Change of State & Latent Heat

An ice cream melting in the harsh Indian summer is matter changing its state right before your eyes. This lesson explains how and why matter switches between solid, liquid and gas when you change the temperature, the precise temperatures at which water does this, and the Kelvin scale that exams expect you to use correctly.

Definition: A change of state (also called interconversion of states) is the conversion of matter from one physical state, solid, liquid or gas, into another, brought about mainly by changing temperature or pressure.

How heating changes a state

When you heat a solid, you are giving its particles more kinetic energy. The particles vibrate faster and faster about their fixed positions. At a certain temperature, the vibration becomes so strong that it overcomes the force of attraction holding the particles in place. The particles break free of their rigid arrangement and begin to slide past one another, the solid has become a liquid.

Definition: The melting point (fusion) is the temperature at which a solid turns into a liquid at atmospheric pressure. The process is called melting or fusion.

For ice, the melting point is 273.15 K (0 °C). (For exams the rounded value 273 K is normally used.)

If you keep heating the liquid, the particles move even faster until, at the boiling point, they have enough energy to escape into the gaseous state.

Definition: The boiling point is the temperature at which a liquid rapidly turns into vapour (gas) throughout its bulk. The process is called boiling or vaporisation.

Water boils at 373 K (100 °C) at normal atmospheric pressure. Boiling is a bulk phenomenon, meaning it happens throughout the entire liquid, not just at the surface, which is why bubbles form everywhere inside boiling water.

How cooling reverses the change

Cooling removes energy from the particles, so the process runs backwards:

• Gas → liquid is called condensation (also liquefaction). The vapour loses energy, particles slow down, attraction pulls them close, and a liquid forms.
• Liquid → solid is called freezing or solidification. The particles slow further, lock into fixed positions, and a solid forms.

A useful fact: the freezing point of a liquid equals the melting point of its solid. Water freezes and ice melts at the same 273 K, the change just runs in opposite directions.

The Kelvin scale (and the mistake everyone makes)

Scientists measure temperature on the Kelvin (K) scale, the SI unit of temperature. To convert:

K = °C + 273 and conversely °C = K − 273

So 0 °C = 273 K (ice melts) and 100 °C = 373 K (water boils).

Worked example:
Question: Convert 25 °C (a comfortable room temperature in India) to the Kelvin scale.
Solution:
Step 1: Use the formula K = °C + 273.
Step 2: Substitute the value: K = 25 + 273.
Step 3: Add: K = 298.
Conclusion: 25 °C is equal to 298 K.

Worked example:
Question: A gas is stored at 300 K. What is this temperature in degrees Celsius?
Solution:
Step 1: Use °C = K − 273.
Step 2: Substitute: °C = 300 − 273.
Step 3: Subtract: °C = 27.
Conclusion: 300 K is equal to 27 °C.

Why it matters: Many exam marks are lost simply by forgetting to add or subtract 273. The Kelvin scale also has a deeper meaning, 0 K (absolute zero) is the temperature at which particle motion is theoretically at its minimum, so it is the natural "true zero" for measuring energy of particles.

Real-world example: Tiny water droplets appear on the outside of a cold soft-drink bottle. Water vapour already present in the warm air touches the cold surface, loses energy, and condenses into liquid drops, a state change happening on your bottle.

Common misconception: "You can skip the +273 when converting because it's a small number." Forgetting it gives a completely wrong answer (e.g. writing 100 K instead of 373 K for boiling water). Always apply K = °C + 273.

Common misconception: "Boiling and evaporation are the same thing." They are not. Boiling is a rapid, bulk process at a fixed temperature (the boiling point), while evaporation is a slow surface process that happens at any temperature below boiling.

During melting and boiling the temperature stays constant — the heat supplied becomes latent heat, used to change the state rather than raise the temperature.

Here is a genuine brain-bender: ice at 0 °C and water at 0 °C are at the same temperature, yet the water holds more energy. Where did the extra energy go, if the thermometer didn't move? The answer is latent heat, hidden heat. This lesson explains latent heat, why steam burns are so dangerous, and the special process of sublimation where solids leap straight to gas.

Definition: Latent heat (Latin latent = hidden) is the heat energy that is absorbed or released during a change of state without any change in temperature.

Why the temperature stays constant during a state change

When you heat ice, its temperature rises until it reaches 0 °C (273 K). Then something strange happens: even though you keep supplying heat, the temperature stops rising and stays fixed at 0 °C until all the ice has melted. Only after the last bit of ice becomes water does the temperature begin to climb again.

So where is the heat going? It is not raising the temperature, instead it is being used to break the force of attraction that holds the particles in their fixed solid positions. This energy gets "stored" inside the particles as potential energy. Because it does not show up on the thermometer, it is called hidden heat. This is exactly why water at 0 °C has more energy than ice at 0 °C, the water has absorbed all that latent heat to free its particles.

The two kinds of latent heat

Definition: The latent heat of fusion is the amount of heat required to change 1 kg of a solid into liquid at its melting point, with no change in temperature.

For ice, the latent heat of fusion is about 3.34 × 10^5 J/kg. This large value is why ice is so effective at keeping drinks cold, it soaks up a great deal of heat just to melt.

Definition: The latent heat of vaporisation is the amount of heat required to change 1 kg of a liquid into vapour (gas) at its boiling point, with no change in temperature.

For water, the latent heat of vaporisation is about 22.5 × 10^5 J/kg, much larger than the latent heat of fusion, because completely separating particles into a gas takes far more energy than just loosening them into a liquid.

Why steam burns are worse than boiling water

Both steam and boiling water are at 100 °C. So why does steam cause a far more severe burn? Because steam carries extra latent heat of vaporisation. When steam touches your skin, it first condenses back into water at 100 °C, releasing all that stored latent heat (22.5 × 10^5 J/kg) onto your skin, and then the hot water cools, releasing still more heat. Boiling water only delivers the second part. The hidden energy in steam makes the difference.

Sublimation

Definition: Sublimation is the change of a substance directly from solid to gas (and the reverse, gas directly to solid, is called deposition) without ever passing through the liquid state.

Some substances skip the liquid stage entirely when heated. Common Indian-context examples include camphor (kapoor) used in pujas, naphthalene balls used to protect clothes, ammonium chloride, and dry ice (solid carbon dioxide).

Real-world example: Naphthalene balls, kept in cupboards to keep moths away, slowly shrink and disappear over weeks without ever leaving a puddle behind. They are subliming, turning directly from solid into gas. Dry ice is used to keep ice cream cold during transport precisely because it turns straight into gas and leaves no liquid mess.

Why it matters: Latent heat explains how our bodies, refrigerators and pressure cookers manage energy, and sublimation is the basis of techniques like freeze-drying. Recognising that energy can be absorbed without a temperature change corrects a deep, common misunderstanding about heat itself.

Common misconception: "Adding heat always raises the temperature." False. During melting and boiling, added heat goes into latent heat (breaking particle forces), and the temperature stays constant until the change of state is complete.

Common misconception: "Boiling water and steam burn equally because both are at 100 °C." Steam burns are far worse because it releases extra latent heat of vaporisation as it condenses on the skin.

Heat an ice cube and it becomes water; heat the water and it becomes steam. These everyday transformations hide one of the most surprising facts in physics — that during melting and boiling, the temperature stops rising even though heat keeps pouring in. This lesson explains how matter changes state and reveals the "hidden heat," latent heat, behind the mystery.

Definition: A change of state is the conversion of matter from one physical state (solid, liquid, or gas) into another by changing temperature or pressure.
Definition: Latent heat is the heat energy absorbed or released during a change of state at constant temperature, used to overcome the forces of attraction between particles rather than to raise the temperature.

How matter changes state with temperature

All changes of state come down to the tug-of-war between particle attraction and particle motion. Adding heat gives particles more kinetic energy, making them move faster and weakening the grip of the attractive forces.

On heating a solid, its particles vibrate faster and faster. At a certain temperature — the melting point — the vibrations become strong enough to break the rigid arrangement, and the solid turns into a liquid. This is called melting (or fusion). The melting point of ice is 273.15 K (0 °C).

On further heating the liquid, particles gain still more energy until, at the boiling point, they break free of each other entirely and escape as a gas. For water this happens at 373 K (100 °C). This change is boiling (vaporisation).

On cooling, the reverse happens: a gas loses energy and becomes liquid (condensation), and a liquid loses energy and becomes solid (freezing).

Why it matters: Knowing the standard temperatures — ice melts at 273.15 K, water boils at 373 K (at normal atmospheric pressure) — and the names of each change is heavily tested and underpins the entire study of heat.

The surprise: temperature stays constant during a change of state

Here is the counter-intuitive heart of this lesson. If you put a thermometer in a beaker of melting ice and keep heating it, the temperature stays stuck at 0 °C until all the ice has melted — even though the flame is supplying heat the whole time. The same happens at the boiling point: water stays at 100 °C while it boils away.

Where is the heat going if not into raising the temperature? It is being used to break the forces of attraction between particles so they can rearrange into the new state. This "hidden" heat that does work without changing the temperature is latent heat (latent means "hidden").

Definition: The latent heat of fusion is the heat required to change 1 kg of a solid into liquid at its melting point, without any rise in temperature.
Definition: The latent heat of vaporisation is the heat required to change 1 kg of a liquid into gas at its boiling point, without any rise in temperature.

In both cases the energy is spent overcoming interparticle attraction, not heating the substance — which is exactly why the thermometer holds steady.

Worked example — the danger of steam

Question: Why does steam at 373 K cause more severe burns than boiling water at 373 K, even though both are at the same temperature?

Solution:
Step 1: Steam and boiling water are both at 100 °C (373 K), so neither is "hotter" than the other.
Step 2: To form steam, water at 100 °C had to absorb a large amount of extra energy — the latent heat of vaporisation.
Step 3: This latent heat is stored in the steam without raising its temperature.
Step 4: When steam touches the skin and condenses back into water, it releases this stored latent heat onto the skin, on top of the ordinary heat of cooling.

Conclusion: Steam delivers extra latent heat of vaporisation to the skin when it condenses, so it causes more severe burns than boiling water at the same temperature.

Real-world example: This is why a face held over a pot of boiling water (steam) can scald far worse than a quick splash of the hot water itself, and why steam is used to heat and sterilise — it carries a large hidden energy payload.

Common misconception: Students assume that if you keep heating ice its temperature must keep rising. In fact, while ice is melting the temperature holds steady at 0 °C until all the ice has melted, because the heat is being used as latent heat to break particle bonds rather than to warm the substance.

Common misconception: Some think latent heat "disappears." It does not — it is stored in the new state (in the spacing/freedom of the particles) and is fully released again when the substance changes back, which is exactly why condensing steam burns so badly.

A splash of boiling water hurts, but a blast of steam at the same temperature can scald far worse. How can two things at 100 °C cause such different injuries? The answer is one of the most elegant applications of latent heat, and this lesson works through it completely.

Definition: Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid into gas at its boiling point, without any rise in temperature.

Why temperature alone does not tell the whole story

Both boiling water and steam can sit at exactly 100 °C (373 K). Temperature measures the kinetic energy of particles — how fast they move — and on that count they are equal. So if temperature were the only thing that mattered, the two should burn equally. The fact that steam burns worse tells us there is extra hidden energy in steam that the thermometer does not reveal: its latent heat of vaporisation.

Why it matters: This question is a perfect test of whether you understand that energy is stored during a change of state, not just shown as temperature.

The worked example

Question: Both boiling water and steam are at 100 °C. Why does steam cause more severe burns?

Solution:
Step 1: Steam is the gaseous state of water at its boiling point, while boiling water is the liquid state at the same temperature.
Step 2: To convert water at 100 °C into steam at 100 °C, a large amount of energy — the latent heat of vaporisation — must be absorbed by the water.
Step 3: This latent heat is stored in the steam without raising its temperature (the thermometer still reads 100 °C).
Step 4: When steam touches the skin and condenses back into water, it releases this stored latent heat onto the skin, in addition to the ordinary heat given out as the resulting hot water cools.

Conclusion: Steam delivers extra latent heat of vaporisation to the skin on condensing, so it causes more severe burns than boiling water at the same temperature.

The intuition — a hidden energy payload

Imagine two delivery trucks arriving at your door, both driving at the same speed (same "temperature"). One truck is empty; the other is loaded with heavy cargo. The loaded truck does far more damage if it hits you, even at the same speed, because it carries extra energy. Steam is the loaded truck — it carries a large cargo of latent heat that boiling water simply does not have. When steam hits the skin and condenses, it dumps that whole cargo, on top of the heat it gives off while cooling.

Real-world example: Cooks and chai makers know to be careful of the steam escaping from a pressure cooker valve far more than the boiling liquid inside, because that steam can cause deep scalds. The same principle is why steam is used industrially to sterilise instruments and to transfer large amounts of heat efficiently.

Common misconception: Students think that because both are at 100 °C, they must burn the same. Temperature is equal, but total heat delivered is not — steam gives up its latent heat on condensing, delivering much more energy to the skin.

Common misconception: Some imagine the latent heat is "lost" once steam forms. It is not lost; it is stored in the steam and released in full when the steam condenses back to water — which is precisely what makes the burn worse.

Matter changes state when temperature or pressure changes; during the change the temperature stays constant because of latent heat.

• Melting (fusion): solid → liquid at the melting point (ice = 273.15 K = 0 °C); freezing is the reverse.
• Boiling (vaporisation): liquid → gas at the boiling point (water = 373 K = 100 °C at normal pressure); condensation is the reverse.
• Sublimation: solid → gas directly (camphor, ammonium chloride, dry ice); reverse is deposition.
• Kelvin conversion: K = °C + 273; °C = K − 273.
• During melting/boiling the temperature stays constant — heat supplied is used to break interparticle forces, not to raise temperature.
• Latent heat of fusion: heat to change 1 kg solid → liquid at melting point without temperature change; latent heat of vaporisation: for liquid → gas at boiling point.
• Steam at 100 °C burns worse than water at 100 °C because steam carries extra latent heat of vaporisation released on condensing.

Cover the answer, recall, then check. 8 cards on state changes and latent heat.

Q1. Name the process solid → liquid and the temperature at which it occurs.
A1. Melting (fusion); it occurs at the melting point.

Q2. What is sublimation? Give two examples.
A2. The direct change of a solid to gas without becoming liquid — e.g. camphor and ammonium chloride (also dry ice).

Q3. Convert 25 °C to Kelvin.
A3. K = °C + 273 = 25 + 273 = 298 K.

Q4. Why does temperature stay constant while ice is melting even though heat is added?
A4. The heat is absorbed as latent heat of fusion to overcome the forces holding the particles together, not to raise temperature.

Q5. Define latent heat of vaporisation.
A5. The heat required to change 1 kg of a liquid into vapour at its boiling point without any rise in temperature.

Q6. Why does steam cause a more severe burn than boiling water at the same 100 °C?
A6. Steam releases its additional latent heat of vaporisation when it condenses on the skin, delivering more heat than water alone.

Q7. What is the boiling point of water in Kelvin at normal pressure?
A7. 373 K (100 °C).

Q8. Name the change gas → liquid and give an everyday example.
A8. Condensation — e.g. water droplets forming on a cold glass or dew on grass.