Water in an open container boils at a lower temperature when heated at the top of a mountain than at sea-level because at the top of a mountain

Step 1: Analyze the question We are asked why water in an open container boils at a lower temperature on top of a mountain than at sea level.

Step 2: The concept A liquid boils when its saturation vapour pressure equals the external (atmospheric) pressure above it. At sea level, atmospheric pressure is about 760 mmHg (101,325 Pa), and water boils at 100°C because its vapour pressure reaches that value at 100°C.

At the top of a mountain, there is less air above you, so atmospheric pressure is lower. The water therefore needs less vapour pressure to start boiling, which it reaches at a lower temperature (for example, about 90°C at roughly 3,000 m).

Step 3: Check each option

• A. Pressure is increased: Wrong. Pressure decreases with altitude.
• B. Pressure is reduced: Correct. Lower atmospheric pressure means a lower boiling point.
• C. Sun's rays add more heat: Wrong. This would not lower the boiling point, and it is not the reason.
• D. Saturation vapour pressure is higher than at sea level: Wrong. The saturation vapour pressure of water depends on temperature, not on altitude. What changes is the external pressure.
• E. The temperature is lower: Wrong. The air temperature may be lower, but that does not explain the lower boiling point. Colder surroundings would actually make boiling harder, not change the boiling point itself.

Step 4: Answer

Tip: Remember "Boiling point follows pressure": high pressure means a higher boiling point (pressure cooker), and low pressure means a lower boiling point (mountain top).