Question

Why does the boiling point of water decrease at higher altitudes?

A) Atmospheric pressure is low at higher altitudes.

B) Temperature is low at higher altitudes.

C) Atmospheric pressure is high at higher altitudes

D) None of the above.

Answer 1

Hint: If you have been trekking, you must have noticed as we move up in altitude, it gets tougher to breathe properly and preparation of food takes less time. 

Complete step by step solution:

Boiling point is defined as the point at which the vapour pressure becomes equal to the atmospheric pressure.

In the case of liquids, its particles always have enough energy to escape from the liquid phase to the gaseous phase.

The vapour pressure is the pressure which is exerted by the gas when a particular number of particles leaving the liquid equals the same number of particles entering the liquid.

And as the temperature increases, more particles get enough energy to escape to the gaseous phase. As a result, this increases the vapour pressure. When the vapour pressure becomes equal to the atmospheric pressure, the liquid starts boiling.

Now, at the higher altitude level the atmospheric pressure is lower, so the vapour pressure of the liquid also needs to be lower to reach boiling point.

Therefore, less heat is required to make the vapour pressure equal to the atmospheric pressure. 

Hence, the boiling point of water is lower at a higher altitude; the correct option is (A).

Note: Let’s see how the boiling point changes as we move up in altitude.

Altitude in Feet	Boiling point in Degrees Celsius
500	99.4
1000	99
1500	98.3
2000	95
5000	94.4
10,000	89.6