Pure Substances & Mixtures: Separation of Mixtures

Its components retain their individual properties.

In a mixture, the components are loosely held together without any chemical reaction. This means each component keeps its own properties. For example, in a mixture of salt and pepper, salt still tastes salty and pepper still tastes hot. Mixtures do NOT have a fixed melting point — it depends on the proportions of the components. Components of a mixture do NOT combine chemically (that would make a compound). Mixtures cannot be written as a chemical formula because they have no fixed composition.

Sugar dissolved in water

Sugar dissolved in water forms a solution in which sugar particles are uniformly spread throughout the water. You cannot see separate particles of sugar and water — this makes it a homogeneous mixture. Sand and water is heterogeneous because sand does not dissolve and you can see the two components. Oil and water form two visible layers, making it heterogeneous. Chalk powder does not dissolve in water and the particles are visible, so it is heterogeneous. In a homogeneous mixture, the composition is uniform throughout.

Winnowing

Winnowing is the method where a mixture is allowed to fall from a height. The heavier particles (like grains) fall straight down, while lighter particles (like husk) are blown away by the wind and collect at a different spot. This method uses the difference in weight between the two components. Sieving uses different sizes of holes to separate particles. Decantation is used to pour out clear liquid after a solid has settled at the bottom. Loading (or coagulation) involves adding alum to help fine particles settle faster.

Magnetic separation

Iron is a magnetic material — it gets attracted to a magnet. Sulphur is non-magnetic. So when a magnet is moved over the mixture of iron and sulphur, the iron filings stick to the magnet and get separated, while sulphur is left behind. This is called magnetic separation. Sieving works on size differences. Winnowing works on weight differences. Sublimation works when one component can change directly from solid to vapour on heating. None of these involve magnetism, which is the key property used here.