Matter in Our Surroundings

2. Separation by a Separating Funnel

A mixture of two immiscible liquids can be separated by using a separating funnel (shown in Figure 85). A separating funnel is a special type of funnel which has a stop-cock (or tap) in its stem to allow the flow of a liquid from it, or to stop the flow of liquid from it. The separation of two immiscible liquids by a separating funnel depends on the difference in their densities. We will now describe how a mixture of two immiscible liquids is separated by using a separating funnel.
The mixture of two immiscible liquids is put in a separating funnel and allowed to stand for some time.The mixture separates into two layers according to the densities of the liquids in it. The heavier liquid or denser liquid (having higher density) forms the lower layer whereas the lighter liquid (having lower density) forms the upper layer (see Figure 85). On opening the stop-cock of separating funnel, the lower layer of heavier liquid comes out first and collected in a beaker. When the lower layer of heavier liquid has completely run off, the stop-cock is closed. The lighter liquid in the upper layer is collected in a separate beaker by opening the stop-cock again.
Water and kerosene oil are two immiscible liquids. So, a mixture of water and kerosene can be separated by using a separating funnel.When the mixture of water and kerosene is put in a separating funnel, it forms two layers. Water, being heavier, forms the lower layer in the separating funnel whereas kerosene,being lighter, forms the upper layer (see Figure 85). On opening the stop-cock of separating funnel, the lower layer of water comes out first and collected in a beaker. When the water layer has completely run off,then the stop-cock is closed. The kerosene is left behind in the separating funnel. It can be removed in a separate beaker by opening the stop-cock again.
A mixture of petrol and water can also be separated by using a separating funnel. Petrol and water are immiscible liquids so they form two layers in the separating funnel. Water being heavier, forms the lower layer whereas petrol being lighter, forms the upper layer. On opening the stop-cock of separating funnel, the lower layer of water comes out first and collected in a beaker. When the water layer has completely run off, then the stop-cock is closed. The petrol is left behind in the separating funnel. It can be collected in a separate beaker by opening the stop-cock again.
Please note that groundnut oil (or cooking oil), and mustard oil are also immiscible with water. So,a mixture of groundnut oil (or cooking oil) and water can be separated by using a separating funnel. Similarly,a mixture of mustard oil and water can also be separated by using a separating funnel. It should be noted that both, groundnut oil and mustard oil, are lighter than water and hence form upper layer in the separating funnel. Please note that there are two differences in the properties of oil (like kerosene oil, mustard oil,groundnut oil, etc.) and water which enable their separation by a separating funnel. These are :
(i) oil and water are immiscible liquids,and
(ii) oil and water have different densities.
Mercury, carbon disulphide, chloroform, benzene and ether are also immiscible with water.So,a mixture of mercury and water ; carbon disulphide and water ; chloroform and water ; benzene and water, or ether and water, can be separated by using a separating funnel. Please remember that mercury, carbon disulphide and chloroform are heavier than water but benzene and ether are lighter than water.
A mixture of more than two immiscible liquids can also be separated by using a separating funnel.For example, a mixture of mercury, water and benzene can be separated in this way. When a mixture of mercury, water and benzene is put in a separating funnel, it separates into three layers. Mercury, being heaviest liquid, forms the bottom layer, water forms the middle layer, and benzene, being lightest, forms the top layer. On opening the stop-cock, mercury will run out first, followed by water and then benzene This type of separation (in which the heavier liquid forms lower layer and lighter liquid forms upper layer) is useful in industry. For example, during the extraction of iron in the blast furnace, the heavier molten iron forms the lower layer at the base of blast furnace whereas the lighter slag (molten waste material) forms a separate layer on the top of molten iron. The two layers can be 'tapped' off separately from the blast furnace.