Chapter 5: Changes Around Us: Physical and Chemical
Folding paper, melting ice, setting curd, rusting iron, a candle burning, rocks crumbling: change is everywhere. In this chapter we learn to tell physical from chemical changes, test for carbon dioxide with lime water, meet the fire triangle, sort changes as reversible or not and desirable or not, and watch nature change rocks very slowly.
In this chapter you will
- tell physical changes from chemical changes
- test for carbon dioxide with lime water
- explain rusting, combustion and the fire triangle
- classify changes as reversible or irreversible, desirable or undesirable
Topics
- Physical and Chemical Changes NCERT 5.1–5.2
Physical change: only shape, size or state changes; no new substance.
- Rusting and Combustion NCERT 5.3
Rusting forms iron oxide; burning magnesium forms magnesium oxide: both are chemical changes.
- Reversible, Permanent and Desirable Changes NCERT 5.4–5.6
Reversible changes can be undone; irreversible ones cannot, whether physical or chemical.
- Slow Natural Changes NCERT 5.7
Weathering breaks rocks, physically (heat, ice, roots) or chemically (iron oxide on basalt).
Key words
- Physical change
- only shape, size or state changes; no new substance
- Chemical change
- a change in which new substances form
- Combustion
- burning in oxygen giving heat and light
- Ignition temperature
- lowest temperature at which a substance catches fire
- Rust
- iron oxide formed on iron in moist air
- Weathering
- slow breaking of rocks
- Erosion
- carrying away of broken rock and soil
Summary
- Physical changes alter shape, size or state; chemical changes form new substances.
- Carbon dioxide turns lime water milky; rusting and burning are chemical changes.
- Combustion needs fuel, oxygen and heat to reach the ignition temperature.
- Changes can be reversible or irreversible, desirable or undesirable; weathering and erosion are slow natural changes.