Internal Energy and Changes of State

Every substance is made of particles that are constantly moving and that pull on or push against one another. The energy tied up in that motion and in those forces is the substance's internal energy.

Internal energy is the total kinetic energy and potential energy of all the particles that make up a system.

The internal energy has two parts:

• Kinetic energy — the energy of the particles' movement. Faster-moving particles have more kinetic energy.
• Potential energy — the energy stored because of the positions of the particles and the forces between them. Pulling particles further apart against their attractions stores potential energy.

A "system" simply means the object or group of particles being considered, such as a beaker of water or a block of ice. Internal energy is a store of energy inside that system, separate from any energy the whole object has because it is moving or lifted up.

Transferring energy to a system by heating always increases its internal energy, because it increases the energy of the particles. What happens to that extra energy depends on the situation.

Heating a system transfers energy to its particles. This raises the temperature of the system, or it produces a change of state.

If the extra energy makes the particles move faster, their average kinetic energy rises and the temperature increases. Temperature is a measure of the average kinetic energy of the particles.

If instead the extra energy is used to break the bonds and forces holding particles in place, the substance changes state. During this the potential energy of the particles rises but their kinetic energy, and so the temperature, stays constant.

A single system can only do one of these at a time. While ice is melting at 0 °C, all the energy going in is breaking the fixed arrangement, so the temperature stays at 0 °C until every particle has melted.

There are six named changes of state. Learning both the name and the direction between states is worth easy marks.

Change	From	To
Melting	Solid	Liquid
Freezing	Liquid	Solid
Boiling / evaporating	Liquid	Gas
Condensing	Gas	Liquid
Subliming	Solid	Gas

Melting and freezing are opposites, as are boiling and condensing. Sublimation is when a solid turns straight into a gas without becoming a liquid first, as solid carbon dioxide (dry ice) does at room temperature.

During any of these changes the particles gain or lose energy and rearrange, but they are never created or destroyed. That fact leads directly to the next slide.

When a substance changes state, the number of particles does not change; they simply move closer together or further apart. Because no particles are gained or lost, the total mass stays the same.

When a substance changes state, its mass is conserved. The mass before the change equals the mass after it.

Worked example — 50 g of ice is left in a sealed, insulated container and completely melts to water. What is the mass of the water?

No particles can leave the sealed container, so the mass is unchanged: the water has a mass of 50 g.

This is why a puddle that evaporates seems to "disappear": the water has not vanished, its particles have spread out into the air as water vapour. If you could capture and weigh that vapour, its mass would equal the mass of the puddle that dried up.

A change of state is a physical change, not a chemical one. This distinction is a common exam point.

A physical change does not make a new substance. The material keeps its chemical identity, and reversing the change recovers the original substance with its original properties.

When water freezes into ice and then melts again, you get back exactly the same water. The molecules are still water molecules throughout; only their arrangement and spacing changed. Contrast this with a chemical change, such as burning wood, which produces new substances (ash, gases) that cannot be turned back into wood.

Feature	Change of state (physical)	Chemical change
New substance made?	No	Yes
Easily reversible?	Yes	Usually not
Particles changed?	No, only rearranged	Yes, rebonded

Because it is physical and reversible, a change of state changes the internal energy of the system but not the substance itself.

1. Saying temperature always rises when you heat something

While a substance is changing state, its temperature stays constant even though energy is being supplied. The energy goes into potential energy, breaking the arrangement of particles, not into kinetic energy.

2. Forgetting potential energy in the definition of internal energy

Internal energy is the total of both kinetic and potential energy of the particles. An answer that mentions only kinetic energy is incomplete and loses marks.

3. Thinking evaporating water is destroyed

Mass is conserved. When a liquid evaporates, its particles spread out into the air as a gas; the mass is not lost, it has just moved.

4. Calling a change of state a chemical change

Melting, boiling and freezing are physical changes. No new substance forms and the original material returns when the change is reversed.

5. Confusing temperature with heat or internal energy

Temperature measures the average kinetic energy of the particles. Internal energy is the total energy of all the particles. A large tank of warm water can hold more internal energy than a small cup of boiling water, despite its lower temperature.