Question

At higher altitude the boiling point of liquid :

• A. Increases
• B. Decreases
• C. Remains the same
• D. Increases then decreases

Hint:

1. {Higher altitude} \(\rightarrow\) {Lower atmospheric pressure}. (Less air pressing down). 2. A liquid boils when its vapor pressure equals the external atmospheric pressure. 3. If external pressure is lower, the liquid doesn't need to get as hot for its vapor pressure to match it. 4. Therefore, {lower atmospheric pressure} \(\rightarrow\) {lower boiling point}. Example: Water boils at \(100^\circ\text{C}\) at sea level, but at less than \(100^\circ\text{C}\) on a high mountain.

Answer 1

The Correct Option is B

Concept: The boiling point of a liquid is the temperature at which its vapor pressure equals the external (atmospheric) pressure surrounding the liquid. The atmospheric pressure changes with altitude. Step 1: Relationship between Altitude and Atmospheric Pressure As altitude increases (i.e., you go higher up, like on a mountain), the atmospheric pressure decreases. This is because there is less air above pressing down. Step 2: Relationship between External Pressure and Boiling Point A liquid boils when its vapor pressure (the pressure exerted by its vapor) becomes equal to the external pressure.
If the external pressure is lower, the liquid needs to achieve a lower vapor pressure to boil.
A lower vapor pressure is achieved at a lower temperature (since vapor pressure increases with temperature). Therefore, if the external atmospheric pressure decreases, the boiling point of the liquid also decreases. Step 3: Applying this to higher altitudes At higher altitudes, the atmospheric pressure is lower. Since the boiling point depends on the external pressure, a lower atmospheric pressure at higher altitudes results in a lower boiling point for liquids. This is why water boils at a temperature less than \(100^\circ\text{C}\) on high mountains, and food takes longer to cook (or a pressure cooker is needed). Step 4: Analyzing the options
(1) Increases: Incorrect. Boiling point decreases.
(2) Decreases: Correct. Lower atmospheric pressure at higher altitudes leads to a lower boiling point.
(3) Remains the same: Incorrect. Boiling point is pressure-dependent.
(4) Increases then decreases: Incorrect.