Boiling Point and Air Pressure

Introduction

Climbers making tea near the summit of Mount Everest find that their water boils at only about 70 °C. The tea tastes weak and eggs take far longer to cook. Nothing is wrong with the water or the stove — the air above them is thinner. The boiling point of a liquid depends on the pressure pushing down on it, and this page explains why using the particle model.

Core explanation

The atmosphere pushes on liquids. Air particles constantly collide with the surface of any liquid, exerting a pressure. At sea level this atmospheric pressure is about 101 kPa. As you climb higher, there is less air above you, so the pressure falls. At the top of Mount Everest it is only about one-third of the sea-level value.

Bubbles must push back. For a liquid to boil, bubbles of vapour must form inside it. A bubble can only survive if the vapour inside it pushes outwards at least as hard as the liquid and air press inwards. The vapour's push comes from its particles colliding with the bubble wall, and it gets stronger as the temperature rises.

Lower pressure, lower boiling point. When the surrounding pressure is lower, the vapour does not need to push as hard, so bubbles can form at a lower temperature. The liquid boils at a lower temperature.

Higher pressure, higher boiling point. When the surrounding pressure is higher, particles need more energy before their bubbles can survive, so the boiling point rises.

Approximate boiling points of water:

Place or situation Approximate pressure Boiling point of water --- --- --- Sea level 101 kPa 100 °C A city about 1600 m high 83 kPa about 95 °C Summit of Mount Everest 34 kPa about 71 °C Inside a pressure cooker about 200 kPa about 120 °C

Why the boiling point is quoted with a pressure. Because boiling point changes with pressure, data books state values at standard pressure. A sharp boiling point is still a characteristic property of a pure substance — as long as the pressure is specified.

Step-by-step reasoning

To predict how boiling point changes:

1. Decide whether the surrounding pressure has increased or decreased. 2. Lower pressure means vapour bubbles can form more easily. 3. So less energy, and a lower temperature, is needed: the boiling point falls. 4. Higher pressure has the opposite effect: the boiling point rises.

Visual explanation

Picture a bubble inside water with arrows around it. Outward arrows show vapour particles hitting the bubble wall; inward arrows show the pressure of the air and liquid. At sea level the inward arrows are long. On a mountain they are shorter, so smaller outward arrows — from slower particles at a lower temperature — are enough to keep the bubble open.

Real-world analogy

Imagine trying to push open a door while people lean on the other side. If three people are leaning on it, you need to be very strong. If only one person is leaning, a weaker push will do. Vapour bubbles are like you pushing; the surrounding pressure is the people leaning on the door.

Real-world example

Pressure cookers seal in the steam, raising the pressure inside to roughly twice atmospheric pressure. Water then boils at about 120 °C, so food cooks much faster. They are widely used at high altitude, where ordinary boiling water is not hot enough to cook beans or rice properly. Their safety valves release excess pressure, which is why they must never be forced open while hot.

Why?

Why does food cook slowly on a mountain even though the water is boiling? Cooking depends on the temperature of the water, not on whether it is bubbling. Water boiling at 71 °C is much cooler than water boiling at 100 °C, so heat is transferred to the food more slowly and the chemical changes of cooking take longer.

Common misconception

"Water always boils at 100 °C." That is only true at standard sea-level pressure. Water can boil at room temperature if the pressure is lowered enough, and remain liquid above 100 °C if the pressure is raised.

Worked example

Question: A hiker at 3000 m finds her water boils at about 90 °C. A friend in a pressure cooker kitchen says his water boils at about 120 °C. Explain both observations.

Reasoning: At 3000 m the air pressure is lower than at sea level, so vapour bubbles can form at a lower temperature. Inside the pressure cooker the trapped steam increases the pressure, so the particles need more energy before bubbles can form.

Answer: Lower pressure lowers the boiling point (90 °C); higher pressure raises it (120 °C).

Quick check

1. Does water boil at a higher or lower temperature at the top of a mountain? Answer: Lower, because the air pressure is lower.

Exam focus

Examiners expect the link "lower pressure → lower boiling point; higher pressure → higher boiling point", with a reason in terms of the pressure acting on the liquid. Quote pressure cookers and high-altitude cooking as applications, and remember that boiling points in data tables are given at standard pressure.

Advanced insight

A liquid boils when its vapour pressure equals the surrounding pressure. Vapour pressure rises steeply with temperature, and water's vapour pressure reaches 101 kPa at exactly 100 °C. Chemists exploit this in vacuum distillation: lowering the pressure lets delicate substances that would decompose at their normal boiling point be boiled and purified at much lower temperatures.

Summary

The boiling point of a liquid depends on the pressure above it. Lower pressure, as found at high altitude, lets vapour bubbles form more easily, so the liquid boils at a lower temperature; water boils at about 71 °C on Everest. Higher pressure, as in a pressure cooker, raises the boiling point. Boiling points in tables are therefore quoted at standard pressure.

Practice questions

1. Explain why the air pressure is lower at the top of a mountain than at sea level. Answer: There is less air above, so fewer air particles push down on each square metre. 2. Why do pressure cookers cook food faster? Answer: The pressure inside is higher, so water boils at a higher temperature (about 120 °C) and the food heats up more quickly. 3. A liquid boils at 60 °C in a laboratory. The pressure above it is then lowered. Predict what happens to its boiling point. Answer: The boiling point falls below 60 °C. 4. A student says, "Boiling water on Everest is at 100 °C." Correct the statement. Answer: Water on Everest boils at about 71 °C because the air pressure is much lower than at sea level.