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Name: Gabe
Status: student
Location: NC
Country: USA

Why does heat make particles expand? I know that it increases the particles movement and all that, but why?

Hi Gabe,

Looking at your question, there may be some misconceptions. You asked why heat makes particles expand. If, by "particles," you mean atoms or molecules, the answer is that heat does NOT make atoms or molecules expand. Heat simply causes atoms and molecules to vibrate more (this is the increased movement you refer to); it does not cause the actual atoms or molecules to expand.

Molecular movement or vibration is a fundamental result that occurs when energy is absorbed by an atom. The energy that is absorbed cannot just disappear; it causes the atom to increase its movement or vibration, and we perceive this as increased heat.

Most substances do expand when heated, but not all do. This expansion is caused (in simple terms) by the atoms or molecules needing more "room" when they vibrate more at higher temperature. Not all substances expand when heated, though. For example, if you heat water at 0°C (32°F) it actually contracts until it reaches 4°C (39°F). Heating further does cause expansion. Similarly, some metal alloys contract when heated over a wide temperature range as a result of complex interactions in the metal's crystals.

Bob Wilson


Heat is a transfer of energy. This energy when absorbed by objects tend to be expressed or manifested as an increase in temperature. Temperature is a measure of the average motion of particles within an object. So heat will often cause particles to move faster.

In the case of solids, the particles in the solid will vibrate in place (they do not move away from their original center, just vibrate back and forth around the same center). So, imagine that a particle of a solid now vibrates faster and takes up more space, the other particles do the same. As such, while the number of particles remain the same, the size of the particle also remains the same, the space taken up -due to the stronger vibration makes each particle take up more space (without actually getting bigger). As a result the object expands in volume.

In gases, heat allows the gases particles to move faster. They will hit the walls of the container with more force. If the walls are allowed to move, then the walls will actually move to a bigger volume because of the much stronger striking of the gas particles against the walls. Again the container becomes bigger.

Liquids do the same.

Greg (Roberto Gregorius)


I am sure, by now, that some of the other scientists at this site have given you good answers, and have distinguished between particles expanding (a misconception) and the bulk material expanding.

In materials science, you will learn that much of matter is crystalline in nature. As materials increase in temperature, the additional vibrational energy of the particle can be great enough that it moves over one position in the lattice structure. This creates a vacancy. If enough vacancies are created on the atomic or molecular scale, a change in the geometry of the bulk material can be measured.

Great question!
---Nathan A. Unterman

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