Separation of Substance

Decantation; oil forms the top layer because it is less dense than water

Step 1: Oil and water are immiscible liquids – they do not dissolve in each other. Step 2: When allowed to stand, they form two distinct layers based on their densities. Step 3: Oil is less dense than water, meaning it is lighter for the same volume, so it floats on top of water. Step 4: Water, being denser, forms the bottom layer. Step 5: The top layer (oil) can then be carefully poured off using decantation. Filtration is incorrect because both oil and water are liquids and would both pass through a filter. Sieving works only for solids and liquids with different particle sizes. Evaporation

Sieving works only when the particles of the two components have different sizes; chalk powder and flour have similar particle sizes

Step 1: Sieving separates components of a mixture based on difference in particle size. Step 2: The sieve has small holes – particles smaller than the holes pass through, while larger particles remain on the sieve. Step 3: When chalk is ground into powder, its particle size becomes very similar to the fine flour particles. Step 4: Since both particles are approximately the same size, they both either pass through or both remain on the sieve – no separation occurs. Step 5: This is why Activity 4 in the chapter asks students to try this and observe the failure. Option B is wrong – sieving is com

The solid particles are too large to pass through the fine pores of the filter paper and remain on it, while the liquid passes through and is collected below

Step 1: A filter paper has very fine pores (tiny holes) in it. Step 2: When a mixture of a solid and liquid is poured onto the filter paper, the liquid molecules are small enough to pass through these fine pores. Step 3: The solid particles are much larger than the pores and cannot pass through, so they are retained on the filter paper. Step 4: The liquid that passes through and is collected in the vessel below is called the filtrate. Step 5: The solid left on the filter paper is called the residue. A real-world example is making paneer – the liquid (whey) passes through the cloth while solid

Evaporation by sunlight causes water to turn into vapour and escape, leaving behind solid salt

Step 1: Sea water contains many dissolved salts, including common salt (sodium chloride). Step 2: When sea water is collected in large shallow pits, it is exposed to sunlight and wind. Step 3: The heat from sunlight causes the water molecules to gain energy and escape as water vapour – this is evaporation. Step 4: Since salt cannot evaporate at these temperatures, it remains behind in the pit as the water level gradually decreases over days. Step 5: Eventually, all the water evaporates and a mixture of salts is left, from which common salt is obtained by further purification. Option B is wrong