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Ice at 0 °C keeps absorbing heat yet stays at 0 °C until it has all melted. Spilt water disappears from the floor without ever boiling. This post explains where this topic sits in the new books, how the particle model explains melting, boiling and evaporation, and how to calculate latent heat, with a practice routine at the end.
Where is this chapter in 2026–27?
The CBSE syllabus for 2025–26 listed "Matter in Our Surroundings" first for Class 9: the particulate nature of matter, the three states, and changes of state including melting, freezing, evaporation (cooling by evaporation), condensation and sublimation.
The CBSE Class IX syllabus for 2026–27 follows the new NCERT book *Exploration*, which has no such chapter. The pieces now sit here:
Particle model, states, melting point, boiling point, evaporation: Grade 8 *Curiosity*, Chapter 7, Particulate Nature of Matter.
Sublimation (camphor): Class 9 Chapter 5, Exploring Mixtures and their Separation, as a separation method.
Latent heat with numbers: Class 11 Physics, Chapter 10, Thermal Properties of Matter.
If your board still follows the older book, all of this is examinable in Class 9. On the new book, it is background for Chapter 5 and a head start on Class 11.
What decides whether something is solid, liquid or gas?
Grade 8 *Curiosity* puts it in terms of interparticle attractions. In a solid, particles are tightly packed and held in place, so they only vibrate; the solid keeps its shape and volume. In a liquid, attractions are a little weaker, so particles move past each other: fixed volume, no fixed shape. In a gas, attractions are negligible and particles move freely, filling any container. Heating makes particles vibrate or move faster until they break free of their neighbours: that is melting, and at a higher temperature, boiling.
Why does temperature stop rising while ice melts?
While a solid melts, the heat you supply goes into pulling particles apart, not into making them move faster. Temperature is linked to how fast particles move, so it stays put until the change of state is complete. The heat absorbed per kilogram during a change of state is the latent heat: Q = mL.
The Class 11 NCERT chapter gives the values for water: the latent heat of fusion is 3.33 × 10⁵ J/kg and the latent heat of vaporisation is 22.6 × 10⁵ J/kg. It also notes that this is why burns from steam are usually more serious than burns from boiling water: steam at 100 °C carries far more heat than water at 100 °C.
Worked example. How much heat does 200 g (0.2 kg) of ice at 0 °C need to end up as steam at 100 °C? Use the specific heat capacity of water, 4186 J/(kg K), from the same chapter. A change of 1 K equals a change of 1 °C.
Melt the ice: Q₁ = mL_f = 0.2 × 3.33 × 10⁵ = 66,600 J.
Heat the water from 0 °C to 100 °C: Q₂ = mcΔT = 0.2 × 4186 × 100 = 83,720 J.
Boil it away: Q₃ = mL_v = 0.2 × 22.6 × 10⁵ = 452,000 J.
Total: 66,600 + 83,720 + 452,000 = 602,320 J, about 602 kJ.
Boiling the water away takes more than five times the heat needed to warm it from 0 °C to 100 °C. If you draw temperature against heat supplied, you get two flat stretches, one at 0 °C and one at 100 °C, joined by a sloping line. The calculation itself is Class 11 work, but the flat stretches are exactly what Class 9 and Grade 8 ask you to explain.
How is evaporation different from boiling?
*Curiosity* draws the line clearly. Boiling happens at one temperature, the boiling point, with vapour forming throughout the liquid, which is why you see bubbles. Evaporation happens at any temperature, slowly, and only at the surface.
Using the particle model, you can reason out what speeds evaporation up:
Larger surface area: more particles at the surface can escape. Clothes dry faster spread out than folded.
Higher temperature: more particles have enough energy to escape.
Wind: moving air carries away the escaped particles, so fewer return.
Drier air: the air can take up more vapour, so fewer particles return to the liquid.
Why evaporation cools. The particles that escape are the faster ones, so the ones left behind have less energy on average and the liquid cools. Sweat cools your skin this way, and water seeping through an earthen pot evaporates from its outside surface and cools the water inside.
Which ideas do students get wrong?
Osborne and Cosgrove (1983) interviewed children about evaporating, condensing, boiling and melting, and found their ideas could be quite different from scientists' views, and that science teaching could sometimes influence those ideas in ways teachers did not intend. Check your own answers against these four statements, all of which are wrong:
"The bubbles in boiling water are air." They are water vapour.
"Water on a cold glass leaked through it." It is water vapour from the air condensing.
"Evaporation needs boiling." It happens at all temperatures, just slowly.
"Temperature rises steadily while ice melts." It stays at 0 °C until all the ice has melted.
Changes of state are easier to reason about when you can watch particles speed up and break apart. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read, so you can predict what happens at each stage before you watch it.
A practice routine
Explain in three sentences, using particles, why ice stays at 0 °C while it melts.
List the four factors that speed up evaporation and give one everyday example for each.
Do the latent-heat calculation above for 500 g of ice, then check your answer by scaling the 200 g result.
Move on to the next chapter in the new book with our guide to mixtures and separation in Class 9. When you reach Class 11, our post on calorimetry and heat transfer takes these ideas further.
Tonight, put a thermometer in a glass of ice water and note the reading every two minutes until the ice is gone.
