Boiling Water Away Costs Five Times More Than Heating It
Learn the six changes of state and their names, why a heating curve has flat stretches where the temperature refuses to rise, what latent heat of fusion and vaporisation actually pay for, and how evaporation differs from boiling.
Why does a pan of boiling water take so long to boil dry?
Because turning water into steam costs far more energy than heating it did.
Taking of water from freezing point all the way to boiling point needs about . Boiling that same away into steam — with the temperature not rising by a single degree — needs about , more than five times as much.
All that energy goes into pulling the molecules apart rather than speeding them up, which is why the thermometer sits stubbornly at while the pan empties. This page covers the second part of the ICSE Class 8 Chemistry chapter on matter: the six changes of state, the heating curve, latent heat, and evaporation.
Taking of water from freezing point all the way to boiling point needs about . Boiling that same away into steam — with the temperature not rising by a single degree — needs about , more than five times as much.
All that energy goes into pulling the molecules apart rather than speeding them up, which is why the thermometer sits stubbornly at while the pan empties. This page covers the second part of the ICSE Class 8 Chemistry chapter on matter: the six changes of state, the heating curve, latent heat, and evaporation.
What are the six changes of state and their proper names?
Three pairs, each pair being a change and its reverse.
Melting (fusion) — solid to liquid, at a fixed temperature called the melting point. Ice becomes water at ; wax melts in a burning candle; ghee melts in a warm kitchen.
Freezing (solidification) — liquid to solid, at the same temperature, now called the freezing point. Water becomes ice at ; molten wax sets as the candle cools.
Vaporisation (boiling) — liquid to gas, at a fixed temperature called the boiling point. Water becomes steam at at normal atmospheric pressure.
Condensation (liquefaction) — gas to liquid. Steam turning back to water on a cold lid; dew forming on a chilled steel glass; mist on a window.
Sublimation — solid straight to gas, with no liquid stage at all. Camphor, naphthalene balls, iodine and ammonium chloride all do this. A camphor tablet left in a cupboard slowly disappears without ever becoming wet.
Deposition — gas straight to solid, the reverse of sublimation. Heat iodine in a test tube and its violet vapour re-forms as solid crystals on the cool upper walls; frost forms directly on a cold surface in the same way.
The symmetry worth noticing. For a given substance, the melting point and freezing point are the same temperature — for water either way. What differs is only the direction in which heat is moving: melting absorbs heat, freezing releases it.
And every one of these is a physical change. The molecules are unaltered, so all six are reversible and no new substance is formed. Ice, water and steam remain throughout, which is why this chapter sits in chemistry as the clearest example of what a physical change is.
Melting (fusion) — solid to liquid, at a fixed temperature called the melting point. Ice becomes water at ; wax melts in a burning candle; ghee melts in a warm kitchen.
Freezing (solidification) — liquid to solid, at the same temperature, now called the freezing point. Water becomes ice at ; molten wax sets as the candle cools.
Vaporisation (boiling) — liquid to gas, at a fixed temperature called the boiling point. Water becomes steam at at normal atmospheric pressure.
Condensation (liquefaction) — gas to liquid. Steam turning back to water on a cold lid; dew forming on a chilled steel glass; mist on a window.
Sublimation — solid straight to gas, with no liquid stage at all. Camphor, naphthalene balls, iodine and ammonium chloride all do this. A camphor tablet left in a cupboard slowly disappears without ever becoming wet.
Deposition — gas straight to solid, the reverse of sublimation. Heat iodine in a test tube and its violet vapour re-forms as solid crystals on the cool upper walls; frost forms directly on a cold surface in the same way.
The symmetry worth noticing. For a given substance, the melting point and freezing point are the same temperature — for water either way. What differs is only the direction in which heat is moving: melting absorbs heat, freezing releases it.
And every one of these is a physical change. The molecules are unaltered, so all six are reversible and no new substance is formed. Ice, water and steam remain throughout, which is why this chapter sits in chemistry as the clearest example of what a physical change is.
Why does a heating curve have flat stretches?
Because during a change of state the supplied heat does no heating at all — it goes entirely into separating the molecules.
Reading the curve for ice taken from below freezing to steam. Plot temperature against time while heating steadily, and five distinct stages appear.
- Rising — ice below warms up to .
- **Flat at — the ice melts. Ice and water coexist and the temperature does not move.
- Rising** — the water warms from to .
- **Flat at — the water boils. Water and steam coexist and the temperature does not move.
- Rising** — the steam itself is heated above .
How to read the curve. The flat portions give the melting point and the boiling point directly. The sloping portions are where temperature is actually changing. And the relative lengths of the two flat stretches tell you which change of state needs more energy — the boiling plateau is much the longer one.
Why the temperature holds steady. Temperature measures the average kinetic energy of the molecules. During melting, the heat supplied is used to break the molecules out of their fixed positions — overcoming the intermolecular force rather than making the molecules move faster. Since the average kinetic energy does not rise, neither does the temperature.
The moment the last of the ice has melted, there is no more force left to overcome, and the heat starts raising the kinetic energy again — so the curve resumes climbing.
A cooling curve is the mirror image, with flat stretches where heat is being given out during freezing and condensation.
The misconception this kills. Adding heat always raises temperature is false, and the flat stretches are the proof. Heat and temperature are different quantities, and a change of state is where the difference becomes impossible to ignore.
Reading the curve for ice taken from below freezing to steam. Plot temperature against time while heating steadily, and five distinct stages appear.
- Rising — ice below warms up to .
- **Flat at — the ice melts. Ice and water coexist and the temperature does not move.
- Rising** — the water warms from to .
- **Flat at — the water boils. Water and steam coexist and the temperature does not move.
- Rising** — the steam itself is heated above .
How to read the curve. The flat portions give the melting point and the boiling point directly. The sloping portions are where temperature is actually changing. And the relative lengths of the two flat stretches tell you which change of state needs more energy — the boiling plateau is much the longer one.
Why the temperature holds steady. Temperature measures the average kinetic energy of the molecules. During melting, the heat supplied is used to break the molecules out of their fixed positions — overcoming the intermolecular force rather than making the molecules move faster. Since the average kinetic energy does not rise, neither does the temperature.
The moment the last of the ice has melted, there is no more force left to overcome, and the heat starts raising the kinetic energy again — so the curve resumes climbing.
A cooling curve is the mirror image, with flat stretches where heat is being given out during freezing and condensation.
The misconception this kills. Adding heat always raises temperature is false, and the flat stretches are the proof. Heat and temperature are different quantities, and a change of state is where the difference becomes impossible to ignore.
Formula
What is latent heat, and how much energy does a change of state need?
Latent heat is the heat absorbed or released during a change of state at constant temperature. The word describes it exactly: the heat is hidden, producing no temperature change to show for itself.
where is the mass and is the specific latent heat — the heat needed per unit mass.
Latent heat of fusion is the heat needed to change a solid to a liquid at its melting point. For ice,
Latent heat of vaporisation is the heat needed to change a liquid to a gas at its boiling point. For water,
Worked example 1 — melting ice. Heat needed to melt of ice already at :
Worked example 2 — boiling water. Heat needed to turn of water at into steam:
Worked example 3 — comparing the two.
Boiling a gram of water away needs nearly seven times the energy of melting a gram of ice — because escaping the liquid entirely means breaking free of the attraction completely, while melting only means loosening it.
Worked example 4 — back to the opening claim. Water has a specific heat capacity of about per gram per degree, so warming from to takes
against the to boil it away:
So boiling the water off costs more than five times as much as heating it through its entire liquid range.
And the same heat is released coming back. Freezing of water gives out ; condensing of steam gives out . That released latent heat is why steam scalds far worse than boiling water at the very same temperature — on touching skin it condenses, and dumps the whole per gram into it.
where is the mass and is the specific latent heat — the heat needed per unit mass.
Latent heat of fusion is the heat needed to change a solid to a liquid at its melting point. For ice,
Latent heat of vaporisation is the heat needed to change a liquid to a gas at its boiling point. For water,
Worked example 1 — melting ice. Heat needed to melt of ice already at :
Worked example 2 — boiling water. Heat needed to turn of water at into steam:
Worked example 3 — comparing the two.
Boiling a gram of water away needs nearly seven times the energy of melting a gram of ice — because escaping the liquid entirely means breaking free of the attraction completely, while melting only means loosening it.
Worked example 4 — back to the opening claim. Water has a specific heat capacity of about per gram per degree, so warming from to takes
against the to boil it away:
So boiling the water off costs more than five times as much as heating it through its entire liquid range.
And the same heat is released coming back. Freezing of water gives out ; condensing of steam gives out . That released latent heat is why steam scalds far worse than boiling water at the very same temperature — on touching skin it condenses, and dumps the whole per gram into it.
How does evaporation differ from boiling, and what speeds it up?
Evaporation happens at every temperature and only at the surface; boiling happens at one fixed temperature throughout the liquid.
Setting the two side by side:
- Temperature. Evaporation: at all temperatures. Boiling: only at the boiling point.
- Where. Evaporation: at the surface only. Boiling: throughout the bulk, which is why bubbles rise from the base.
- Rate. Evaporation: slow and silent. Boiling: rapid and vigorous.
- Heat source. Evaporation: latent heat drawn from the surroundings. Boiling: heat supplied continuously from outside.
- Control. Evaporation: cannot be stopped. Boiling: stops when the heat is removed.
Both need the latent heat of vaporisation — that is what links this section to the last. The difference is only where the per gram comes from: a flame in boiling, and the surroundings in evaporation.
Which is why evaporation cools. Only the fastest surface molecules have enough energy to escape. Their departure lowers the average kinetic energy of those remaining, so the liquid's temperature falls and it draws heat from whatever it touches. This is the mechanism behind sweating, an earthen pot of water, and a wet cloth on a fever patient's forehead.
Four factors that speed evaporation up:
- Temperature — higher temperature means more molecules have escape energy. Clothes dry faster in the sun.
- Surface area — more exposed surface means more molecules in a position to escape. Spread washing out rather than bundling it.
- Humidity — drier air means fewer molecules returning, so a higher net rate. Clothes take far longer to dry during the monsoon.
- Wind — moving air carries escaped molecules away before they can return. This is also why a fan cools you while blowing room-temperature air.
And the nature of the liquid. Spirit or petrol evaporates far faster than water and feels much colder on the skin, because its intermolecular force is weaker — less energy is needed for a molecule to break free. Such liquids are called volatile.
The boundary case. Cover a glass of water and evaporation nearly stops, because escaped molecules are trapped above the surface and return as fast as they leave. Evaporation is a net process, not a one-way one.
Setting the two side by side:
- Temperature. Evaporation: at all temperatures. Boiling: only at the boiling point.
- Where. Evaporation: at the surface only. Boiling: throughout the bulk, which is why bubbles rise from the base.
- Rate. Evaporation: slow and silent. Boiling: rapid and vigorous.
- Heat source. Evaporation: latent heat drawn from the surroundings. Boiling: heat supplied continuously from outside.
- Control. Evaporation: cannot be stopped. Boiling: stops when the heat is removed.
Both need the latent heat of vaporisation — that is what links this section to the last. The difference is only where the per gram comes from: a flame in boiling, and the surroundings in evaporation.
Which is why evaporation cools. Only the fastest surface molecules have enough energy to escape. Their departure lowers the average kinetic energy of those remaining, so the liquid's temperature falls and it draws heat from whatever it touches. This is the mechanism behind sweating, an earthen pot of water, and a wet cloth on a fever patient's forehead.
Four factors that speed evaporation up:
- Temperature — higher temperature means more molecules have escape energy. Clothes dry faster in the sun.
- Surface area — more exposed surface means more molecules in a position to escape. Spread washing out rather than bundling it.
- Humidity — drier air means fewer molecules returning, so a higher net rate. Clothes take far longer to dry during the monsoon.
- Wind — moving air carries escaped molecules away before they can return. This is also why a fan cools you while blowing room-temperature air.
And the nature of the liquid. Spirit or petrol evaporates far faster than water and feels much colder on the skin, because its intermolecular force is weaker — less energy is needed for a molecule to break free. Such liquids are called volatile.
The boundary case. Cover a glass of water and evaporation nearly stops, because escaped molecules are trapped above the surface and return as fast as they leave. Evaporation is a net process, not a one-way one.
Exam tip
Exam tip: name what the latent heat is used for
When asked why the temperature stays constant during melting or boiling, say the heat is used to overcome the intermolecular force and separate the molecules, not to raise their kinetic energy. Stopping at the heat is used up loses the mark.
Use and show the substitution with units: . Convert kilograms to grams first if is quoted per gram.
Quote the two constants correctly: **fusion of ice , vaporisation of water . Mixing them up is a costly slip, since they differ by nearly seven times.
Name the change of state with its proper term — fusion, solidification, vaporisation, condensation, sublimation, deposition — and give an example. Camphor and naphthalene for sublimation; iodine crystals re-forming for deposition.
State that the melting point and freezing point are the same temperature, differing only in the direction of heat flow.
On a heating curve, label the flat portions as the melting and boiling points and say the substance exists in two states together there.
For evaporation versus boiling, answer in pairs across both, not as separate lists.
And have the steam-scald explanation ready: condensing steam releases its latent heat** into the skin, which boiling water at the same temperature does not.
Use and show the substitution with units: . Convert kilograms to grams first if is quoted per gram.
Quote the two constants correctly: **fusion of ice , vaporisation of water . Mixing them up is a costly slip, since they differ by nearly seven times.
Name the change of state with its proper term — fusion, solidification, vaporisation, condensation, sublimation, deposition — and give an example. Camphor and naphthalene for sublimation; iodine crystals re-forming for deposition.
State that the melting point and freezing point are the same temperature, differing only in the direction of heat flow.
On a heating curve, label the flat portions as the melting and boiling points and say the substance exists in two states together there.
For evaporation versus boiling, answer in pairs across both, not as separate lists.
And have the steam-scald explanation ready: condensing steam releases its latent heat** into the skin, which boiling water at the same temperature does not.
Did you know
Why does a pressure cooker cook food faster than an open pan?
Food in an open pan of boiling water is being cooked at , and no amount of extra flame will make it hotter. Turn the gas up and the water simply boils away faster, still at — the flat stretch of the heating curve refuses to move.
A pressure cooker changes the boiling point itself. Trapped steam raises the pressure inside, and at higher pressure a molecule needs more energy to escape the liquid, so the water has to reach a higher temperature before it can boil. The food is now cooking well above .
The whistle is the pressure valve releasing the excess, which is what holds the inside at a steady raised pressure rather than letting it climb indefinitely.
The same physics runs the other way on a mountain. Lower atmospheric pressure means water boils below , so rice cooks slowly and badly — not because there is less heat available, but because the plateau has moved down.
A pressure cooker changes the boiling point itself. Trapped steam raises the pressure inside, and at higher pressure a molecule needs more energy to escape the liquid, so the water has to reach a higher temperature before it can boil. The food is now cooking well above .
The whistle is the pressure valve releasing the excess, which is what holds the inside at a steady raised pressure rather than letting it climb indefinitely.
The same physics runs the other way on a mountain. Lower atmospheric pressure means water boils below , so rice cooks slowly and badly — not because there is less heat available, but because the plateau has moved down.
Key takeaways
Changes of state and latent heat: quick revision
- Six changes of state: melting/fusion (solid to liquid), freezing/solidification (liquid to solid), vaporisation/boiling (liquid to gas), condensation (gas to liquid), sublimation (solid straight to gas — camphor, naphthalene, iodine), deposition (gas straight to solid — iodine crystals re-forming, frost).
- The melting point and freezing point are the same temperature; only the direction of heat flow differs. All six are physical and reversible.
- A heating curve for ice to steam has five stages, with flat stretches at and giving the melting and boiling points.
- The temperature holds steady during a change of state because the heat overcomes the intermolecular force instead of raising kinetic energy.
- Latent heat is heat absorbed or released at constant temperature: .
- of ice is ; of water is .
- Melting of ice needs ; boiling of water needs — about seven times more per gram.
- Heating of water from to takes only , so boiling it away costs about 5.4 times as much.
- Steam scalds worse than boiling water because condensing steam releases its latent heat into the skin.
- Evaporation: all temperatures, surface only, slow, latent heat from the surroundings, unstoppable. Boiling: one fixed temperature, throughout the bulk, rapid, heat supplied externally.
- Evaporation cools because the fastest molecules leave and the average kinetic energy of the rest falls.
- It is faster with higher temperature, larger surface area, lower humidity, more wind, and for a volatile liquid such as spirit.
Work through a few problems now — and check every answer against the fact that vaporisation always costs far more than fusion for the same mass.
- The melting point and freezing point are the same temperature; only the direction of heat flow differs. All six are physical and reversible.
- A heating curve for ice to steam has five stages, with flat stretches at and giving the melting and boiling points.
- The temperature holds steady during a change of state because the heat overcomes the intermolecular force instead of raising kinetic energy.
- Latent heat is heat absorbed or released at constant temperature: .
- of ice is ; of water is .
- Melting of ice needs ; boiling of water needs — about seven times more per gram.
- Heating of water from to takes only , so boiling it away costs about 5.4 times as much.
- Steam scalds worse than boiling water because condensing steam releases its latent heat into the skin.
- Evaporation: all temperatures, surface only, slow, latent heat from the surroundings, unstoppable. Boiling: one fixed temperature, throughout the bulk, rapid, heat supplied externally.
- Evaporation cools because the fastest molecules leave and the average kinetic energy of the rest falls.
- It is faster with higher temperature, larger surface area, lower humidity, more wind, and for a volatile liquid such as spirit.
Work through a few problems now — and check every answer against the fact that vaporisation always costs far more than fusion for the same mass.