Class 7 Science Chapter 5 Physical and Chemical Changes — Formulas & Key Points

Core Definitions and Classification Table

• Physical Change: A change in which no new substance is formed; generally reversible; examples include melting ice, dissolving sugar, cutting paper.
• Chemical Change: A change in which one or more new substances are formed; generally irreversible; examples include burning paper, rusting iron, cooking food.
• Reversible Change: A change that can be undone to get back the original substance; most physical changes are reversible.
• Irreversible Change: A change that cannot be undone; most chemical changes are irreversible.
• Rusting: The slow chemical change in which iron reacts with oxygen and moisture to form hydrated iron(III) oxide (rust).
• Crystallisation: The process of obtaining pure solid crystals from a solution by controlled evaporation and cooling.
• Galvanisation: The process of coating iron or steel with a protective layer of zinc to prevent rusting.

Physical vs Chemical Changes — Comparison Table

• Physical Change — No new substance is formed | Chemical Change — One or more new substances with different properties are formed
• Physical Change — Generally reversible | Chemical Change — Generally irreversible or difficult to reverse
• Physical Change — Only physical properties like state, shape, size change | Chemical Change — Chemical composition and properties change
• Physical Change — Examples: melting ice, dissolving salt, tearing paper, stretching rubber band | Chemical Change — Examples: burning wood, rusting iron, cooking egg, ripening fruit
• Physical Change — Energy change is usually small | Chemical Change — Energy change is usually large (heat, light may be released or absorbed)
• Physical Change — No change in mass (Law of Conservation of Mass still holds visibly) | Chemical Change — Mass is conserved but distributed among new substances

Rusting of Iron — Process and Prevention Methods

• Conditions for Rusting: (i) Presence of oxygen from air (ii) Presence of moisture or water (iii) Both must be present simultaneously
• Prevention by Painting: Applying paint creates a barrier that prevents contact of iron with oxygen and moisture
• Prevention by Oiling/Greasing: Thin layer of oil or grease prevents air and water from reaching the iron surface
• Prevention by Galvanisation: Coating iron with a thin layer of zinc; zinc is more reactive and gets oxidised first, protecting iron underneath
• Prevention by Chromium Plating: Similar to galvanisation but uses chromium; gives shiny appearance and rust protection
• Prevention by Alloying: Making stainless steel by mixing iron with chromium and nickel prevents rusting

Crystallisation Process — Step-by-Step Table

• Step 1 — Dissolving: The impure solid is dissolved in minimum amount of suitable solvent (usually water) by heating to make a saturated solution
• Step 2 — Filtration: The hot saturated solution is filtered to remove insoluble impurities
• Step 3 — Cooling: The clear filtrate is allowed to cool slowly; as temperature drops, solubility decreases and pure crystals start forming
• Step 4 — Separation: The crystals are separated by filtration; the liquid (mother liquor) contains most impurities
• Step 5 — Drying: Crystals are dried to remove adhering liquid
• Advantage over Evaporation: In simple evaporation to dryness, both pure substance and impurities solidify together; crystallisation allows separation because impurities remain dissolved

Galvanisation and Coating Methods

• Galvanisation Definition: Process of coating iron or steel with a thin layer of metallic zinc to prevent rusting
• Method: Iron object is dipped in molten zinc or zinc is electroplated onto the iron surface
• Why Zinc: Zinc is more reactive than iron (higher in reactivity series), so it reacts with oxygen first, forming a protective zinc oxide layer
• Common Galvanised Items: Water pipes, roofing sheets, buckets, wire fencing, nuts and bolts, car bodies
• Difference from Painting: Painting is physical barrier; galvanisation provides both barrier and sacrificial protection (zinc reacts instead of iron)
• Duration: Galvanised coating lasts many years; paint needs frequent renewal

Indicators and Tests for Chemical Changes

• Test for Carbon Dioxide: Pass the gas through lime water (calcium hydroxide solution); if CO₂ is present, lime water turns milky due to formation of insoluble calcium carbonate
• Chemical Equation: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (calcium carbonate makes the solution milky)
• Test for Water Vapour (Method 1): Use anhydrous copper sulphate (white powder); it turns blue in presence of water, forming hydrated copper sulphate
• Test for Water Vapour (Method 2): Use anhydrous cobalt chloride paper (blue); it turns pink in presence of moisture
• Observation During Burning: When a candle or any organic substance burns, water vapour and carbon dioxide are produced; this can be confirmed by the above tests
• Sign of Chemical Change: Evolution of gas, change in colour, change in temperature, formation of precipitate are all indicators

Important Examples Classified by Change Type

• Physical Changes: melting of ice, freezing of water, boiling of water, condensation of steam, dissolving sugar in water, dissolving salt in water, cutting of paper, tearing of cloth, breaking of glass, stretching of rubber band, magnetising iron, sublimation of camphor, formation of shadows, glowing of electric bulb
• Chemical Changes: burning of paper, burning of wood, burning of candle, rusting of iron, ripening of fruits, souring of milk, cooking of food, digestion of food, respiration, photosynthesis, fermentation of sugar to alcohol, curdling of milk (by adding lemon), burning of cooking gas, formation of manure from leaves
• Tricky Cases — Dissolving salt: physical (salt can be recovered by evaporation), Burning candle: melting is physical but burning is chemical
• Reversible Physical: melting-freezing, boiling-condensation, sublimation-deposition, magnetisation-demagnetisation
• Irreversible Physical: breaking glass, cutting paper (though no new substance, cannot be easily reversed)
• Energy Changes: burning releases heat and light (exothermic chemical change); melting absorbs heat (endothermic physical change)

Memory Tricks and Mnemonics for Quick Revision

• NEW-REV rule: Chemical change makes NEW substance, physical change is REVersible—if you remember NEW and REV you can classify most changes
• RUST = React Under Slow Transformation (iron + oxygen + moisture = rust slowly)
• GALV = Guard Against Long-term Vulnerability (galvanisation guards iron)
• For Crystallisation steps: D-F-C-S-D = Dissolve, Filter, Cool, Separate, Dry (in that order)
• CO₂ test: 'Lime Milky' rhymes with 'Carbon Silky' (imagine silky white lime water turning milky)
• Anhydrous copper sulphate: 'White to Blue means Water is True' (white powder turns blue = water present)
• Physical vs Chemical: 'If it LOOKS different but IS the same = Physical; if it IS different = Chemical'

Common Mistakes and Notation Errors to Avoid

• Mistake 1: Saying 'all physical changes are reversible'—breaking glass is physical but irreversible; correct statement is 'generally reversible'
• Mistake 2: Writing 'rusting needs oxygen' without mentioning moisture—both oxygen AND moisture are necessary
• Mistake 3: Confusing melting of candle wax (physical) with burning of candle wax (chemical)—melting just changes state, burning produces CO₂ and H₂O
• Mistake 4: Calling crystallisation a chemical process—it is a physical separation method; no new substance forms
• Mistake 5: Writing 'rust is iron oxide'—the complete correct name is 'hydrated iron(III) oxide' or 'rust'; plain iron oxide is a different compound
• Mistake 6: Stating painting and galvanisation are same—painting is barrier method (physical), galvanisation is sacrificial protection (chemical concept)
• Mistake 7: Forgetting to mention 'insoluble impurities removed by filtration' in crystallisation steps
• Mistake 8: Not specifying 'anhydrous' when writing copper sulphate test—hydrated (blue) copper sulphate will not work; must be anhydrous (white)

Three Solved Mini-Examples Applying Core Concepts

One-Glance Last-Minute Revision Box

• Physical Change = No new substance + Generally reversible | Examples: melting ice, dissolving salt, cutting paper
• Chemical Change = New substance formed + Generally irreversible | Examples: burning, rusting, cooking, ripening
• Rusting = Iron + Oxygen + Moisture → Hydrated iron(III) oxide (reddish-brown flaky coating)
• Rust Prevention Methods: Painting, Oiling, Galvanisation, Chromium plating, Alloying (stainless steel)
• Galvanisation = Coating iron with zinc layer; zinc is more reactive so oxidises first, protecting iron
• Crystallisation Steps: Dissolve → Filter → Cool → Separate → Dry (D-F-C-S-D)
• Crystallisation = Physical process to purify solids; impurities remain dissolved, pure crystals form on cooling
• CO₂ Test: Lime water turns milky (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O)
• Water Test: Anhydrous CuSO₄ (white) → Hydrated CuSO₄ (blue) OR Anhydrous CoCl₂ paper (blue) → pink
• Indicators of Chemical Change: Gas evolution, colour change, temperature change, precipitate formation, light emission
• Key Equations: None in this chapter (it is observational/conceptual, not formula-heavy like physics or chemistry higher classes)
• Common Confusions: Candle melting (physical) vs burning (chemical); dissolving (physical, reversible) vs reacting (chemical, irreversible)