Boiling
8 Minutes
Boiling of water for students explains how atmospheric pressure and vapor pressure affect the boiling point. A science lesson in English.
- Three states of matter and what happens during boiling
- An experiment boiling 30-degree Celsius water in a vacuum
- How vapor pressure and atmospheric pressure make water boil
What this lesson teaches
Boiling is the process in which a liquid turns into a gas when its internal vapor pressure becomes equal to the external atmospheric pressure pushing down on it. While water commonly boils at 100 degrees Celsius under standard air pressure, boiling depends entirely on the balance between these two pressures rather than heat alone.
Forces acting on water
- Intermolecular attraction, or Van der Waals force, holds water molecules lightly together and prevents them from easily escaping.
- Atmospheric pressure is the downward force created when Earth's gravity pulls air molecules toward the ground, measuring about one bar at sea level.
- Water molecules absorb heat from their surroundings to gain kinetic energy, which is the energy of motion that lets them move around.
- Vapor pressure is the outward pressure produced by moving water molecules trying to break free and turn into gas.
How water boils
Liquid water boils only when its vapor pressure matches the atmospheric pressure around it. At a normal room temperature of 25 to 30 degrees Celsius, water has a vapor pressure of only 0.03 bar, which is much lower than the one bar of air pressure pressing on its surface. To cause boiling, either the vapor pressure inside the liquid must be increased, or the external atmospheric pressure must be decreased.
Two methods to reach the boiling point
- Adding heat to water until it reaches 100 degrees Celsius gives the molecules enough kinetic energy to raise their vapor pressure to one bar.
- Placing water inside a sealed chamber and pumping out the air lowers the surrounding atmospheric pressure until it drops to the water's natural vapor pressure.
- In both situations, bubbles form and the liquid vigorously transforms into vapor the moment the two pressures become equal.
Boiling without heating
When water at 30 degrees Celsius is placed in a vacuum chamber and the air is sucked out, the atmospheric pressure drops to 0.03 bar. The water begins to boil at room temperature without any flame or heating element applied. Because no extra heat was supplied to the system, the boiling water remains at 30 degrees Celsius and does not become hot.