A gas is a state of matter without a fixed shape or volume, composed of molecules moving freely and randomly within their container. Heat is the transfer of thermal energy due to a temperature difference. When heat is added to a gas, this energy flows into the system, increasing the gas’s total internal energy. This transfer causes significant changes in the gas’s microscopic and macroscopic properties.
The Immediate Molecular Effect
Adding thermal energy to a gas directly increases the average kinetic energy of its constituent molecules. This relationship is a fundamental concept of the Kinetic Molecular Theory, which describes gas behavior. Because kinetic energy is directly related to mass and the square of velocity, an increase in energy must result in an increase in the average speed of the gas particles.
The molecules, now moving faster, collide with one another and with the walls of their container more frequently and with greater force. The energy is distributed among the molecules, causing a rise in the temperature of the gas. This temperature increase is the macroscopic manifestation of the rise in average kinetic energy at the molecular level.
For simple gases, this added energy is primarily converted into translational kinetic energy. Diatomic or polyatomic gases also absorb this energy into rotational and vibrational kinetic energy.
The Result When Volume Stays Constant
When heat is added to a gas contained within a rigid vessel, the volume cannot change. This is known as an isochoric process. The energized molecules, moving faster due to the heat input, strike the container walls more often per unit of time.
Each collision delivers a greater impulse, which is a stronger force applied to the fixed area of the container wall. Since pressure is defined as force per unit area, the combination of more frequent and forceful impacts causes the gas pressure to rise directly in proportion to the absolute temperature. This relationship explains warnings against exposing aerosol cans to high temperatures.
The metal walls of the can provide a constant volume, and heating it can cause a significant pressure buildup. This internal pressure can quickly exceed the structural limit of the container, leading to rupture or explosion. A similar effect occurs in a pressure cooker, where rising steam temperature increases pressure to shorten cooking times. The cooker’s rigid walls ensure the pressure change, while a safety valve prevents dangerous over-pressurization.
The Result When Pressure Stays Constant
If the gas is in a container with flexible walls, such as a balloon or a cylinder with a movable piston, the outcome of adding heat changes completely. In this scenario, known as an isobaric process, the internal gas pressure must remain balanced with the external pressure, typically the surrounding atmosphere.
The increased kinetic energy from heating still causes the molecules to move faster and hit the container walls more forcefully. However, instead of the pressure rising, the flexible walls move outward, increasing the volume of the container. This expansion continues until the frequency of collisions with the now larger wall area returns the internal pressure to the level of the external pressure.
This expansion means that the gas volume increases directly in proportion to its absolute temperature. For example, in a hot air balloon, burners heat the air inside the envelope, causing it to expand and become less dense than the surrounding air, which creates the buoyant force needed for lift. Similarly, a balloon brought from a cold environment into a warm room will visibly inflate as the internal air molecules gain energy and push the elastic material outward.