What is the technique used to separate salt in seawater? (A) distillation (B) filtration (C) sublimation (D) evaporation
Hint:The sea water contains many dissolved salts. Salt is a mineral and sodium chloride is one major part of it. The salt is separated from seawater using special techniques. The salt and water makes a true solution, thus they can’t be separated by simple physical techniques. Salt is used as a preservative and added in food to make it tasty.
Complete step by step answer: As we must know, evaporation is a process of conversion of water into water vapours at a temperature below the boiling point of water. The boiling point of water is 100°C. The salt can be separated from the sea water by a simple process of evaporation. On a commercial scale, large water tanks are built and filled with seawater, then left under the sun for a few days. After a few days it is observed that the water has evaporated and the salt is found at the bottom of that tank. That salt is then collected and purified for making it usable in cooking and other purposes. The principle behind evaporation is simple, the water molecules on absorbing the heat from the Sun get converted into water vapours and dissipate in the environment leaving the salt behind as the salt has a much higher boiling point. If we want to separate potable water from the sea water, we can go for another technique i.e. distillation. In distillation the seawater is kept in a distillation flask and then heated. Due to the heat the temperature rises and reaches the boiling point of water. At this temperature the water starts evaporating and converting into water vapours. The water vapours then travel through the condenser, which condenses the water vapours into pure water. This pure water is collected in a container. However, filtration techniques cannot be used to separate salt from seawater because salt and water is a true solution and thus the particles cannot be separated by simple filtration. In the same way, sublimation can also not be used for separation of Salt from seawater because sublimation is the process of conversion of a solid substance directly into gaseous state. This technique is not appropriate in this condition. Hence, the correct option is D.
Note: By the method of evaporation of sea water we cannot directly get sodium chloride as a product. Seawater contains many impurities such as ions of other elements. For obtaining pure sodium chloride we need to purify the salt obtained by the evaporation of seawater. The salt is not only found in the sea water but it is also found in the rocks. Mining of rocks is done to obtain the rock salt.