Science · Class 7 · Chapter 5

Changes Around Us: Physical and Chemical

• Physical change — Shape, size, or state changes; no new substance.
• Chemical change — One or more new substances form.
• Combustion — Reacting with oxygen, releasing heat and/or light.
• Ignition temperature — The lowest temperature a substance catches fire at.
• Reversible change — The original substance can be recovered.
• Irreversible change — The original form cannot be recovered.
• Weathering — Rocks broken down by physical and chemical action.
• Erosion — Broken-down material carried away by wind or water.

Chapter 5 · Concept 1 of 9

Physical Changes

• Folding paper, inflating a balloon, crushing chalk into powder — the material stays the same substance even though its shape or size changes.
• In one line — A physical change alters only physical properties — shape, size, or state — with no new substance formed.

Chapter 5 · Concept 2 of 9

Chemical Changes

• Blowing breath into plain water causes no change — but blowing into lime water turns it milky.
• A new substance forms — Calcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate (insoluble, milky) + Water.

Chapter 5 · Concept 3 of 9

Vinegar and Baking Soda

• Bubbles = a new gas — Mixing vinegar or lemon juice with baking soda produces fizzing — carbon dioxide gas escapes, confirmed by turning lime water milky.

Chapter 5 · Concept 4 of 9

Combustion and the Fire Triangle

• Combustion happens when a substance reacts with oxygen, releasing heat and often light.

Chapter 5 · Concept 5 of 9

Ignition Temperature

• What it means — The minimum temperature at which a substance catches fire.
• Seeing it happen — Focusing sunlight through a magnifying glass onto paper heats a small spot until it smokes and ignites.

Chapter 5 · Concept 6 of 9

Burning a Candle: Both at Once

• Physical changes — Solid wax melting, flowing up the wick, and evaporating.
• Chemical change — The burning of wax vapour itself, producing the flame — a genuinely new process, happening at the same time as the physical ones.

Chapter 5 · Concept 7 of 9

Reversible and Irreversible Changes

• Reversible — The original substance or object can be recovered — melted ice can be refrozen, water vapour can be condensed back.
• Irreversible — The original form cannot be recovered — chopped vegetables, or popcorn made from corn.

Chapter 5 · Concept 8 of 9

Desirable and Undesirable Changes

• Desirable — Milk turning to curd, fruit ripening, cooking food.
• Undesirable — Rusting of iron, food decay — though the same kind of change (decomposition) can be desirable elsewhere, like in a compost pit.

Chapter 5 · Concept 9 of 9

Weathering and Erosion

• Weathering — Temperature swings, root growth, and freezing water (physical) — plus reactions with water and air (chemical) — slowly break rocks down into soil.
• Erosion — Wind and flowing water carry away broken rock, soil, and sediment — a physical change that reshapes the land over time.

Quick Recap

Check Your Understanding

1. 1Which statements describe a physical change? (i) The state may or may not change. (ii) A substance with different properties forms. (iii) No new substance is formed. (iv) The substance undergoes a chemical reaction.
2. 2State True or False, correcting the false ones: (i) Melting wax is necessary for a candle to burn. (ii) Condensing water vapour is a chemical change. (iii) Composting leaves is a chemical change. (iv) Mixing baking soda with lemon juice is a chemical change.
3. 3Is water changing to ice, and water changing to steam, a physical or chemical change? Explain.