Q1.
Observe Fig. 5.9, it shows how salt crystals are obtained from seawater. Can you describe the process in your own words?
Answer
Seawater → saturated solution → salt crystals. It is crystallization driven by the evaporation of the solvent, carried out on a huge scale in shallow salt pans along the coast.
- Seawater is let into shallow pans. Seawater is a dilute solution — roughly 3.5 g of dissolved salts in every 100 g of water — so it is nowhere near saturated to begin with. The pans are made wide and shallow so that a very large surface is exposed to the sun and wind.
- The water evaporates and the brine becomes a saturated solution. Water molecules leave from the surface, but the salt cannot; so the mass of solvent falls while the mass of solute stays the same and the concentration climbs steadily. Once the water can hold no more salt at that temperature, the brine is saturated.
- Further evaporation deposits salt crystals. Every further gram of water that escapes leaves behind salt it can no longer hold, and the excess separates out on the floor of the pan as cubic crystals of common salt. These are raked up, washed and dried.
Why evaporation rather than cooling: Cooling only helps for a solute whose solubility changes sharply with temperature. Common salt dissolves to about 36 g per 100 g of water at 10 °C and only about 37 g at 80 °C (Table 5.4) — practically a flat curve. Cooling seawater would therefore give almost no crystals. Removing the solvent is the only route that works, and the sun does it for free.
Did you know? Along India’s coasts this was an old craft: karkatch salt was made by evaporating seawater in pans, and panga salt by boiling concentrated sea brine — the two methods yielding crystals of different sizes.