Why this chapter matters for UPSC: Corrosion, galvanisation, and metal protection are relevant to infrastructure and technology. The distinction between physical and chemical changes is fundamental for chemistry and environmental science questions.

PART 1 — Quick Reference Tables

Physical vs Chemical Changes

Feature Physical Change Chemical Change
New substance formed? No — same substance, different form Yes — entirely new substance(s) with different properties
Reversible? Usually reversible Usually irreversible
Chemical composition Unchanged Changed
Examples Melting ice, dissolving salt in water, cutting paper, breaking glass, change of state Burning, rusting, cooking food, curdling milk, photosynthesis, digestion

PART 2 — Detailed Notes

Rusting and Corrosion

Key Term

Rusting (chemical change):

Iron + oxygen + water → iron oxide (rust) 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → Fe₂O₃.nH₂O (hydrated iron oxide = rust)

Conditions required: Both oxygen AND water are needed — iron does NOT rust in:

  • Dry air (no water)
  • Boiled/degassed water sealed from air (no dissolved oxygen)

Prevention of rusting:

  1. Painting: Barrier between iron and air/water (used for bridges, ships — frequent repainting needed)
  2. Oiling/greasing: Blocks air and water contact
  3. Galvanisation: Coating iron/steel with zinc — zinc corrodes preferentially protecting iron underneath; used for roofing sheets, pipes, buckets
  4. Electroplating: Depositing a protective metal layer using electricity (chrome on car parts, tin on food cans, gold on jewellery)
  5. Alloying: Mixing iron with other metals — stainless steel (iron + chromium + nickel) does NOT rust
  6. Cathodic protection: Connecting iron structure to a more reactive metal (zinc, magnesium) which corrodes instead of iron; used for underground pipelines and ship hulls

Economic cost of corrosion:

  • Globally: ~3–4% of GDP lost annually to corrosion damage
  • India: Infrastructure projects (bridges, railways, coastal buildings) face significant corrosion costs
  • Coastal areas (high humidity + salt water) have accelerated corrosion rates

Galvanisation vs Electroplating:

Galvanisation Electroplating
Process Hot-dipping in molten zinc Electric current deposits metal ions
Metals used Zinc only (on iron/steel) Any metal can be deposited
Used for Roofing, pipes, fencing Jewellery, electronic parts, car parts, food cans

Chemical Changes and Their Significance

Explainer

Crystallisation: Process of obtaining pure crystals of a substance from its solution by slow evaporation or cooling.

  • Salt crystals from seawater (solar evaporation in salt pans)
  • Sugar crystals from sugar solution
  • Application: Purification of substances; growing large pure crystals for electronics (silicon wafers, quartz crystals)

Burning (combustion): Fuel + oxygen → CO₂ + H₂O + energy (heat and light)

  • Complete combustion → CO₂ + H₂O (clean burning)
  • Incomplete combustion → CO (carbon monoxide, toxic) + soot (particulates) — indoor air pollution from chulhas

Burning vs rusting: Both are oxidation reactions (combination with oxygen), but:

  • Burning: Fast; produces heat and light; flame
  • Rusting: Slow; no visible heat/light

Cooking (chemical change):

  • Proteins are denatured (unfolded) by heat → texture changes permanently
  • Sugars caramelise
  • Starches gelatinise
  • Maillard reaction (between amino acids and sugars) → browning, new flavours
  • Cannot be reversed — cooked food cannot become raw

Neutralisation: Acid + Base → Salt + Water HCl + NaOH → NaCl + H₂O (Chemical change — new substance NaCl formed)

Exam Strategy

Prelims traps:

  • Dissolving salt in water = physical change (can recover salt by evaporation) — NOT chemical
  • Rusting = chemical change (iron oxide is a NEW substance, very different from iron)
  • Rusting requires BOTH water AND oxygen — neither alone causes rusting
  • Galvanisation uses ZINC (NOT chromium, NOT nickel)
  • Stainless steel = iron + chromium (+nickel) — resists corrosion; NOT galvanised steel

Previous Year Questions

Prelims:

  1. Which of the following is a chemical change?
    (a) Melting of wax
    (b) Dissolving sugar in water
    (c) Rusting of iron
    (d) Changing shape of a piece of clay

  2. Galvanisation, used to prevent rusting of iron, involves coating the iron with:
    (a) Chromium
    (b) Zinc
    (c) Tin
    (d) Nickel