Class 7 Science • Chapter 05

Physical and Chemical Changes

Vardaan Learning Institute • Detailed Chapter Notes

🔄 1. Classification of Changes

⚡ Changes in Matter

⬇

PHYSICAL CHANGE

✅ Reversible (usually)
NO new substance formed
Only shape, size, state changes
Chemical composition SAME

Examples:
Melting ice → water
Cutting paper
Dissolving sugar in water
Stretching rubber band
Breaking a glass

CHEMICAL CHANGE

❌ Irreversible (usually)
NEW substance(s) formed
Properties completely different
Chemical composition CHANGES

Examples:
Burning of paper/wood
Rusting of iron
Cooking food (egg, bread)
Souring of milk
Photosynthesis, Respiration

📊 2. Comparison Table

Feature	Physical Change	Chemical Change
New substance formed?	❌ No	✅ Yes
Reversible?	Usually ✅ Yes	Usually ❌ No
Chemical composition	Remains the same	Changes completely
Energy change	Small or no change	Energy absorbed or released
Signs of change	Change in shape/size/state	Gas evolved, colour change, heat/light produced, precipitate formed, odour change
Examples	Freezing water, tearing paper, dissolving salt, melting wax	Burning, rusting, digestion, cooking, explosions, ripening of fruit

📸 AI Image Prompt

A two-panel side-by-side educational illustration: Left panel "Physical Change" — show three examples: (a) An ice cube melting into water with a double-headed arrow (reversible) showing the same molecules in solid and liquid form, (b) a piece of paper being cut into smaller pieces (same chemical, just smaller), (c) a copper wire being bent into different shapes (same material). The panel has a blue frame and label "Physical Change — Same substance, different form". Right panel "Chemical Change" — show three examples: (a) A piece of iron rod with reddish-brown rust forming on it (Fe₂O₃), titled "Rusting = new substance", (b) a candle flame burning and turning to ash and CO₂ gas (with CO₂ bubbles shown), (c) an egg being fried on a hot pan — egg white solidifying permanently. The panel has an orange frame and label "Chemical Change — New substance formed, IRREVERSIBLE". Clean, colorful, educational illustration style.

Fig. 5.1 — Comparison of physical and chemical changes

🦀 3. Signs That a Chemical Change Has Occurred

            When you see ANY of these signs, a chemical change is likely happening:
            🫧 Gas evolved — bubbles form (e.g. acid on chalk → CO₂ released)
🌈 Colour change — a new, permanent colour appears (e.g. iron rusts from grey →
                    orange-brown)
🌡️ Heat/Light produced — fire, glow, warmth generated (e.g. burning wood gives
                    light + heat)
🪨 Precipitate formed — solid forms in a clear liquid (e.g. mixing silver nitrate +
                    salt water → white solid AgCl precipitate)
👃 Odour/Smell change — new smell develops (e.g. burning rubber, rotten eggs)

        

🦀 4. Rusting of Iron — A Chemical Change

Iron (Fe)

Oxygen (O₂)

Water (H₂O)

⬇

Fe₂O₃ · xH₂O (Rust — hydrated iron oxide)

            Conditions needed for rusting: Both oxygen AND water (moisture) must be present. In dry air
            or under water alone → iron does NOT rust easily.


            Prevention of rusting:
            Painting/oiling/greasing — keeps air and moisture away
Galvanisation — coating iron with zinc (zinc sacrifices itself to protect iron)
                
Electroplating — depositing a protective metal coat (tin, chromium, nickel)
Alloying — mixing iron with carbon and chromium → stainless steel
                    (doesn't rust!)

        

🧪 5. Crystallisation — A Physical Change

            Crystallisation is the process of obtaining pure crystals of a solid substance
            from its solution by slow evaporation. Example: obtaining pure salt crystals from seawater by evaporation.
            This is a physical change — no new substance is formed. The salt can be re-dissolved
            easily.
        

📝 6. Quick Revision

            Physical change: No new substance, reversible, same chemical composition. E.g.
                    melting ice, dissolving sugar
Chemical change: New substance formed, usually irreversible. E.g. burning, rusting,
                    cooking
Signs of chemical change: gas evolved, colour change, heat/light, precipitate, odour change
Rusting: Iron + O₂ + H₂O → Fe₂O₃ (rust). Needs BOTH oxygen and moisture
Prevent rusting: paint, oil, galvanise (zinc coat), electroplate, make stainless steel
Galvanisation = coating iron with zinc to prevent rust
Crystallisation = physical change — salt recovered unchanged from solution by
                    evaporation

        