Two Buckets at the Same Temperature Hold Very Different Heat
Learn why heat and temperature are not the same quantity, what heat does to a body, how a rise in temperature changes molecular motion, and how to convert between the Celsius and Kelvin scales.
If two buckets of water are both at 30 degrees, do they hold the same heat?
No. A bucket holding of water at contains twice as much heat as one holding at the same .
The temperature tells you how hot each is. The heat tells you how much thermal energy is in there in total — and that depends on how much water there is as well as how hot it is.
A burning matchstick has a far higher temperature than a bucket of warm water, and far less heat. This page covers the first part of the ICSE Class 8 Physics chapter on heat transfer: the difference between heat and temperature, what heat does, the molecular picture, and the two temperature scales.
The temperature tells you how hot each is. The heat tells you how much thermal energy is in there in total — and that depends on how much water there is as well as how hot it is.
A burning matchstick has a far higher temperature than a bucket of warm water, and far less heat. This page covers the first part of the ICSE Class 8 Physics chapter on heat transfer: the difference between heat and temperature, what heat does, the molecular picture, and the two temperature scales.
What exactly is the difference between heat and temperature?
Heat is a form of energy — the total thermal energy of all the molecules of a body. Temperature is the degree of hotness, a measure of the average kinetic energy of those molecules.
Setting them side by side:
- What it measures. Heat: total thermal energy. Temperature: average energy per molecule.
- SI unit. Heat: the joule (). Temperature: the kelvin (), with the degree Celsius () in common use.
- Instrument. Heat: a calorimeter. Temperature: a thermometer.
- Depends on mass? Heat: yes. Temperature: no.
- What it causes. Heat flows from a hotter body to a colder one; temperature decides which way it flows.
The relationship between them. Heat is the cause and a temperature change is usually the effect — supply heat and the temperature normally rises. But temperature is what determines the direction of flow: heat always moves from higher temperature to lower, never the other way on its own.
A test case worth remembering. Drop a hot iron nail into a large tub of cool water and the nail cools while the water barely warms. The nail was at a much higher temperature, but the tub contains far more heat and can absorb the nail's without much change. Temperature decided the direction; the amounts of heat decided how much each changed.
And the boundary case. During a change of state — ice melting, water boiling — heat is being supplied and the temperature does not rise at all. The energy goes into breaking the molecules apart instead of speeding them up. That single fact is the clearest proof that heat and temperature are different quantities.
Setting them side by side:
- What it measures. Heat: total thermal energy. Temperature: average energy per molecule.
- SI unit. Heat: the joule (). Temperature: the kelvin (), with the degree Celsius () in common use.
- Instrument. Heat: a calorimeter. Temperature: a thermometer.
- Depends on mass? Heat: yes. Temperature: no.
- What it causes. Heat flows from a hotter body to a colder one; temperature decides which way it flows.
The relationship between them. Heat is the cause and a temperature change is usually the effect — supply heat and the temperature normally rises. But temperature is what determines the direction of flow: heat always moves from higher temperature to lower, never the other way on its own.
A test case worth remembering. Drop a hot iron nail into a large tub of cool water and the nail cools while the water barely warms. The nail was at a much higher temperature, but the tub contains far more heat and can absorb the nail's without much change. Temperature decided the direction; the amounts of heat decided how much each changed.
And the boundary case. During a change of state — ice melting, water boiling — heat is being supplied and the temperature does not rise at all. The energy goes into breaking the molecules apart instead of speeding them up. That single fact is the clearest proof that heat and temperature are different quantities.
What does heat do to a body?
Supplying heat produces one of three effects, and which one you get depends on the situation.
1. A rise in temperature. The commonest effect. Heat a pan of water on a stove and its temperature climbs. Remove heat and the temperature falls — leave the same water in a fridge and it cools.
2. Thermal expansion. Almost every substance expands on heating and contracts on cooling, because the molecules move further apart. A tight metal lid on a glass jar loosens when held under hot water, because the metal expands more than the glass. Electric wires strung between poles are left slightly slack in summer so they do not snap when they contract in winter.
3. A change of state. Enough heat converts a solid to a liquid (melting) and a liquid to a gas (boiling). Ice becomes water, water becomes steam. Remove heat and the changes reverse — condensation and freezing.
A fourth effect worth knowing. Heat can also cause a chemical change in some substances, as when sugar turns brown on heating. That change is not reversible, unlike the three above.
Which effect appears when. Heat a block of ice at and you see all three in turn: first its temperature rises to , then it melts at a steady , then the water's temperature rises to , then it boils at a steady . The expansion happens quietly throughout.
Notice that the temperature pauses twice during that sequence. Those pauses are where the heat is going into a change of state rather than into a rise in temperature.
1. A rise in temperature. The commonest effect. Heat a pan of water on a stove and its temperature climbs. Remove heat and the temperature falls — leave the same water in a fridge and it cools.
2. Thermal expansion. Almost every substance expands on heating and contracts on cooling, because the molecules move further apart. A tight metal lid on a glass jar loosens when held under hot water, because the metal expands more than the glass. Electric wires strung between poles are left slightly slack in summer so they do not snap when they contract in winter.
3. A change of state. Enough heat converts a solid to a liquid (melting) and a liquid to a gas (boiling). Ice becomes water, water becomes steam. Remove heat and the changes reverse — condensation and freezing.
A fourth effect worth knowing. Heat can also cause a chemical change in some substances, as when sugar turns brown on heating. That change is not reversible, unlike the three above.
Which effect appears when. Heat a block of ice at and you see all three in turn: first its temperature rises to , then it melts at a steady , then the water's temperature rises to , then it boils at a steady . The expansion happens quietly throughout.
Notice that the temperature pauses twice during that sequence. Those pauses are where the heat is going into a change of state rather than into a rise in temperature.
What happens to the molecules when the temperature rises?
They move faster, and their average kinetic energy increases. That increase is the rise in temperature.
How they move depends on the state, as the earlier chapter on matter established.
- In a solid, molecules only vibrate about fixed positions. Heating makes them vibrate more violently and through a larger distance, which pushes their average separation up — and that is thermal expansion.
- In a liquid, molecules slide past one another. Heating makes them move faster and further apart.
- In a gas, molecules fly freely. Heating raises their speed sharply, which is why gases expand far more than solids for the same temperature rise.
Where a change of state fits in. Keep heating a solid and the vibration eventually becomes strong enough to break the molecules out of their fixed positions. At that point the added heat goes into overcoming the intermolecular force rather than into speeding the molecules up — so the temperature holds steady while the solid melts.
The lower limit. Since temperature measures average molecular kinetic energy, there must be a temperature at which molecular motion is at its least possible. Nothing can be colder than that, because there is no energy left to remove. That limiting temperature is absolute zero, and it is the starting point of the Kelvin scale in the next section.
A common misconception. Molecules do not become hot individually. There is no such thing as one hot molecule — heat and temperature are properties of a large collection of molecules, describing their total and average energy. A single molecule simply has a speed.
How they move depends on the state, as the earlier chapter on matter established.
- In a solid, molecules only vibrate about fixed positions. Heating makes them vibrate more violently and through a larger distance, which pushes their average separation up — and that is thermal expansion.
- In a liquid, molecules slide past one another. Heating makes them move faster and further apart.
- In a gas, molecules fly freely. Heating raises their speed sharply, which is why gases expand far more than solids for the same temperature rise.
Where a change of state fits in. Keep heating a solid and the vibration eventually becomes strong enough to break the molecules out of their fixed positions. At that point the added heat goes into overcoming the intermolecular force rather than into speeding the molecules up — so the temperature holds steady while the solid melts.
The lower limit. Since temperature measures average molecular kinetic energy, there must be a temperature at which molecular motion is at its least possible. Nothing can be colder than that, because there is no energy left to remove. That limiting temperature is absolute zero, and it is the starting point of the Kelvin scale in the next section.
A common misconception. Molecules do not become hot individually. There is no such thing as one hot molecule — heat and temperature are properties of a large collection of molecules, describing their total and average energy. A single molecule simply has a speed.
Formula
How do you convert between Celsius and Kelvin?
and in reverse
The Kelvin scale is the SI scale of temperature, and it begins at absolute zero — the lowest temperature possible.
The two fixed points, both defined by a change of state of water at normal atmospheric pressure:
- Melting point of ice:
- Boiling point of water:
Worked examples.
Room temperature of :
A cold-storage temperature of :
Normal body temperature given as :
Absolute zero itself:
A temperature difference is the same number on both scales. Warm water from to and the rise is . In kelvin those readings are and , and
The **same , because the two scales have identically sized divisions and differ only in where they start.
Notice the notation.** Kelvin temperatures are written — no degree sign and no degrees in words. Celsius keeps the degree sign, as .
And no temperature is ever negative in kelvin, because absolute zero is the floor. A negative answer in kelvin means the arithmetic went wrong.
Exam tip
Exam tip: answer heat questions with mass, not just hotness
When a question compares two bodies, decide first whether it is asking about heat or temperature. If mass is mentioned, heat is almost certainly the point.
Give both units correctly: heat in joules, temperature in kelvin or degrees Celsius. Writing heat in loses the mark.
Write kelvin without a degree sign — , never .
For a conversion, add or subtract and show the line: . Do not try it mentally on a negative Celsius value; that is where sign errors happen.
Remember a temperature difference is numerically the same in and in — so a rise of 30 degrees needs no conversion at all.
For effects of heat, list all three — rise in temperature, expansion, change of state — with one example each.
And when asked why the temperature stays constant during melting, say the heat is used to overcome the intermolecular force, not to raise the molecular kinetic energy.
Give both units correctly: heat in joules, temperature in kelvin or degrees Celsius. Writing heat in loses the mark.
Write kelvin without a degree sign — , never .
For a conversion, add or subtract and show the line: . Do not try it mentally on a negative Celsius value; that is where sign errors happen.
Remember a temperature difference is numerically the same in and in — so a rise of 30 degrees needs no conversion at all.
For effects of heat, list all three — rise in temperature, expansion, change of state — with one example each.
And when asked why the temperature stays constant during melting, say the heat is used to overcome the intermolecular force, not to raise the molecular kinetic energy.
Did you know
Why is there a lowest possible temperature but no highest?
Temperature measures how much kinetic energy the molecules have on average. You can always add more energy — so there is no ceiling; a substance can in principle be made hotter and hotter without limit.
Going the other way is different. Every joule you remove slows the molecules a little further, and eventually there is nothing left to take. At that point molecular motion is at its absolute minimum and no amount of further cooling can achieve anything.
That floor is absolute zero, or , and it is why the Kelvin scale was built to start there. On the Celsius scale, zero is just the temperature at which ice happens to melt — a useful landmark, but an arbitrary one, and it leaves the genuinely meaningful limit sitting at an awkward negative number.
This is also why gas laws and other physics formulas insist on kelvin: they involve the actual amount of molecular energy, and only a scale starting at zero energy can express that as a plain ratio.
Going the other way is different. Every joule you remove slows the molecules a little further, and eventually there is nothing left to take. At that point molecular motion is at its absolute minimum and no amount of further cooling can achieve anything.
That floor is absolute zero, or , and it is why the Kelvin scale was built to start there. On the Celsius scale, zero is just the temperature at which ice happens to melt — a useful landmark, but an arbitrary one, and it leaves the genuinely meaningful limit sitting at an awkward negative number.
This is also why gas laws and other physics formulas insist on kelvin: they involve the actual amount of molecular energy, and only a scale starting at zero energy can express that as a plain ratio.
Key takeaways
Heat, temperature and the Kelvin scale: quick revision
- Heat is the total thermal energy of a body, measured in joules with a calorimeter, and it depends on mass.
- Temperature is the degree of hotness — the average molecular kinetic energy — measured in kelvin with a thermometer, and it does not depend on mass.
- of water at holds twice the heat of at ; a matchstick flame is hotter than warm water but holds far less heat.
- Heat always flows from higher to lower temperature.
- Three effects of heat: a rise in temperature, thermal expansion (a jar lid loosening under hot water), and a change of state (ice to water to steam).
- Heating a solid makes its molecules vibrate harder and further apart — that is expansion. In liquids they slide faster; in gases they fly faster, which is why gases expand most.
- During a change of state the temperature pauses, because the heat overcomes the intermolecular force instead of raising kinetic energy.
- , so , , and .
- Reversing it, , and is absolute zero.
- A temperature difference is the same number on both scales: and .
Try a set of conversions and heat-versus-temperature comparisons now — the questions that look like trick questions are usually just testing which of the two quantities is being asked about.
- Temperature is the degree of hotness — the average molecular kinetic energy — measured in kelvin with a thermometer, and it does not depend on mass.
- of water at holds twice the heat of at ; a matchstick flame is hotter than warm water but holds far less heat.
- Heat always flows from higher to lower temperature.
- Three effects of heat: a rise in temperature, thermal expansion (a jar lid loosening under hot water), and a change of state (ice to water to steam).
- Heating a solid makes its molecules vibrate harder and further apart — that is expansion. In liquids they slide faster; in gases they fly faster, which is why gases expand most.
- During a change of state the temperature pauses, because the heat overcomes the intermolecular force instead of raising kinetic energy.
- , so , , and .
- Reversing it, , and is absolute zero.
- A temperature difference is the same number on both scales: and .
Try a set of conversions and heat-versus-temperature comparisons now — the questions that look like trick questions are usually just testing which of the two quantities is being asked about.