What will happen to the boiling point of pure water when it is heated in a place 30m below sea level?

1. Analyze the question The question asks how the boiling point of pure water changes at a place 30 m below sea level.

2. Explain the concept Water boils when its saturated vapour pressure equals the external (atmospheric) pressure above it. So the boiling point depends on the surrounding pressure:

• Higher pressure → higher boiling point
• Lower pressure → lower boiling point (e.g. on a mountain top)

The standard boiling point of 100 °C applies at normal atmospheric pressure at sea level (760 mmHg).

3. Step-by-step solution

• Atmospheric pressure increases as you go down, because there is more air above you (P = ρgh applies to the column of air).
• At 30 m below sea level, the air column above is slightly taller than at sea level, so the pressure is slightly greater than 760 mmHg.
• Greater external pressure means the water's vapour pressure must reach a higher value before boiling, so more heat (a higher temperature) is needed.
• The boiling point is therefore above 100 °C. The effect is small for only 30 m, but it is in that direction.

4. Answer The boiling point will be more than 100 °C, which is option A.

• B is wrong because lower boiling points happen at high altitude (lower pressure).
• C is wrong because 100 °C holds only at standard sea-level pressure.
• D is wrong because pure water at constant pressure boils at a steady temperature.

5. Exam tip Remember: "High pressure, high boiling point." A pressure cooker works on this idea, and so does boiling food slowly on a mountain (low pressure, lower boiling point). Below sea level means more pressure, so the boiling point goes up.