When salt is dissolved in water, the freezing point of the water

Analyze the question: The question asks what happens to the freezing point of water when salt is dissolved in it.

Explain the concept: This is called freezing point depression, a colligative property. Adding a solute (such as salt) to a solvent (water) lowers the temperature at which the solvent freezes. The dissolved particles interfere with water molecules as they arrange themselves into the ordered structure of ice. The water must therefore be cooled to a lower temperature before it can freeze.

Step-by-step solution:

1. Pure water freezes at 0°C.
2. Dissolve salt (NaCl) in the water. The Na⁺ and Cl⁻ ions spread among the water molecules.
3. These ions disrupt the formation of the ice crystal lattice, so freezing is harder.
4. The water now has to be cooled below 0°C before it freezes. For example, sea water freezes at about -2°C.
5. So the freezing point decreases.

Checking the options:

• A. Increases: wrong, because the freezing point falls.
• B. Decreases: correct.
• C. Remains the same: wrong, because impurities change it.
• D. Increases and later decreases: wrong, because there is no such pattern.

Real-life application: In cold countries, salt is spread on icy roads to melt the ice. Ice cream makers also mix salt with ice to reach temperatures below 0°C.

Tip: Remember "Salt Lowers": a solute lowers the freezing point and raises the boiling point. This is why salty water boils at a slightly higher temperature, too.