CHAPTER 3 SEPARATION OF SUBSTANCES
There are many reasons as to why we might want to separate substances from a mixture. Some of them are:
There are a number of methods that can be used to separate a mixture of two solids. Some of them are:
In this method, the impurities are picked up by hand in order to separate them from the useful components. When the impurities are large in size and less in amount, handpicking can be used. This method is employed for separating dirt or stones from wheat and pulses.
When seeds have to be separated from the stalks, threshing is used. In order to free the grains from the stalks, the latter is beaten after drying them in the sun. Handpicking cannot be done in this case as each stalk has many grains attached to it. Therefore, threshing is used for this purpose.
Winnowing is used to separate mixtures when one component is lighter and the other one is heavier. The mixture is dropped from a height so that the lighter particles are carried away by the wind while the heavier particles fall straight down on the ground. This method is widely used by farmers to separate the wheat from the chaff.
In this process we pass the mixture through a sieve in order to remove the fine particles of impurity. A sieve is simply a mesh or a net through which the mixture is passed. This process is used in construction sites to separate sand from pebbles.
Separation of insoluble solids from liquids
1. Sedimentation, decantation and filtration
Sedimentation is used when the impurities i.e. the insoluble solid is heavier than the liquid and therefore it settles down to the bottom. This process is called sedimentation.
Decantation is when the liquid (along with the dust) is removed.
Filtration is when a filter medium which allows the fluid to pass through but retains the solid particles is used to filter out the fine solid particles from a liquid. Usually filter papers or strainers are used for filtration.
Separation of soluble solids
1. Evaporation and condensation
The process of conversion of water into vapour is called evaporation. Salt is obtained from sea water through this very process. Sea water is allowed to stand in shallow pits and when sunlight heats up the water, it slowly evaporates and the salt is left behind.
Condensation is the reverse process of evaporation. When water vapour converts into liquid form, the process is called condensation.