Ice Absorbs an Enormous Amount of Heat Without Getting Any Warmer
See why a thermometer in melting ice refuses to move, define latent heat and specific latent heat of fusion, read the flat portions of a heating and cooling curve, explain why ice cools a drink far better than ice-cold water, and solve combined heat numericals.
Why does a thermometer in melting ice stay at zero for so long?
Put a thermometer into a beaker of crushed ice and water and heat it gently. **For several minutes the reading does not budge from C, even though the flame is supplying heat the whole time. Only when the last piece of ice has gone does the temperature begin to rise.
The heat has not vanished. It has been used to change the ice into water instead of to make anything warmer.**
That is the whole subject of this part of the chapter, and it breaks the rule you learnt in the previous one. There, every joule of heat produced a temperature rise, and told you how much. **Here heat is absorbed with , so that formula gives zero and a different one is needed.
The heat absorbed during a change of state is called latent heat, from a word meaning hidden — hidden because a thermometer cannot see it.
And the amount is startlingly large.** To melt one kilogram of ice already at C takes J. **The same J would raise a kilogram of water by degrees, which is nearly the whole range from freezing to boiling. So melting ice costs as much energy as heating the resulting water almost to boiling.
Two consequences follow, and both are examinable.
- A heating curve has flat portions.** Plot temperature against time while heating ice steadily and you get a rise, a flat stretch at C, another rise, a flat stretch at C, and a final rise — one flat stretch for each change of state
- Ice cools a drink far better than ice-cold water does, because the ice must take its latent heat from the drink before it can start warming up at all
The reverse is equally true and less obvious. Freezing water releases that same latent heat to its surroundings, which is why a pond freezes only slowly and why the air feels less bitter while snow is actually falling.
This page covers the second part of the ICSE Class 10 Physics chapter on heat: change of phase, latent heat and specific latent heat of fusion, heating and cooling curves, and numericals on the latent heat of fusion.
The heat has not vanished. It has been used to change the ice into water instead of to make anything warmer.**
That is the whole subject of this part of the chapter, and it breaks the rule you learnt in the previous one. There, every joule of heat produced a temperature rise, and told you how much. **Here heat is absorbed with , so that formula gives zero and a different one is needed.
The heat absorbed during a change of state is called latent heat, from a word meaning hidden — hidden because a thermometer cannot see it.
And the amount is startlingly large.** To melt one kilogram of ice already at C takes J. **The same J would raise a kilogram of water by degrees, which is nearly the whole range from freezing to boiling. So melting ice costs as much energy as heating the resulting water almost to boiling.
Two consequences follow, and both are examinable.
- A heating curve has flat portions.** Plot temperature against time while heating ice steadily and you get a rise, a flat stretch at C, another rise, a flat stretch at C, and a final rise — one flat stretch for each change of state
- Ice cools a drink far better than ice-cold water does, because the ice must take its latent heat from the drink before it can start warming up at all
The reverse is equally true and less obvious. Freezing water releases that same latent heat to its surroundings, which is why a pond freezes only slowly and why the air feels less bitter while snow is actually falling.
This page covers the second part of the ICSE Class 10 Physics chapter on heat: change of phase, latent heat and specific latent heat of fusion, heating and cooling curves, and numericals on the latent heat of fusion.
What is latent heat, and what does specific latent heat of fusion mean?
Latent heat is the heat absorbed or released during a change of state at constant temperature, and the specific latent heat of fusion is that amount for one kilogram of a substance melting.
What a change of phase is. Matter exists in three states, and heating or cooling can carry it from one to another.
- Melting or fusion — solid to liquid, at the melting point, absorbing heat
- Freezing or solidification — liquid to solid, at the same temperature, releasing heat
- Boiling or vaporisation — liquid to gas, at the boiling point, absorbing heat
- Condensation — gas to liquid, releasing heat
The defining feature of every one of them is that the temperature does not change while the change of state is going on. All the heat supplied goes into separating the particles from one another rather than into making them move faster.
Why the temperature stays constant, in terms of the particles. In a solid the particles are held in fixed positions by strong forces of attraction.
- Heating the solid below its melting point makes the particles vibrate faster, which raises the temperature
- At the melting point the vibration is violent enough to break the fixed arrangement, and the heat supplied from then on is used to overcome the forces of attraction and let the particles move past one another
- Overcoming those forces increases the potential energy of the particles, not their kinetic energy — and temperature measures the average kinetic energy
- So the thermometer reads a constant value until every particle has been freed
Latent heat. The latent heat of a substance is the heat absorbed or given out during a change of state without any change of temperature.
Specific latent heat of fusion. The specific latent heat of fusion of a substance is the heat required to change unit mass of it from the solid state to the liquid state at its melting point, without any change of temperature.
- **Its SI unit is the joule per kilogram (J kg)
- For ice its value is J kg**, which is often written as J per gram or J kg
- The same amount is released when one kilogram of water at C freezes into ice
How large that number is, put beside the previous part of the chapter. The specific heat capacity of water is J kg K, so
**Melting a kilogram of ice takes the same heat as raising a kilogram of water through K. That single comparison explains almost every phenomenon in this part.
One point of vocabulary that questions test. The melting point of a substance is the temperature at which it changes from solid to liquid; the freezing point is the temperature at which it changes back. For a pure substance the two are the same temperature** — C for water — and the direction of the heat flow is what distinguishes melting from freezing, not the temperature.
And a boundary case worth noticing. Ice at C and water at C are at the same temperature but do not contain the same heat. **A kilogram of water at C holds J more than a kilogram of ice at C, and that hidden difference is exactly the latent heat. It is the clearest illustration in the whole syllabus that heat and temperature are different quantities.**
What a change of phase is. Matter exists in three states, and heating or cooling can carry it from one to another.
- Melting or fusion — solid to liquid, at the melting point, absorbing heat
- Freezing or solidification — liquid to solid, at the same temperature, releasing heat
- Boiling or vaporisation — liquid to gas, at the boiling point, absorbing heat
- Condensation — gas to liquid, releasing heat
The defining feature of every one of them is that the temperature does not change while the change of state is going on. All the heat supplied goes into separating the particles from one another rather than into making them move faster.
Why the temperature stays constant, in terms of the particles. In a solid the particles are held in fixed positions by strong forces of attraction.
- Heating the solid below its melting point makes the particles vibrate faster, which raises the temperature
- At the melting point the vibration is violent enough to break the fixed arrangement, and the heat supplied from then on is used to overcome the forces of attraction and let the particles move past one another
- Overcoming those forces increases the potential energy of the particles, not their kinetic energy — and temperature measures the average kinetic energy
- So the thermometer reads a constant value until every particle has been freed
Latent heat. The latent heat of a substance is the heat absorbed or given out during a change of state without any change of temperature.
Specific latent heat of fusion. The specific latent heat of fusion of a substance is the heat required to change unit mass of it from the solid state to the liquid state at its melting point, without any change of temperature.
- **Its SI unit is the joule per kilogram (J kg)
- For ice its value is J kg**, which is often written as J per gram or J kg
- The same amount is released when one kilogram of water at C freezes into ice
How large that number is, put beside the previous part of the chapter. The specific heat capacity of water is J kg K, so
**Melting a kilogram of ice takes the same heat as raising a kilogram of water through K. That single comparison explains almost every phenomenon in this part.
One point of vocabulary that questions test. The melting point of a substance is the temperature at which it changes from solid to liquid; the freezing point is the temperature at which it changes back. For a pure substance the two are the same temperature** — C for water — and the direction of the heat flow is what distinguishes melting from freezing, not the temperature.
And a boundary case worth noticing. Ice at C and water at C are at the same temperature but do not contain the same heat. **A kilogram of water at C holds J more than a kilogram of ice at C, and that hidden difference is exactly the latent heat. It is the clearest illustration in the whole syllabus that heat and temperature are different quantities.**
How do you read a heating curve and a cooling curve for water?
The sloping parts are where the temperature is rising and only one state is present; the flat parts are where a change of state is happening at a constant temperature.
The heating curve, obtained by heating ice steadily from below its melting point and plotting temperature against time. It has five distinct portions.
- **A rising portion, from below C up to C. Only ice is present, and the heat is raising its temperature. The slope here is governed by the specific heat capacity of ice, J kg K
- A flat portion at C. Ice and water coexist, and all the heat supplied is the latent heat of fusion. The temperature does not change until the last of the ice has melted
- A rising portion, from C to C. Only water is present. The slope here is governed by the specific heat capacity of water, J kg K
- A flat portion at C. Water and steam coexist, and all the heat is the latent heat of vaporisation
- A final rising portion above C. Only steam is present
Two features of the curve that a question will ask you to explain.
Why the water portion is less steep than the ice portion. For the same rate of heating, a substance with a larger specific heat capacity warms more slowly. Water's is twice ice's , so the water section climbs at about half the rate of the ice section. The change of slope at C is therefore not an accident of drawing — it is the ratio of the two specific heat capacities.
Why the boiling plateau is much longer than the melting plateau.** The specific latent heat of vaporisation of water is about J kg, against J kg for fusion — nearly seven times as much. So at a steady rate of heating, boiling away the same mass takes nearly seven times as long as melting it. The two flat stretches are drawn at very different lengths for that reason.
The cooling curve is the same picture read backwards, obtained by cooling steam steadily.
- A falling portion while steam cools to C
- **A flat portion at C while steam condenses, releasing its latent heat of vaporisation
- A falling portion** while water cools from C to C
- **A flat portion at C while water freezes, releasing its latent heat of fusion
- A falling portion** while the ice cools below C
The important difference between the two curves is the direction of the heat flow. On the heating curve the flat portions are heat being absorbed; on the cooling curve they are heat being given out. The temperatures at which they occur are identical, because a pure substance melts and freezes at the same temperature.
How to identify the state from a point on the curve, which is the standard diagram-based question:
- On a sloping portion, only one state is present — ice below the first plateau, water between the plateaus, steam above the second
- On a flat portion, two states coexist, and the substance is part way through a change of state
- At the very start of a plateau the substance is entirely in the earlier state; at the very end it is entirely in the later one
One boundary case that explains a laboratory observation. A mixture of ice and water stays at C however much you stir it or however gently you heat it, and that is why a beaker of melting ice is used as a fixed point for checking a thermometer. The temperature is held constant by the change of state itself, which makes it far more reliable than any attempt to hold a temperature by careful heating.
The heating curve, obtained by heating ice steadily from below its melting point and plotting temperature against time. It has five distinct portions.
- **A rising portion, from below C up to C. Only ice is present, and the heat is raising its temperature. The slope here is governed by the specific heat capacity of ice, J kg K
- A flat portion at C. Ice and water coexist, and all the heat supplied is the latent heat of fusion. The temperature does not change until the last of the ice has melted
- A rising portion, from C to C. Only water is present. The slope here is governed by the specific heat capacity of water, J kg K
- A flat portion at C. Water and steam coexist, and all the heat is the latent heat of vaporisation
- A final rising portion above C. Only steam is present
Two features of the curve that a question will ask you to explain.
Why the water portion is less steep than the ice portion. For the same rate of heating, a substance with a larger specific heat capacity warms more slowly. Water's is twice ice's , so the water section climbs at about half the rate of the ice section. The change of slope at C is therefore not an accident of drawing — it is the ratio of the two specific heat capacities.
Why the boiling plateau is much longer than the melting plateau.** The specific latent heat of vaporisation of water is about J kg, against J kg for fusion — nearly seven times as much. So at a steady rate of heating, boiling away the same mass takes nearly seven times as long as melting it. The two flat stretches are drawn at very different lengths for that reason.
The cooling curve is the same picture read backwards, obtained by cooling steam steadily.
- A falling portion while steam cools to C
- **A flat portion at C while steam condenses, releasing its latent heat of vaporisation
- A falling portion** while water cools from C to C
- **A flat portion at C while water freezes, releasing its latent heat of fusion
- A falling portion** while the ice cools below C
The important difference between the two curves is the direction of the heat flow. On the heating curve the flat portions are heat being absorbed; on the cooling curve they are heat being given out. The temperatures at which they occur are identical, because a pure substance melts and freezes at the same temperature.
How to identify the state from a point on the curve, which is the standard diagram-based question:
- On a sloping portion, only one state is present — ice below the first plateau, water between the plateaus, steam above the second
- On a flat portion, two states coexist, and the substance is part way through a change of state
- At the very start of a plateau the substance is entirely in the earlier state; at the very end it is entirely in the later one
One boundary case that explains a laboratory observation. A mixture of ice and water stays at C however much you stir it or however gently you heat it, and that is why a beaker of melting ice is used as a fixed point for checking a thermometer. The temperature is held constant by the change of state itself, which makes it far more reliable than any attempt to hold a temperature by careful heating.
Why does ice cool a drink better than water at the same temperature?
Because the ice must absorb its latent heat of fusion from the drink before it can begin to warm up at all, and that latent heat is enormous compared with anything a temperature change can supply.
Worked comparison — the case made in numbers. Compare g of ice at C with g of water at C, each added to a drink and each ending at C.
**The water at C** simply warms up:
**The ice at C** must first melt and then warm up:
Nine times as much heat removed from the drink, for the same mass at the same starting temperature. That is why a glass with ice in it cools so much faster and stays cold so much longer, and it is the clearest possible demonstration that latent heat is real energy.
Other phenomena involving the latent heat of fusion, each with its explanation:
Snow on the mountains melts slowly, and a sudden thaw causes floods. Every kilogram of snow needs J before it becomes water, so an ordinary spring supplies that heat over weeks and the melt water runs off gradually. An unusually warm spell supplies the same heat far faster, the snow melts together, and the rivers below cannot carry the sudden volume.
It feels less bitterly cold while snow is actually falling. As water vapour and water droplets in the cloud freeze into snow, they release their latent heat to the surrounding air, which keeps the air temperature from dropping as far as it otherwise would. The coldest weather often comes after the snowfall, not during it, once that release has stopped.
Fish survive in a frozen pond. Ice forms at the surface first and, as it does, releases latent heat into the water below, which slows further cooling. The ice layer also insulates the water beneath it, so the pond freezes from the top down and a layer of liquid water remains at the bottom.
Ice cream and cold drinks are kept in a mixture of ice and salt. Adding salt lowers the melting point of ice below C, so the mixture is colder than plain ice and the melting still absorbs latent heat from the surroundings. Both effects work in the same direction.
Ice is wrapped in sawdust, a blanket or a gunny sack while being transported. That sounds like keeping it warm, and it is the opposite. The ice can only melt if latent heat reaches it from outside, so an insulating wrapping slows the supply of that heat and the ice lasts far longer.
Water in a bottle left in a freezer can crack the bottle. Water is unusual in expanding when it freezes, so the ice occupies more volume than the water did. This is a consequence of freezing rather than of latent heat itself, and it is worth keeping the two apart — the latent heat explains how long the freezing takes, the expansion explains the broken bottle.
A refrigerator works by the same principle in reverse. The refrigerant is made to evaporate inside the cabinet, absorbing latent heat from the food, and to condense outside it, releasing that heat to the room. The heat is not destroyed; it is pumped from inside to outside, which is why the back of a refrigerator is warm and why a refrigerator with its door left open warms a closed kitchen rather than cooling it.
One honest qualification about the drink. The ice cools the drink but also dilutes it as it melts, since the melted ice becomes part of the liquid. A sealed ice pack cools without diluting, which is why it is used for an injury rather than loose ice — the physics is identical, only the mixing is prevented.
Worked comparison — the case made in numbers. Compare g of ice at C with g of water at C, each added to a drink and each ending at C.
**The water at C** simply warms up:
**The ice at C** must first melt and then warm up:
Nine times as much heat removed from the drink, for the same mass at the same starting temperature. That is why a glass with ice in it cools so much faster and stays cold so much longer, and it is the clearest possible demonstration that latent heat is real energy.
Other phenomena involving the latent heat of fusion, each with its explanation:
Snow on the mountains melts slowly, and a sudden thaw causes floods. Every kilogram of snow needs J before it becomes water, so an ordinary spring supplies that heat over weeks and the melt water runs off gradually. An unusually warm spell supplies the same heat far faster, the snow melts together, and the rivers below cannot carry the sudden volume.
It feels less bitterly cold while snow is actually falling. As water vapour and water droplets in the cloud freeze into snow, they release their latent heat to the surrounding air, which keeps the air temperature from dropping as far as it otherwise would. The coldest weather often comes after the snowfall, not during it, once that release has stopped.
Fish survive in a frozen pond. Ice forms at the surface first and, as it does, releases latent heat into the water below, which slows further cooling. The ice layer also insulates the water beneath it, so the pond freezes from the top down and a layer of liquid water remains at the bottom.
Ice cream and cold drinks are kept in a mixture of ice and salt. Adding salt lowers the melting point of ice below C, so the mixture is colder than plain ice and the melting still absorbs latent heat from the surroundings. Both effects work in the same direction.
Ice is wrapped in sawdust, a blanket or a gunny sack while being transported. That sounds like keeping it warm, and it is the opposite. The ice can only melt if latent heat reaches it from outside, so an insulating wrapping slows the supply of that heat and the ice lasts far longer.
Water in a bottle left in a freezer can crack the bottle. Water is unusual in expanding when it freezes, so the ice occupies more volume than the water did. This is a consequence of freezing rather than of latent heat itself, and it is worth keeping the two apart — the latent heat explains how long the freezing takes, the expansion explains the broken bottle.
A refrigerator works by the same principle in reverse. The refrigerant is made to evaporate inside the cabinet, absorbing latent heat from the food, and to condense outside it, releasing that heat to the room. The heat is not destroyed; it is pumped from inside to outside, which is why the back of a refrigerator is warm and why a refrigerator with its door left open warms a closed kitchen rather than cooling it.
One honest qualification about the drink. The ice cools the drink but also dilutes it as it melts, since the melted ice becomes part of the liquid. A sealed ice pack cools without diluting, which is why it is used for an injury rather than loose ice — the physics is identical, only the mixing is prevented.
Formula
How do you solve a numerical involving latent heat of fusion?
**Split the process into stages, use for each change of state and for each temperature change, and add.**
where is the specific latent heat, in J kg. For a process with both a change of state and a temperature change:
**Take the specific latent heat of fusion of ice as J kg**, the specific heat capacity of water as J kg K and that of ice as J kg K.
Worked example 1 — melting alone. Find the heat required to melt kg of ice at C into water at C.
No temperature change at all, so contributes nothing.
Worked example 2 — melting and then heating. Find the heat required to convert g of ice at C into water at C.
Two stages, in order.
**Stage 1 — melt the ice at C:**
**Stage 2 — heat the resulting water from to C:**
Total:
Notice the proportions. Melting took four times as much heat as the entire -degree warming that followed. Anyone who leaves out the latent-heat stage gets an answer five times too small, which is the most expensive error in this part of the chapter.
Worked example 3 — warming the ice first. Find the heat required to convert g of ice at C into water at C.
**Stage 1 — warm the ice from to C, using the specific heat capacity of ice**:
**Stage 2 — melt it at C:**
Total:
**Using instead of in the first stage is a standard trap. While the substance is ice, its specific heat capacity is ice's — the value changes at the moment the state changes.
Worked example 4 — the reverse process.** Calculate the heat released when g of water at C is converted into ice at C.
**Stage 1 — cool the water from to C:**
**Stage 2 — freeze it at C**, which releases the latent heat:
Total released:
Worked example 5 — a refrigerator. How much heat must a refrigerator remove to turn kg of water at C into ice at C?
Four fifths of the work is the freezing itself, which is why making ice takes so much longer than merely chilling water.
Worked example 6 — a mixture with ice, which is the hardest standard type. How much ice at C must be added to g of water at C to bring the whole to C, assuming no heat is lost to the surroundings?
**Heat lost by the water in cooling from to C:**
Heat gained by the ice, which only melts — it does not warm up, since the final temperature is C:
Equating them:
Check that the answer is sensible. Only g of ice absorbs all the heat given up by g of water cooling thirty degrees — less than half the mass, because melting is so much more expensive per kilogram than cooling. An answer larger than the mass of the water would be a signal that the latent heat had been left out.
The layout that never loses marks. Number the stages, write the formula for each, compute each separately, and add at the end. Each stage is separately markable, so an arithmetic slip in one still leaves the others intact — provided the working shows them.
where is the specific latent heat, in J kg. For a process with both a change of state and a temperature change:
**Take the specific latent heat of fusion of ice as J kg**, the specific heat capacity of water as J kg K and that of ice as J kg K.
Worked example 1 — melting alone. Find the heat required to melt kg of ice at C into water at C.
No temperature change at all, so contributes nothing.
Worked example 2 — melting and then heating. Find the heat required to convert g of ice at C into water at C.
Two stages, in order.
**Stage 1 — melt the ice at C:**
**Stage 2 — heat the resulting water from to C:**
Total:
Notice the proportions. Melting took four times as much heat as the entire -degree warming that followed. Anyone who leaves out the latent-heat stage gets an answer five times too small, which is the most expensive error in this part of the chapter.
Worked example 3 — warming the ice first. Find the heat required to convert g of ice at C into water at C.
**Stage 1 — warm the ice from to C, using the specific heat capacity of ice**:
**Stage 2 — melt it at C:**
Total:
**Using instead of in the first stage is a standard trap. While the substance is ice, its specific heat capacity is ice's — the value changes at the moment the state changes.
Worked example 4 — the reverse process.** Calculate the heat released when g of water at C is converted into ice at C.
**Stage 1 — cool the water from to C:**
**Stage 2 — freeze it at C**, which releases the latent heat:
Total released:
Worked example 5 — a refrigerator. How much heat must a refrigerator remove to turn kg of water at C into ice at C?
Four fifths of the work is the freezing itself, which is why making ice takes so much longer than merely chilling water.
Worked example 6 — a mixture with ice, which is the hardest standard type. How much ice at C must be added to g of water at C to bring the whole to C, assuming no heat is lost to the surroundings?
**Heat lost by the water in cooling from to C:**
Heat gained by the ice, which only melts — it does not warm up, since the final temperature is C:
Equating them:
Check that the answer is sensible. Only g of ice absorbs all the heat given up by g of water cooling thirty degrees — less than half the mass, because melting is so much more expensive per kilogram than cooling. An answer larger than the mass of the water would be a signal that the latent heat had been left out.
The layout that never loses marks. Number the stages, write the formula for each, compute each separately, and add at the end. Each stage is separately markable, so an arithmetic slip in one still leaves the others intact — provided the working shows them.
Exam tip
Which steps protect the marks in a latent heat numerical?
Break the process into stages before calculating anything, and name which formula belongs to each stage.
- Write the stages out first: warm the ice, melt it, warm the water — in the order they happen
- **Use where the temperature is constant** and where it changes. Never use both for the same stage
- Change the specific heat capacity when the state changes — for ice, for water
- Convert masses to kilograms before substituting
- **Take J kg for the fusion of ice, and say so
- For a mixture, put the latent heat on the side where the ice is, and check whether the ice also warms up afterwards
- If the final temperature is C the ice only melts** and there is no term for it
- Check that a mixture's answer is sensible — a small mass of ice absorbs a great deal of heat
- On a curve question, say which states are present at a sloping portion and at a flat one
- Give the unit with every answer, and state whether the heat is absorbed or released
The misconception to name. Latent heat does not raise the temperature, and heat supplied at a constant temperature has not been wasted. It has been used to overcome the forces of attraction between the particles, increasing their potential energy rather than their kinetic energy. Writing that "no heat is absorbed during melting because the temperature does not change" reverses the physics completely — the temperature does not change because heat is being absorbed for a different purpose.
A second trap. Using the specific heat capacity of water for a stage in which the substance is ice. **Ice has J kg K and water has **, so warming g of ice through K needs J and not J. The value changes exactly at the melting point, and the stage-by-stage layout is what makes the switch impossible to forget.
- Write the stages out first: warm the ice, melt it, warm the water — in the order they happen
- **Use where the temperature is constant** and where it changes. Never use both for the same stage
- Change the specific heat capacity when the state changes — for ice, for water
- Convert masses to kilograms before substituting
- **Take J kg for the fusion of ice, and say so
- For a mixture, put the latent heat on the side where the ice is, and check whether the ice also warms up afterwards
- If the final temperature is C the ice only melts** and there is no term for it
- Check that a mixture's answer is sensible — a small mass of ice absorbs a great deal of heat
- On a curve question, say which states are present at a sloping portion and at a flat one
- Give the unit with every answer, and state whether the heat is absorbed or released
The misconception to name. Latent heat does not raise the temperature, and heat supplied at a constant temperature has not been wasted. It has been used to overcome the forces of attraction between the particles, increasing their potential energy rather than their kinetic energy. Writing that "no heat is absorbed during melting because the temperature does not change" reverses the physics completely — the temperature does not change because heat is being absorbed for a different purpose.
A second trap. Using the specific heat capacity of water for a stage in which the substance is ice. **Ice has J kg K and water has **, so warming g of ice through K needs J and not J. The value changes exactly at the melting point, and the stage-by-stage layout is what makes the switch impossible to forget.
Did you know
Why does a pressure cooker work and an ice pack stay cold for so long?
Both devices exploit the same fact: a change of state moves an enormous amount of heat at a fixed temperature. One uses it to deliver heat and the other to remove it.
Start with the ice pack. A pack of ice at C pressed on a swollen ankle stays cold far longer than a pack of chilled water would, and the reason is the number in this chapter. **Every gram of ice must absorb J before it becomes water at the same temperature, and all of that comes from the injury. The pack holds its temperature at C for as long as any ice remains, which is exactly what is wanted — a steady cold rather than a falling one.
And the steadiness is the point. A chilled water pack warms continuously and its cooling effect fades. A melting ice pack cannot warm up until the last of the ice is gone, so its temperature is held constant by the change of state. That is the same reason a beaker of melting ice makes a reliable fixed point for a thermometer.
Now the reverse, in steam.** Condensing steam releases its latent heat of vaporisation — nearly J per gram, almost seven times the latent heat of fusion. So steam at C carries far more heat than boiling water at C, even though a thermometer cannot tell them apart.
Which explains two everyday things at once.
- A steam burn is far worse than a boiling-water burn. The steam condenses on the skin and delivers its latent heat there, on top of the heat it then gives up while cooling
- Steam heats food quickly, which is why idli and dhokla are cooked in a steamer and why steam is used for heating in industry
A pressure cooker adds a second effect to the first. Raising the pressure inside raises the boiling point of water above C, so the food cooks at a higher temperature and is surrounded by condensing steam delivering latent heat. Both effects shorten the cooking time, and at high altitudes, where the pressure is low and water boils below C, a cooker becomes almost essential.
One more case, and it is the one that seems back to front. In very cold weather, orange growers spray their trees with water to protect them from frost. Spraying cold water on a freezing night sounds like the worst possible idea. **But as the water freezes on the fruit it releases J per gram into the surroundings**, and that release holds the temperature at C — above the point at which the fruit itself would be damaged. The ice layer is doing the protecting.
And the same reasoning explains why a hard frost often follows a snowfall rather than accompanying it. While snow is forming, latent heat is being released into the air and the temperature is held up. Once the snowfall stops, that source of heat stops with it, and the air is free to cool sharply. The coldest night is the clear one after the storm, and the reason is a number about ice rather than anything about wind or cloud.
Start with the ice pack. A pack of ice at C pressed on a swollen ankle stays cold far longer than a pack of chilled water would, and the reason is the number in this chapter. **Every gram of ice must absorb J before it becomes water at the same temperature, and all of that comes from the injury. The pack holds its temperature at C for as long as any ice remains, which is exactly what is wanted — a steady cold rather than a falling one.
And the steadiness is the point. A chilled water pack warms continuously and its cooling effect fades. A melting ice pack cannot warm up until the last of the ice is gone, so its temperature is held constant by the change of state. That is the same reason a beaker of melting ice makes a reliable fixed point for a thermometer.
Now the reverse, in steam.** Condensing steam releases its latent heat of vaporisation — nearly J per gram, almost seven times the latent heat of fusion. So steam at C carries far more heat than boiling water at C, even though a thermometer cannot tell them apart.
Which explains two everyday things at once.
- A steam burn is far worse than a boiling-water burn. The steam condenses on the skin and delivers its latent heat there, on top of the heat it then gives up while cooling
- Steam heats food quickly, which is why idli and dhokla are cooked in a steamer and why steam is used for heating in industry
A pressure cooker adds a second effect to the first. Raising the pressure inside raises the boiling point of water above C, so the food cooks at a higher temperature and is surrounded by condensing steam delivering latent heat. Both effects shorten the cooking time, and at high altitudes, where the pressure is low and water boils below C, a cooker becomes almost essential.
One more case, and it is the one that seems back to front. In very cold weather, orange growers spray their trees with water to protect them from frost. Spraying cold water on a freezing night sounds like the worst possible idea. **But as the water freezes on the fruit it releases J per gram into the surroundings**, and that release holds the temperature at C — above the point at which the fruit itself would be damaged. The ice layer is doing the protecting.
And the same reasoning explains why a hard frost often follows a snowfall rather than accompanying it. While snow is forming, latent heat is being released into the air and the temperature is held up. Once the snowfall stops, that source of heat stops with it, and the air is free to cool sharply. The coldest night is the clear one after the storm, and the reason is a number about ice rather than anything about wind or cloud.
Exam relevance
How does latent heat prepare you for JEE and NEET?
This is foundation work for Class 11 Thermal Properties of Matter and Thermodynamics, and for Class 11 and 12 Chemistry, so it feeds JEE Main, JEE Advanced and NEET.
**Where leads. Class 11 keeps it unchanged and adds the latent heat of vaporisation to the calculations, so a single problem may involve warming ice, melting it, warming the water, boiling it and superheating the steam — five stages instead of the two or three you handle here. The method does not change; only the number of terms does. JEE Main sets exactly such multi-stage calorimetry numericals.
Where the constant-temperature idea leads. In Chemistry the same quantity becomes the enthalpy of fusion and the enthalpy of vaporisation, quoted per mole rather than per kilogram and carrying a sign to show whether heat is absorbed or released. The physical content is identical, and NEET Chemistry uses it in thermochemistry and in questions on intermolecular forces — because a large latent heat means strong forces between the particles.
Where the heating curve leads. Class 11 uses it to introduce phase diagrams and the triple point, and Chemistry uses the same curve to compare substances: a substance with strong intermolecular forces has a high melting point and a large latent heat. Reading a curve to identify the state is a standard diagram-based question in both subjects.
Where the particle explanation leads. The statement that latent heat increases the potential energy of the particles rather than their kinetic energy becomes the distinction between internal energy and temperature in thermodynamics. A body can absorb heat with no change in temperature and therefore no change in its kinetic energy, which is the point assertion-reason questions in JEE and NEET are built on.
Where the refrigerator example leads. Class 11 treats the refrigerator and the heat pump as thermodynamic devices with a coefficient of performance, and the principle — evaporate inside to absorb latent heat, condense outside to release it — is stated in exactly these terms.
Where the ice-and-salt observation leads. Chemistry explains it as depression of freezing point, a colligative property, and gives it a formula in Class 12. The qualitative fact you learn here is the first half of that topic, and it is a reliable NEET item.
Question types to expect.** At this level: numericals, combined melting-and-heating problems, mixtures involving ice, curve interpretation, and explanations of everyday phenomena. In competitive papers: multi-stage calorimetry including vaporisation, enthalpy changes per mole, phase diagrams, and colligative-property calculations.
The single trap that costs marks. Omitting the latent-heat stage. **Converting g of ice at C into water at C needs J, of which J is the melting — so leaving it out gives an answer five times too small. In Class 11 the same omission in a five-stage problem loses most of the marks even when every other stage is right.
A second trap. Carrying the wrong specific heat capacity across a change of state. While the substance is ice, use ; once it is water, use — and Class 11 adds a third value for steam. Writing the stages out separately is what prevents this, and it is the habit competitive problems reward most.
Board versus competitive emphasis. The ICSE paper marks the definition with its unit, the staged working, the substitution and the explanation of a phenomenon; a competitive paper marks a single final answer, often after five stages, or an enthalpy in kilojoule per mole. The transferable habit is asking, at every step, whether the temperature is changing or the state is changing** — because that one question decides which of the two formulas applies, and no calorimetry problem at any level can be started without it.
**Where leads. Class 11 keeps it unchanged and adds the latent heat of vaporisation to the calculations, so a single problem may involve warming ice, melting it, warming the water, boiling it and superheating the steam — five stages instead of the two or three you handle here. The method does not change; only the number of terms does. JEE Main sets exactly such multi-stage calorimetry numericals.
Where the constant-temperature idea leads. In Chemistry the same quantity becomes the enthalpy of fusion and the enthalpy of vaporisation, quoted per mole rather than per kilogram and carrying a sign to show whether heat is absorbed or released. The physical content is identical, and NEET Chemistry uses it in thermochemistry and in questions on intermolecular forces — because a large latent heat means strong forces between the particles.
Where the heating curve leads. Class 11 uses it to introduce phase diagrams and the triple point, and Chemistry uses the same curve to compare substances: a substance with strong intermolecular forces has a high melting point and a large latent heat. Reading a curve to identify the state is a standard diagram-based question in both subjects.
Where the particle explanation leads. The statement that latent heat increases the potential energy of the particles rather than their kinetic energy becomes the distinction between internal energy and temperature in thermodynamics. A body can absorb heat with no change in temperature and therefore no change in its kinetic energy, which is the point assertion-reason questions in JEE and NEET are built on.
Where the refrigerator example leads. Class 11 treats the refrigerator and the heat pump as thermodynamic devices with a coefficient of performance, and the principle — evaporate inside to absorb latent heat, condense outside to release it — is stated in exactly these terms.
Where the ice-and-salt observation leads. Chemistry explains it as depression of freezing point, a colligative property, and gives it a formula in Class 12. The qualitative fact you learn here is the first half of that topic, and it is a reliable NEET item.
Question types to expect.** At this level: numericals, combined melting-and-heating problems, mixtures involving ice, curve interpretation, and explanations of everyday phenomena. In competitive papers: multi-stage calorimetry including vaporisation, enthalpy changes per mole, phase diagrams, and colligative-property calculations.
The single trap that costs marks. Omitting the latent-heat stage. **Converting g of ice at C into water at C needs J, of which J is the melting — so leaving it out gives an answer five times too small. In Class 11 the same omission in a five-stage problem loses most of the marks even when every other stage is right.
A second trap. Carrying the wrong specific heat capacity across a change of state. While the substance is ice, use ; once it is water, use — and Class 11 adds a third value for steam. Writing the stages out separately is what prevents this, and it is the habit competitive problems reward most.
Board versus competitive emphasis. The ICSE paper marks the definition with its unit, the staged working, the substitution and the explanation of a phenomenon; a competitive paper marks a single final answer, often after five stages, or an enthalpy in kilojoule per mole. The transferable habit is asking, at every step, whether the temperature is changing or the state is changing** — because that one question decides which of the two formulas applies, and no calorimetry problem at any level can be started without it.
Key takeaways
What must you be able to do from this part?
One extra formula, one constant temperature and one staged layout.
- A change of state happens at a constant temperature, with heat absorbed on melting and boiling and released on freezing and condensing
- Latent heat is the heat absorbed or given out during a change of state without any change of temperature
- The heat goes into overcoming the forces of attraction between the particles, raising their potential energy rather than their kinetic energy — and temperature measures kinetic energy
- Specific latent heat of fusion is the heat needed to change unit mass of a solid into liquid at its melting point, in **J kg
- For ice it is J kg**, and the same amount is released when water at C freezes
- **That equals the heat needed to raise a kilogram of water through K**, since
- **Water at C holds J per kilogram more than ice at C
- for a change of state, and when a temperature change is involved too
- Melting kg of ice** takes kJ; ** g of ice at C to water at C** takes J
- ** g of ice at C to water at C** takes J, using for the ice stage
- ** g of water at C to ice at C** releases J
- **Freezing kg of water from C** needs J removed
- ** g of ice at C cools g of water from C to C
- g of ice removes J from a drink reaching C**, against J for the same mass of water at C — nine times as much
- The heating curve has five portions: ice warming, a flat melt at C, water warming, a flat boil at C, and steam warming
- The water section is less steep than the ice section, because is twice
- The boiling plateau is much longer than the melting one, because the latent heat of vaporisation is nearly seven times as large
- On a sloping portion one state is present; on a flat portion two states coexist
- Phenomena: slow mountain thaws and sudden floods, milder air while snow falls, fish surviving under pond ice, ice-and-salt mixtures, ice wrapped in sawdust lasting longer, and a refrigerator pumping latent heat from inside to outside
The most convincing self-test costs one ice cube. Put it in a glass of water with a thermometer, note how long the reading stays at zero, and then work out from roughly how much heat must have come out of the water in that time.
- A change of state happens at a constant temperature, with heat absorbed on melting and boiling and released on freezing and condensing
- Latent heat is the heat absorbed or given out during a change of state without any change of temperature
- The heat goes into overcoming the forces of attraction between the particles, raising their potential energy rather than their kinetic energy — and temperature measures kinetic energy
- Specific latent heat of fusion is the heat needed to change unit mass of a solid into liquid at its melting point, in **J kg
- For ice it is J kg**, and the same amount is released when water at C freezes
- **That equals the heat needed to raise a kilogram of water through K**, since
- **Water at C holds J per kilogram more than ice at C
- for a change of state, and when a temperature change is involved too
- Melting kg of ice** takes kJ; ** g of ice at C to water at C** takes J
- ** g of ice at C to water at C** takes J, using for the ice stage
- ** g of water at C to ice at C** releases J
- **Freezing kg of water from C** needs J removed
- ** g of ice at C cools g of water from C to C
- g of ice removes J from a drink reaching C**, against J for the same mass of water at C — nine times as much
- The heating curve has five portions: ice warming, a flat melt at C, water warming, a flat boil at C, and steam warming
- The water section is less steep than the ice section, because is twice
- The boiling plateau is much longer than the melting one, because the latent heat of vaporisation is nearly seven times as large
- On a sloping portion one state is present; on a flat portion two states coexist
- Phenomena: slow mountain thaws and sudden floods, milder air while snow falls, fish surviving under pond ice, ice-and-salt mixtures, ice wrapped in sawdust lasting longer, and a refrigerator pumping latent heat from inside to outside
The most convincing self-test costs one ice cube. Put it in a glass of water with a thermometer, note how long the reading stays at zero, and then work out from roughly how much heat must have come out of the water in that time.