Pure water can be made to boil at a temperature lower than 100°C by

Step 1: Analyze the question The question asks how pure water can boil below its normal boiling point of 100°C (which is at standard atmospheric pressure, 760 mmHg).

Step 2: The concept A liquid boils when its saturated vapour pressure equals the external (atmospheric) pressure above it. So:

• Lower external pressure means the vapour pressure reaches it at a lower temperature, so the boiling point falls.
• Higher external pressure means the liquid must be heated more, so the boiling point rises.

Step 3: Check each option

• A. Reducing its quantity: The boiling point is an intensive property. It does not depend on how much water you have. Incorrect.
• B. Decreasing the external pressure: This lowers the boiling point. Correct. At high altitudes, such as on a mountain, water boils below 100°C because the air pressure is lower.
• C. Distilling it: Distillation is a purification method. Pure water already boils at 100°C, and distilling it does not change that. Incorrect.
• D. Increasing the external pressure: This raises the boiling point above 100°C, which is how a pressure cooker works. Incorrect.

Step 4: Answer The correct option is B. decreasing the external pressure.

Tip: Remember "Low pressure, low boiling point; high pressure, high boiling point." Also, adding impurities (like salt) raises the boiling point, while reducing pressure lowers it. Pressure cooker = high pressure = faster cooking at a higher temperature.