Physical and Chemical Changes
NCERT Class 7 • Science • Chapter "Physical and Chemical Changes" | NCERT Class 9 • Science • Chapter "Matter in Our Surroundings"
Physical and Chemical Changes
In a physical change only the form, size, shape or state of a substance changes, and no new substance is formed. In a chemical change the substance itself is converted into one or more new substances with different properties, and the change is usually not easy to reverse.
Telling Them Apart
- A physical change is usually reversible, such as melting ice, dissolving sugar or stretching a rubber band.
- A chemical change is usually irreversible, such as burning paper, rusting of iron or cooking food.
- In a physical change the chemical composition stays the same; in a chemical change it does not.
- Signs of a chemical change include change of colour, evolution of gas, change in temperature, formation of a precipitate and change in smell.
- Burning of a candle involves both: melting of wax is physical while burning of wax vapour is chemical.
- A physical change involves relatively little energy; chemical changes often absorb or release substantial heat.
Rusting and Crystallisation
- Rusting is the slow chemical change in which iron reacts with oxygen and moisture to form a brown flaky oxide.
- Rusting needs both air and water, so iron does not rust in dry air or in boiled water free of air.
- Rust weakens structures, so preventing it protects bridges, vehicles, tools and machines.
- Rusting is prevented by painting, greasing, galvanising with a zinc coating, and by using stainless steel which is an alloy.
- Galvanisation works because zinc corrodes in place of iron and shields the surface beneath.
- Crystallisation is a physical change in which a pure solid separates from a hot saturated solution on cooling.
- Corrosion is the general term for the slow eating away of a metal surface by air, moisture or chemicals.
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance | Not formed | Formed |
| Reversibility | Usually reversible | Usually irreversible |
| Composition | Unchanged | Changed |
| Example | Melting of ice | Rusting of iron |
Rusting — the chemical change in which iron reacts with oxygen and moisture to form a brown flaky iron oxide.
Exam me kaise aata hai
- Rusting needs which two things — air and water
- Coating iron with zinc is called — galvanisation
- Melting of ice is which type of change — physical change
- Formation of a precipitate indicates a — chemical change
UPSC/State PSC ke liye
- Galvanisation protects even when the coating is scratched, because zinc is more reactive and corrodes sacrificially in place of the iron.
- Rusting is faster in coastal areas because salt in the moist air acts as an electrolyte and speeds up the reaction.
Yahan confuse hote hain
✗ All irreversible changes are chemical | Some physical changes like cutting paper are also hard to reverse | ✓
✗ Rusting happens in dry air | Rusting needs both air and moisture | ✓
✗ Crystallisation is a chemical change | Crystallisation is a physical change | ✓
Ek nazar me
- Physical change alters form only; chemical change makes a new substance.
- Colour change, gas, heat, precipitate and smell signal a chemical change.
- Rusting needs air and water and is prevented by painting, greasing and galvanising.
- Crystallisation is a physical change used for purification.
Ab practice karein
Matter aur changes se har exam me seedhe questions bante hain. ExamAtlas ke mock test se abhi practice karein.