Chemical Changes

• Chemical change — new substances with different properties formed.
• Often irreversible — burnt wood cannot become wood again.
• Signs: colour change, gas evolved, heat/light, precipitate, smell.
• Examples: rusting, burning, digestion, curdling milk, photosynthesis.
• Rusting — iron + oxygen + water → rust (hydrated iron oxide).
• NCERT Activity — burn magnesium ribbon (teacher demo).

Level 1 — Core idea

1. Burning — fuel + O₂ → CO₂ + H₂O + ash (new products).

2. Rusting — iron → reddish-brown flaky rust; weakens gate/grill.

3. Digestion — starch → sugars; proteins → amino acids in body.

Level 2 — Process and representation

1. Diagram (text) — iron nail in moist air → rust after days.

2. Curd from milk — lactic acid formation; new taste and texture.

3. Magnesium burn — dazzling white flame; white MgO ash.

Level 3 — Applications and NCERT links

1. Real world — painting iron gate prevents rust; galvanisation with zinc.

2. NCERT — vinegar + baking soda → CO₂ bubbles (chemical).

3. Energy — many chemical changes release or absorb energy.

Observing rust formation on iron nails (NCERT-style)

Materials: three iron nails, water, oil, dry air jar, labels.
Step 1 — Nail A: exposed to moist air and water.
Step 2 — Nail B: dipped in oil (blocks water/air).
Step 3 — Nail C: dry jar with calcium chloride (dry air).
Step 4 — Wait 5–7 days; observe.
Step 5 — Nail A rusts; B and C little or no rust.
Step 6 — Conclude: both **water and oxygen** needed for rusting.
Step 7 — Prevention: paint, grease, galvanise — barrier to air/water.
Conclusion: rusting is chemical change forming new substance iron oxide.

• Define chemical change with two signs.
• Write conditions needed for rusting.
• Is burning paper physical or chemical?
• Name one way to prevent rust on bicycle chain.
• What new substance forms when magnesium burns?
• What three things needed for rusting?

Open the Practice tab for graded questions on Chemical Changes.