier, insoluble components in a liquid mixture settle down at the bottom of the container.

Decantation

The process of pouring out the clear liquid from the top without disturbing the settled solid (sediment).

Filtration

A method for separating insoluble solids from a liquid by passing the mixture through a filter, such as filter paper.

Evaporation

The process of converting a liquid into its vapour. It is used to separate a soluble solid dissolved in a liquid.

Condensation

The process of converting a vapour (gas) back into its liquid form on cooling.

Must Remember

• The choice of separation method depends on the properties (size, shape, weight, solubility) of the components of the mixture.
• Handpicking is suitable when impurities are in small quantities and are easily visible.
• Winnowing works on the principle that lighter particles are blown away by wind, while heavier particles fall down.
• Sieving separates components based on the difference in their particle sizes.
• Sedimentation and Decantation are used to separate an insoluble solid from a liquid.
• Filtration is a better method than decantation for separating fine insoluble particles.
• Evaporation is used to separate a soluble solid from a liquid (e.g., salt from water). The liquid is lost in this process.
• To recover both the soluble solid and the liquid, we use evaporation followed by condensation.
• A solution is called 'saturated' when it cannot dissolve any more of the solute at a given temperature.

Separating Insoluble Solids from Liquids

Often, we need to separate solids that do not dissolve in a liquid, like sand or mud from water. Three common methods used in sequence are Sedimentation, Decantation, and Filtration.

1. Sedimentation: Imagine you have a glass of muddy water. If you let it stand undisturbed for some time, you will notice that the heavier mud particles settle down at the bottom. This process of heavier components settling down is called sedimentation. The layer of settled particles is called the sediment.

1. Decantation: Once the sediment has formed, the water on top becomes clearer. You can now carefully pour this clear water into another container without disturbing the sediment at the bottom. This process of pouring off the upper clear liquid is called decantation. While this removes most of the mud, some very fine particles might still remain suspended in the water, making it slightly cloudy.

1. Filtration: To get completely clear water, we use filtration. This method is more effective than decantation for removing fine, insoluble particles. For this, we pass the mixture through a filter, like a filter paper. The filter paper has very tiny pores that allow the liquid (the filtrate) to pass through but block the solid particles (the residue). When you pour the muddy water through a filter paper folded into a cone, the mud particles are left behind on the paper, and you collect crystal clear water in the beaker below.

Comparison of Simple Separation Methods

Quick Examples

• {"title":"Separating Sand and Salt","bodyMarkdown":"To separate a mixture of sand and salt: \n1. Add water to the mixture and stir. Salt will dissolve, but sand will not. \n2. Use decantation and filtration to separate the sand from the salt solution. \n3. Use evaporation to heat the salt solution. Water will turn into vapour, leaving the salt behind."}
• {"title":"Making Paneer (Cottage Cheese)","bodyMarkdown":"To make paneer, a few drops of lemon juice are added to boiling milk, causing it to curdle. This forms a mixture of solid paneer and a liquid (whey). The paneer is then separated from the liquid by straining the mixture through a fine cloth or a strainer. This is an example of filtration."}

Exam Tip

In exams, questions often ask you to choose the 'best' method to separate a given mixture. First, identify the components (e.g., solid-solid, solid-liquid). Then, check their properties: Are they soluble? Are they different in size or weight? For 'sand and water', filtration is better than decantation. For 'salt and water', evaporation is the answer. Justifying 'why' you chose a method will fetch you full marks.

Practice Questions with Solutions

• Q: How would you separate husks from rice grains at home? A: Winnowing. The lighter husks will be blown away by the wind (or by blowing air), while the heavier rice grains will fall straight down.
• Q: What is the process used to obtain salt from seawater? A: Evaporation. Seawater is collected in shallow pits, and the sun's heat causes the water to evaporate, leaving the salt behind.
• Q: You have a mixture of oil and water. Which method would you use to separate them and why? A: Decantation. Since oil and water are immiscible (they don't mix) and oil is lighter than water, it will form a separate layer on top. We can carefully pour off the top layer of oil.
• Q: What is a saturated solution? A: A solution in which no more solute can be dissolved at a particular temperature. Adding more solute will cause it to settle at the bottom.

Frequently Asked Questions

Frequently Asked Questions