What Happens to Particles When They Are Heated: A Guide to Thermal Motion

Heating transfers energy to a substance. The energy can increase particle motion: gas particles move faster, while particles in a solid vibrate more strongly around their usual positions. In a liquid, particles move around one another with greater kinetic energy.

The effect depends on the substance and its conditions. Temperature usually rises as particles gain kinetic energy. During melting or boiling, however, added energy can change how particles are arranged instead, so the temperature can remain steady while the substance changes state.

For a gas, the average particle speed increases with the square root of its absolute temperature, measured in kelvins NASA Glenn Research Center. This relationship describes a gas at comparable conditions and refers to average speed, not the speed of every individual particle.

Particles move; they do not get bigger

Heating does not make atoms or molecules themselves expand. It changes their motion and can change the average spacing between them. In materials that are free to expand, increased average spacing causes the material to expand.

In a gas, particles travel freely between collisions. When a gas is heated, its particles have greater average kinetic energy and move faster. If the gas can expand, it may take up more space. If it is held in a fixed volume, its pressure can rise as particles collide with the container walls more forcefully and often.

In a solid, particles do not move freely from place to place. They vibrate around positions in the solid’s structure. Heating increases the strength of those vibrations, and the average spacing can grow slightly, causing the solid to expand. Liquids can also expand when heated, with the amount depending on the substance and its conditions.

Some materials expand by different amounts in different directions, and some behave unusually over particular temperature ranges. Expansion comes from changes in particle spacing, not from particles growing larger.

Temperature describes an average

Particles in a sample do not all move at the same speed. At any moment, some move more slowly and others more quickly. Collisions transfer energy between them, so the distribution of speeds keeps changing.

Temperature describes a bulk property of a sample. For an ideal gas, it is directly related to the average translational kinetic energy per particle. It does not tell you the speed of one particular molecule. A higher temperature means a higher average kinetic energy, while individual particles still have a range of speeds OpenStax Physics.

For a gas of the same kind, the average speed rises in proportion to the square root of its Kelvin temperature. Multiplying the absolute temperature by four multiplies the average speed by two. The temperature must be expressed in kelvins for this comparison. Celsius and Fahrenheit scales have zero points that do not represent the absence of thermal motion, so their values cannot be used directly in this ratio.

One example: heating a gas

Suppose a gas has an average particle speed of 500 m/s at a given temperature. If its absolute temperature is raised to four times its starting value, the average speed becomes about 1,000 m/s, assuming the gas remains in a comparable state. The particles do not all reach 1,000 m/s; that figure describes the average.

At twice the Kelvin temperature, the average speed is multiplied by the square root of two, or about 1.4. Speed rises more slowly than temperature because kinetic energy depends on the square of speed.

Energy can change a substance’s state

Heating often raises temperature, but not at every stage. When a substance melts or boils, added energy can change the arrangement of its particles. During the phase change, the temperature may stay constant until the transition is complete.

In a solid, particles occupy positions in a structured arrangement and vibrate around them. As energy is added, the particles vibrate more strongly. At melting, energy loosens the structure enough for particles to move past one another as a liquid. During boiling, energy helps particles escape from the liquid into the gas phase.

During a phase change, energy changes the particles’ interactions and arrangement; it does not simply make every particle move faster. Once the phase change is complete, further heating can raise the temperature of the new state.

Melting and boiling temperatures depend on the substance and the surrounding pressure. A familiar temperature for a phase change applies only under the conditions for which it is specified.

Heat and temperature are different

Heat is energy transferred between objects or regions because of a temperature difference. Temperature describes a condition of a material, linked to the average energy of its particles. Adding heat can raise temperature, cause expansion, or drive a phase change. The effect depends on the substance and its circumstances.

The amount of temperature change also depends on how much material receives the energy and on the material’s properties. The same energy transfer can produce a larger temperature rise in a small sample than in a large one. Energy may also spread through a substance at different rates depending on its structure.

This helps explain why surfaces at the same temperature can feel different. The sensation depends partly on how quickly energy moves between the surface and your skin. A metal surface and a wooden surface can have the same temperature but feel different because they transfer heat differently.

Key Takeaways

• Heating can increase particle motion: gas particles move faster, while particles in solids vibrate more strongly.
• Particles do not get bigger. Materials that are free to expand can expand as their average particle spacing increases.
• Temperature describes a sample, not the speed of one particle. Particles in the same sample have a range of speeds.
• For a gas of the same kind, average speed scales with the square root of absolute temperature NASA Glenn Research Center.
• During melting or boiling, added energy can change particle arrangement while temperature remains steady through the transition OpenStax Physics.