States of matter & phase changes (particle view)
The idea
When ice melts or water boils, not a single particle is created, destroyed, or transformed — the same H₂O molecules simply change how they are arranged and how fast they move. You already picture solids as packed vibrating particles, liquids as close sliding ones, and gases as far-flying ones. A phase change is the journey between those pictures, and energy is the ticket: heating loosens the attractions holding particles in place, while cooling lets the attractions pull them back into order.
The most surprising rule appears on a thermometer. While a substance is actually melting or boiling, its temperature stops rising even though you keep heating it. Many people assume the temperature climbs the whole time, but during a phase change the incoming energy is spent prying particles away from each other rather than speeding them up — and speed is what temperature measures. Only when the change is complete does the temperature resume climbing. The same logic runs in reverse: freezing and condensing release energy while the temperature holds steady.
Worked example
You steadily heat a block of ice that starts at −10 °C until it has all turned to steam. Ice melts at 0 °C and water boils at 100 °C. Describe what the thermometer shows from start to finish, and explain where and why it pauses.
- From −10 °C up to 0 °C the ice particles vibrate faster and faster in their fixed pattern, so the thermometer climbs steadily.
- At 0 °C the reading freezes in place: every bit of incoming energy now goes into breaking the rigid arrangement so particles can slide, not into extra speed. The mixture sits at 0 °C until the last ice crystal melts.
- Once it is all liquid, the energy speeds the particles up again and the temperature climbs from 0 °C toward 100 °C.
- At 100 °C comes the second pause: energy is spent separating particles completely so they can fly off as gas. The water stays at 100 °C until it has all boiled away, and only the steam can then get hotter.
- Sanity-check against experience: a drink full of ice stays near 0 °C no matter how warm the day, exactly because melting pins the temperature down.
Answer. The temperature climbs from −10 °C, pauses at 0 °C during melting, climbs to 100 °C, then pauses again during boiling — the two plateaus are energy reorganizing particles instead of speeding them up.
Check your understanding
- Why does the temperature of melting ice stay at 0 °C even though the stove keeps pouring energy in?
- What happens to the energy released when steam condenses on a cold window, and where does it go?
- How would a heating curve look different for a substance that melts at 50 °C instead of 0 °C?
- How would you convince a skeptical friend that the molecules in ice, liquid water, and steam are all identical?
Build the foundations first
States of matter & phase changes (particle view) builds on these concepts. If any feel shaky, start there.