Supplying thermal energy to a substance doesn’t always make it hotter. To see why, you need to split its internal energy into the two things it’s actually made of.
📘 What you need to know
Internal energy is the sum of the total kinetic energy and total intermolecular potential energy of the particles in a substance
Gaining or losing thermal energy changes a substance’s internal energy — but not always its temperature
An increase in average kinetic energy means faster-moving particles, and corresponds to a rise in temperature
An increase in potential energy means particles moving further apart — this does not change the temperature
This is exactly what happens during a change of state, like melting or boiling
What Is Internal Energy?
Internal energy is defined as the sum of the total kinetic energy and the total intermolecular potential energy of all the particles within a substance. When thermal energy is transferred to a substance, one of two things can happen — sometimes both at once:
the average kinetic energy of the particles increases, so they vibrate or move around faster
the potential energy of the particles increases, so they move further apart from one another
Internal energy has two components — only a rise in kinetic energy shows up as a rise in temperature
Internal Energy and Temperature
Temperature is a measure of the average kinetic energy of a substance’s particles — nothing else. That means only a change in average kinetic energy produces a change in temperature. A change in potential energy, on its own, leaves the temperature exactly where it was.
Due to thermal expansion, a rising temperature usually does increase potential energy slightly too, since particles spread out a little as they heat up. But when potential energy changes on its own — with kinetic energy held constant — temperature simply doesn’t move. This is exactly what happens during every change of state, such as melting or boiling.
Quick recap: Internal energy = total KE + total PE of the particles. A change in average KE changes temperature. A change in PE alone does not.
WE 1
A sealed container of ice at 0 °C is heated at a constant rate. For the first several minutes, its temperature stays fixed at 0 °C even though energy is being continuously supplied. Explain this in terms of internal energy.
Reasoning
The thermal energy supplied is being used to increase the potential energy of the particles, partially overcoming the intermolecular forces holding the solid lattice together, as the ice melts.
Why temperature doesn’t change
Because the average kinetic energy of the particles isn’t increasing during this process, the temperature stays constant even though internal energy is rising.
Energy goes into breaking bonds, not into speeding particles up
WE 2
Two identical blocks of the same metal, both starting as solids, are supplied with equal amounts of thermal energy. Block A stays solid throughout; Block B fully melts during the process. Which block shows the greater rise in temperature, and why?
Reasoning
All the energy supplied to Block A goes into increasing the kinetic energy of its particles, since no phase change occurs.
Contrast with Block B
Some of the energy supplied to Block B is instead used to increase potential energy as it melts, leaving less energy available to raise kinetic energy.
Block A shows the greater temperature rise
💡 Top tips
A change in internal energy does not automatically mean a change in temperature — always ask whether kinetic or potential energy is changing
Keep internal energy and temperature conceptually separate: one is a total, the other reflects only one part of that total
State changes are the clearest example of internal energy rising while temperature holds steady
Thermal expansion is a small potential-energy effect that happens alongside — not instead of — kinetic energy changes
⚠ Common mistakes
Assuming that supplying thermal energy to a substance must always raise its temperature
Treating internal energy and temperature as if they were the same quantity
Forgetting that intermolecular potential energy — not just kinetic energy — contributes to internal energy
Concluding that no energy is being transferred just because temperature isn’t changing during a phase change
Up next: Thermal Equilibrium — where we look at what happens when two substances at different temperatures come into contact.
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