Question
Classify the following as physical or chemical changes: (a) melting of ice, (b) burning of paper, (c) dissolving sugar in water, (d) rusting of iron, (e) tearing a cloth. How can you tell the difference?
(CBSE Class 6-7 Science)
Solution — Step by Step
Physical change: Only the form, size, or state changes. No new substance is formed. The change is usually reversible.
Chemical change: A completely new substance with different properties is formed. The change is usually irreversible.
| Change | Type | Reason |
|---|---|---|
| Melting of ice | Physical | Water changes state (solid to liquid), same substance H₂O |
| Burning of paper | Chemical | Paper becomes ash and CO₂ — completely new substances |
| Dissolving sugar in water | Physical | Sugar molecules are still sugar, just spread out in water |
| Rusting of iron | Chemical | Iron reacts with oxygen and moisture to form iron oxide (rust) — a new substance |
| Tearing a cloth | Physical | Cloth pieces are still cloth — only size changed |
Look for these clues:
- Change in colour (iron turns reddish-brown when it rusts)
- Gas produced (bubbles when vinegar meets baking soda)
- Heat or light released (burning produces both)
- Smell produced (food rotting)
- Irreversibility (you cannot un-burn paper)
If none of these signs are present, it is likely a physical change.
Physical vs Chemical Change Decision Tree
flowchart TD
A["Is a new substance formed?"] -->|"Yes"| B["Chemical change"]
A -->|"No"| C["Physical change"]
B --> D["Signs: colour change, gas, heat/light, smell"]
C --> E["Examples: melting, freezing, dissolving, tearing"]
B --> F["Usually irreversible"]
C --> G["Usually reversible"]Why This Works
The core difference is about composition. In a physical change, the chemical composition stays the same — only arrangement or state changes. In a chemical change, bonds break and new bonds form, creating entirely new substances with new properties.
Melting ice is still H₂O. But burning paper converts cellulose into CO₂ and water — completely different molecules.
Common Mistake
Dissolving sugar in water is a physical change, not chemical. The sugar molecules are intact — they are just surrounded by water molecules. If you evaporate the water, you get your sugar back. However, dissolving a reactive metal in acid IS a chemical change because a new substance forms. The word “dissolving” alone does not tell you the type — check whether new substances form.