Observing Melting and Freezing Notes & Questions | Class 3 Chemistry | Drishti OS

Observing Melting and Freezing

What you'll learn

Melting and freezing are changes that can be reversed — water can go from ice to liquid and back to ice again.
Heating causes melting (solid to liquid); cooling causes freezing (liquid to solid).
We see these changes all around us: ice melting in a drink, frost forming on cold windows, ice cream melting in the sun.
The material (water) stays the same substance the whole time — only its state changes.

Key concepts

Level 1 — Core idea

Verbal: Freeze water into ice, let it melt back into water on the counter, then put it in the freezer again — it turns to ice once more. Nothing was lost, only the state changed.

Symbolic: Solid <-> Liquid (heat added = melt; heat removed = freeze).

Visual: Frost on a cold window forms as tiny water droplets freeze into ice crystals overnight.

Level 2 — Going deeper

This back-and-forth change between solid and liquid, caused by adding or removing heat, is called a reversible change of state.

NCERT anchor

NCERT Looking Around 3 — everyday observations of ice, frost, and melting foods.

Worked example

If you freeze, melt, then freeze water again, is it still water?

Step 1 — Freezing and melting only change the state (solid/liquid).
Step 2 — No new substance is formed; it is water throughout.
Answer: **Yes**, it remains water each time.

What causes frost to form on a cold window?

Step 1 — Water vapour or droplets touch the very cold glass.
Step 2 — They lose heat and freeze into tiny ice crystals.
Answer: **Freezing** causes the frost.

Common mistakes

Mistake	Why	Fix
Melted ice cannot refreeze	Thinking the change is permanent	Melting and freezing can repeat — it is reversible
Melting/freezing are never seen at home	Not observing carefully	They can be seen daily — ice, frost, melting food
Cooling melts things	Swapping cause and effect	Cooling causes freezing; heating causes melting
Water becomes a new substance when frozen	Confusing change of state with new substance	It is still water — only the state changed

Quick check

Can melted ice be frozen again?
What causes melting? What causes freezing?
Give one everyday example of freezing.
Stretch: Explain why frost on a window is an example of freezing.

Revision tip: Remember: state changes are usually reversible — heat it, it melts; cool it, it freezes again.

Open the Practice tab for graded questions on Observing Melting and Freezing.

Observing Melting and Freezing