Changes of state (melting, freezing, evaporation)
The idea
You already know matter can be a solid, a liquid, or a gas. A change of state happens when one of these turns into another. Melting turns a solid into a liquid, like an ice cube becoming water in your warm hand. Freezing turns a liquid into a solid, like juice becoming an ice pop. Evaporation turns a liquid into a gas that drifts invisibly into the air, like a puddle slowly disappearing.
The key idea: it is the same stuff before and after, just in a new state. Warming pushes matter toward melting and evaporation; cooling pushes it toward freezing. Water freezes and ice melts at the same temperature, 0 °C — below it, liquid water turns solid; above it, ice turns liquid. Evaporation also has a partner change called condensation, when invisible water gas in the air turns back into liquid drops, like the mist on a cold window.
When a puddle dries up, it is tempting to say the water is gone. It is not gone. It evaporated: it turned into an invisible gas called water vapor and floated into the air. The very same water can show up again later as drops on a cold glass or as rain. Drying is a change of state, not a disappearing act.
Worked example
You pour liquid juice into a mold and put it in the freezer overnight. The next hot afternoon you eat the ice pop outside, and a few drops fall onto the sidewalk. By evening the drops are gone. Name each change of state in this story.
- In the freezer, the liquid juice turns into a solid ice pop, and a liquid turning into a solid is called freezing.
- Out in the hot afternoon, the solid pop turns soft and drippy: the solid is becoming liquid again, which is melting.
- The fallen drops sit as liquid on the warm sidewalk, and by evening they have turned into invisible water vapor in the air — a liquid turning into a gas is evaporation.
- Notice the pattern: cooling caused the freezing, while warmth caused both the melting and the evaporation, and the juice's water was the same stuff the whole time.
Answer. The story shows freezing in the freezer, then melting in the afternoon heat, then evaporation of the drops on the sidewalk.
Check your understanding
- Why does a puddle dry faster on a hot, breezy day than on a cool, still one?
- How would you explain where the water goes when wet laundry dries on a clothesline?
- Melting and freezing both happen at 0 °C for water — what decides which one actually happens?
- What clues would tell you that the drops on the outside of a cold glass came from the air, not from inside the glass?