Physical and Chemical Changes
Overview
Ice melts, becomes water — same water. Paper tears — still paper. But paper burns — becomes ash, cannot go back! The first change is physical, the second is chemical. Let us learn to identify which is which.
Typed Notes
Physical change — only a change in form
In a physical change, the shape, size, or state (solid-liquid-gas) of a substance changes — but the substance itself remains the same: ice melts — becomes water — freezes again — becomes ice (still the same H2O); paper tears — into pieces — but it is still paper; sugar dissolves in water — water tastes sweet — sugar can be recovered back (by evaporation); iron is magnetised — demagnetise it — the magnetism is lost. Physical changes are usually reversible — the energy change involved is small.
Chemical change — a new substance is formed
In a chemical change, a new substance is formed — with different properties: iron rusts (turns brown) — cannot be turned back into iron; paper burns — produces CO2 + ash + light and heat; milk turns into curd (bacteria bring about the change); flour becomes bread; food is digested. Signs: (1) colour changes (rust); (2) a gas is produced (smoke, bubbles); (3) heat and light are released (burning); (4) a new taste or smell appears (curd). Usually the change cannot be reversed.
Rusting and galvanisation
When iron comes into contact with both oxygen and water together — brown rust (iron oxide) is formed. Remember: only air is not enough, only water is not enough — both are needed. Prevention: (1) paint (gates, railings); (2) oil or grease (machines); (3) galvanisation — dipping the iron object in molten zinc so that a thin layer of zinc coats it — zinc itself corrodes first and protects the iron underneath — even if the surface is scratched. Uses: rooftop sheets, railway tracks, road poles. Ship hulls — magnesium block (sacrificial protection). Stainless steel — iron + chromium + nickel — does not rust — used for utensils.
Crystallisation — obtaining pure crystals
An impure salt or sugar is dissolved in hot water — then the solution is allowed to cool slowly or the water is allowed to evaporate — clean, large, regularly shaped crystals are obtained — this process is called crystallisation. Uses: obtaining salt from the sea, refining sugar, preparing pure samples in the laboratory. This is a physical change — not a chemical change (only the form changes; the substance is absorbed and then separated out). The largest crystals are those of sugar candy (rock sugar or mishri).
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Physical change: only shape, size, or state changes — no new substance is formed; usually reversible: ice⇄water, tearing paper, dissolving sugar in water, magnetising iron.
- 02Chemical change: a new substance is formed — properties change; usually irreversible: rusting, burning, curd formation, cooking food, digestion.
- 03Rusting: iron + oxygen + water (both together) → brown rust. Prevention: paint, oil, grease, galvanisation (coating with a layer of zinc — rooftop sheets, railway tracks).
- 04Crystallisation: obtaining pure crystals from a solution — large, clean crystals of sugar or salt — sea salt, pure samples in the laboratory.
- 05In a chemical change, heat, light, and sound can also be released (burning), and a new gas can also be produced (CO2 — dough rising, curd formation).
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the difference between a physical change and a chemical change? Give two examples of each.
What are the signs (indicators) of a chemical change?
What conditions are needed for rusting? How can it be prevented?
What is galvanisation? Where is it used?
When a sugar solution is allowed to cool, why do crystals form?
Is the conversion of milk into curd a physical or chemical change? Why?
Physical change: only shape, size, or state changes; no new substance is formed; usually reversible (ice, tearing, dissolving). Chemical change: a new substance is formed; usually irreversible (rusting, burning, curd formation, cooking). Rusting = iron + oxygen + water — prevention: paint, oil, galvanisation (zinc). Crystallisation gives pure crystals.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which of the following is a physical change?
Q2.What conditions are necessary for rusting?
Q3.In galvanisation, iron is coated with which metal?
Q4.Is the conversion of milk into curd a physical or chemical change?
Mark karo — dashboard par progress dikhega.