What this chapter is about
In our daily life, we often need to separate things that are mixed together. Think about when your mother picks out small stones from rice before cooking. Or when tea is poured through a strainer to remove the tea leaves. These are all methods of separation.
This chapter teaches you different ways to separate mixtures. A mixture is when two or more things are put together but do not combine to form something new. You can still see or get back the original things. You will learn why we need to separate mixtures and which method works best for different situations.
After studying this chapter, you should be able to choose the right method to separate different kinds of mixtures. You will understand how these methods work based on the properties of the things being separated, like their size, weight, or whether they dissolve in water.
Key ideas
- A mixture contains two or more substances that are just mixed together but not joined chemically. Each substance keeps its own properties.
- We separate mixtures to remove unwanted things (like stones from rice), to get useful parts (like cream from milk), or to get back pure substances.
- Handpicking works when the pieces are large enough to see and pick by hand, like removing bad grains or stones.
- Sieving means passing a mixture through a net with small holes. Smaller pieces fall through while larger ones stay on top.
- Winnowing uses wind or blowing air to separate lighter parts from heavier parts, like removing husk from grain.
- Sedimentation means letting heavy particles settle at the bottom when mixed with water. Decantation means carefully pouring out the clear water on top without disturbing what has settled.
- Filtration separates things that do not dissolve in water from the water itself, using filter paper or a cloth.
- Evaporation is used to get back a solid that has dissolved in water by heating the mixture until the water turns to vapour and escapes.
Formulas and facts to remember
- Handpicking: Used when unwanted particles are large, visible, and fewer in number than the main substance.
- Sieving: Separates particles of different sizes using a mesh or net with holes of a particular size.
- Winnowing: Separates lighter particles from heavier ones using moving air or wind.
- Sedimentation: Heavy insoluble particles settle down when a mixture is left undisturbed in water.
- Decantation: Pouring out the clear liquid gently after sedimentation without disturbing the settled material.
- Filtration: Passing a mixture through filter paper or cloth to separate insoluble solids from a liquid.
- Evaporation: Heating a solution so the liquid turns to vapour, leaving the dissolved solid behind.
- A substance that dissolves in water is called soluble. One that does not dissolve is called insoluble.
Worked examples
Example 1: Separating rice and husk
Ravi's grandmother bought rice from a local mill. It has some husk mixed in it. Which method should she use?
Solution: Husk is very light compared to rice grains. She should use winnowing. She can put the rice in a flat basket and let it fall slowly from a height while the wind is blowing. The light husk will blow away while the heavier rice grains fall straight down into a container.
Example 2: Getting salt from salty water
Meena dissolved two spoons of salt in a glass of water. Now she wants the salt back. How can she do it?
Solution: Salt dissolves completely in water. To get it back, Meena should use evaporation. She can pour the salty water into a wide dish and heat it gently. As the water heats up, it will turn into water vapour and escape into the air. The salt cannot turn to vapour, so it will be left behind in the dish as white crystite.
Example 3: Cleaning muddy water
After heavy rain, Arjun collected water in a bucket. The water looks brown because of mud. How can he make it cleaner?
Solution: First, let the bucket stand still for some time. The heavy mud particles will slowly settle at the bottom. This is sedimentation. Then, carefully pour the cleaner water on top into another container without shaking the bucket. This is decantation. If the water still has fine particles, pour it through a clean cloth to filter out the remaining dirt.
Common mistakes
- Thinking winnowing works for any mixture → It only works when one part is much lighter than the other and wind can blow it away.
- Trying to filter out salt from salty water → Salt dissolves in water, so it passes through the filter along with the water. Use evaporation instead.
- Confusing sedimentation with filtration → Sedimentation means letting particles settle by gravity. Filtration means passing through a barrier like cloth or paper.
- Using handpicking when there are too many small unwanted pieces → Handpicking is slow and works only for large, visible, fewer pieces. Use sieving for many small pieces.
- Forgetting that the choice of method depends on the properties of the substances → Always think about size, weight, and whether something dissolves before choosing a method.
Quick revision
- Handpicking: for large, visible unwanted pieces that are few in number.
- Sieving: separates by size using holes in a mesh.
- Winnowing: uses wind to blow away lighter particles.
- Sedimentation and decantation: let heavy particles settle, then pour off clear liquid.
- Filtration: separates insoluble solids from liquids using paper or cloth.
- Evaporation: gets back dissolved solids by heating until liquid turns to vapour.