How can common salt be purified?

Common salt can be effectively purified through the process of crystallization. This method is particularly useful because common salt, often derived from sources like seawater, is typically an impure product.

Understanding Crystallization for Salt Purification

Crystallization is a fundamental purification technique where a solid forms from a solution, and its atoms or molecules arrange themselves into a highly ordered structure known as a crystal. For purifying common salt, this process leverages the principle that solubility changes with temperature, and impurities behave differently during crystal formation.

The purification of impure common salt, such as that obtained from seawater, involves several key steps to yield pure sodium chloride crystals.

The Process of Purifying Impure Salt

Here's a breakdown of the steps involved in purifying common salt via crystallization:

1. Dissolution: The impure common salt is first dissolved in a suitable solvent, typically water, to create a saturated solution. Using hot water can help dissolve the maximum amount of salt while leaving many insoluble impurities behind.
2. Filtration: The hot, saturated salt solution is then filtered. This step is crucial for removing insoluble impurities, such as sand, clay, or other particulate matter, that did not dissolve in the water. The clear filtrate (the liquid that passes through the filter) now contains dissolved salt and any soluble impurities.
3. Concentration (Evaporation): The clear solution is gently heated to evaporate a portion of the water. This process increases the concentration of the salt in the solution, bringing it closer to its saturation point, or even supersaturation, as the water evaporates. Careful heating ensures that the salt does not "salt out" prematurely in an uncontrolled manner.
4. Cooling and Crystallization: The concentrated solution is then allowed to cool slowly and undisturbed. As the solution cools, the solubility of common salt (sodium chloride) decreases, causing the pure salt molecules to come out of the solution and arrange themselves into well-formed, pure crystals. Many soluble impurities, which are present in smaller quantities or have different solubility characteristics, tend to remain in the solution (the mother liquor).
5. Separation and Drying: The newly formed pure salt crystals are then separated from the remaining mother liquor, usually by filtration or decantation. The crystals can be rinsed with a small amount of pure, cold water to remove any surface impurities and then dried.

By following these steps, common salt, which is initially a highly impure product (as highlighted by its origin from seawater), can be effectively purified to obtain high-grade table salt.