Ice in the freezer, water in a glass and steam rising from a kettle are all the same water, but in different states: solid, liquid and gas.
The difference lies in how actively the particles move and how closely they are packed, and gaining or losing heat changes the particles' motion and so the state.
While ice melts or water boils, the temperature does not rise even though heat keeps going in, because the heat is used to change how the particles are arranged.
Raise the temperature from −20 ℃ to 120 ℃ and watch the particles first jiggle in place, then slide past one another, and finally scatter through the whole container.
Three States of Matter
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Water particles at different temperatures: solid (vibrating in place), liquid (sliding past), gas (scattering everywhere)
20℃
💡 Particles in the Three States
①Solid: particles sit in regular rows, attract one another strongly and only vibrate in place, so the shape and volume are fixed.
②Liquid: particles stay close but slide past one another; the volume is fixed but the shape follows the container.
③Gas: particles are far apart and move fast in all directions, so neither shape nor volume is fixed.
④The higher the temperature, the more actively the particles move.
Six Changes of State
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The six changes of state between solid, liquid and gas, and the flow of heat
🔍 Six Changes of State
①Melting: solid → liquid (ice melts into water)
②Freezing: liquid → solid (water freezes into ice)
③Vaporization: liquid → gas (wet laundry dries, water boils)
④Condensation: gas → liquid (droplets form on a cold glass)
⑤Sublimation: solid → gas (dry ice shrinks); deposition: gas → solid (frost forms on a window on a cold day)
Changes of State and Heat Energy
Changes that absorb heat
toward solid → liquid → gas: heat absorbed
Melting, vaporization and sublimation take heat from their surroundings, which is why alcohol on the back of your hand feels cool.
Changes that release heat
toward gas → liquid → solid: heat released
Condensation, freezing and deposition give heat to their surroundings.
🧊 Temperature During a Change of State
①While the state is changing, the temperature stays the same even as heat keeps going in.
②The heat is not used to speed up the particles but to change how they are arranged.
③At 1 atm, water melts at 0 ℃ (melting point) and boils at 100 ℃ (boiling point).
④Melting and boiling points are fixed for each substance, so they help identify substances.
Exam Key Points
Summary
change of state ← heat absorbed or released
Going from solid toward liquid and gas, heat is absorbed; going the other way, heat is released.
🎯 Exam Points
①Solids, liquids and gases differ in how their particles are arranged and move.
②Match each of the six changes with the direction of heat flow (absorbed or released).
③While the state changes, the temperature stays constant (melting point, boiling point).
④In boiling, vaporization happens inside the liquid too; in evaporation, only at the surface.
⑤Examples of sublimation and deposition: dry ice, frost.
Examples and Unit Test
Example 1
What is the change of state when ice melts into water, and does it absorb or release heat energy?
1
A change from solid to liquid is called melting.
2
Melting has to break up the regular arrangement of particles, so it absorbs heat energy from the surroundings.
▸ Melting / absorbs heat energy
Melting ice cools a drink for the same reason: as it melts it absorbs heat from the drink.
Example 2
Why does the temperature stay the same while ice melts (at 0 ℃) even as you keep adding heat?
1
While the state is changing, the heat added is used to change the arrangement of the particles.
2
It is not used to speed the particles up, that is, to raise the temperature, so the temperature stays at 0 ℃ until all the ice has melted.
▸ The heat goes into rearranging the particles, so the temperature stays constant
Set the picture to 0 ℃ or 100 ℃ and a note appears explaining that the temperature holds while the state changes.
Unit-test style
Which statement about changes of state is correct?
①Vaporization is a change that releases heat energy
②Melting and vaporization are changes that absorb heat energy
③The temperature keeps rising during a change of state
④In a gas, particles vibrate in place
⑤Freezing is a liquid changing into a gas
▸ ② Melting and vaporization are changes that absorb heat energy
1
In melting (solid → liquid) and vaporization (liquid → gas) the particle arrangement loosens and heat is absorbed, so ② is correct.
2
Vaporization absorbs heat (①), the temperature stays constant while the state changes (③), particles in a gas scatter quickly (④), and freezing is a liquid turning into a solid (⑤).