Which of the following statements is correct about the effect of dissolved salt on the freezing point of water?

Step 1: Analyze the question The question asks what happens to the freezing point of water when salt is dissolved in it.

Step 2: The concept Dissolving a solute (like salt) in a solvent (like water) lowers the freezing point of the solvent. This is called freezing point depression, and it is a colligative property, meaning it depends on the number of dissolved particles, not on what they are.

Pure water freezes at 0 °C. When salt (NaCl) dissolves, it breaks into Na⁺ and Cl⁻ ions. These ions get in the way of water molecules trying to arrange themselves into the ordered structure of ice. The water therefore has to be cooled below 0 °C before it can freeze.

Step 3: Evaluate the options

• A. The freezing point is lowered: Correct. This is freezing point depression.
• B. The freezing point is increased: Wrong. Impurities do not raise the freezing point. (They raise the boiling point instead.)
• C. No change: Wrong. Dissolved salt does change it.
• D. Depends on the surrounding temperature: Wrong. The freezing point depends on the amount of solute dissolved, not on the surrounding temperature.

Step 4: Real-life Nigerian context This is why people add salt to ice when making "ice block" or freezing drinks. The salty ice-water mixture gets colder than 0 °C, which helps chill things faster. In cold countries, salt is spread on roads in winter to stop ice from forming.

**Answer: A. The freezing point is lowered.

Exam tip: Remember "Salt lowers the freezing point and raises the boiling point." Impurities make water harder to freeze and harder to boil.