Matter exists in four fundamental states: solid, liquid, gas, and plasma. Boiling is a physical process in which a liquid changes into a gas. This transition from one state to another is called a phase change or phase transition. Boiling occurs when a liquid reaches a specific temperature, known as its boiling point. For example, at sea level, water boils at 100°C (212°F). ^[1-4]

What Happens During Boiling

Boiling involves the movement of particles within a liquid. According to the kinetic theory of matter, all particles (such as atoms or molecules) are constantly in motion. In a liquid, these particles are held together by attractive intermolecular forces. As the liquid is heated, the particles absorb energy and move more vigorously. When the temperature reaches the boiling point, the particles have enough energy to overcome the intermolecular forces and escape into the air as gas. This process is closely related to vapor pressure. ^[1-5]

Boiling Point and Vapor Pressure

Vapor pressure refers to the pressure exerted by gas molecules as they escape from the surface of a liquid and collide with the walls of the container. As the temperature of the liquid rises, more particles gain enough energy to vaporize, increasing the vapor pressure. Boiling occurs when the vapor pressure of the liquid equals the surrounding atmospheric pressure — the pressure exerted by the air. ^[1-5]

Boiling Point Values ^[1-4]

The table below shows the boiling point values of a few common substances in both Celsius (°C) and Fahrenheit (°F) scales.

Note: Gasoline has a wide boiling range because it is a mixture of different hydrocarbons.

Factors That Affect Boiling ^[1-4]

1. Atmospheric Pressure

Atmospheric pressure greatly influences the boiling point of a liquid. At sea level, where atmospheric pressure is higher, water boils at 100°C (212°F). However, at higher altitudes, such as in mountainous regions, atmospheric pressure is lower. Under these conditions, a liquid’s vapor pressure can match the reduced external pressure at a lower temperature, causing it to boil more easily. As a result, cooking often takes longer at high altitudes because the boiling temperature is lower.

2. Intermolecular Forces

The strength of intermolecular forces within a liquid affects its boiling point. Liquids with stronger intermolecular attractions require more energy to separate their particles and, therefore, have higher boiling points. For example, water has a relatively high boiling point of 100°C, while ethanol, a type of alcohol, boils at about 78°C. Ethanol molecules are held together by weaker forces compared to water molecules, allowing ethanol to vaporize at a lower temperature.

3. Impurities

The presence of impurities, such as dissolved salts or sugars, can raise the boiling point of a liquid — a phenomenon known as boiling point elevation. For example, when salt is added to water, the solution must be heated to a higher temperature for it to boil.