Chapter 5 — Changes Around Us: Physical and Chemical

Chapter 5, 'Changes Around Us: Physical and Chemical', teaches students to distinguish between physical changes (only shape, size, or state changes; no new substance forms) and chemical changes (new substances are produced). It covers combustion, rusting, reversibility of changes, desirable vs undesirable changes, and natural changes like weathering and erosion.

In a physical change, only physical properties like shape, size, or state alter and no new substance is formed — for example, folding paper, crushing chalk, or melting ice. In a chemical change, one or more new substances are formed through a chemical reaction — for example, blowing exhaled air into lime water forms calcium carbonate, turning the lime water milky.

Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Substances that undergo combustion are called combustible substances. Examples given in the chapter are wood, paper, cotton, and kerosene.

According to the fire triangle described in the chapter, combustion requires: (i) a combustible substance (fuel), (ii) oxygen, and (iii) heat that raises the fuel to its ignition temperature.

Ignition temperature is the minimum temperature at which a substance catches fire. The chapter illustrates this by showing that focusing sunrays through a magnifying glass slowly heats paper until it reaches its ignition temperature and begins to burn, even without an open flame.

The covered candle stops burning because the glass tumbler cuts off the continuous supply of air (specifically oxygen). Once the oxygen inside is used up, combustion cannot continue and the flame is extinguished. The presence of carbon dioxide inside the tumbler can be confirmed by adding lime water to the petri dish — it turns milky.

Burning a candle involves both. The melting of wax, its flow up the wick, and its evaporation are physical changes. The burning of the wax vapour to produce a flame is a chemical change because new substances are formed. So the process involves both physical and chemical changes simultaneously.

Pass the gas through freshly prepared lime water. If carbon dioxide is present, the lime water turns milky (cloudy) due to the formation of calcium carbonate, which is insoluble in water and settles at the bottom.

When vinegar (or lemon juice) is mixed with baking soda (sodium hydrogen carbonate), a fizzing/bubbling sound is heard and gas bubbles form. The gas produced is carbon dioxide (confirmed by passing it through lime water, which turns milky). Since a new substance (carbon dioxide) is formed, this is a chemical change.

Rusting of iron is a chemical change because a new brown-coloured substance called rust (iron oxide) is formed. The original iron is no longer in its original form after rusting.

A reversible change is one where the original object or substance can be recovered — for example, melting ice (it can be refrozen) or boiling water (water vapour can be condensed back). An irreversible change cannot be undone — for example, chopped vegetables cannot return to their original shape, and popcorn cannot turn back into corn.

Weathering is the collective term for physical and chemical changes that break rocks into smaller pieces and eventually form soil. Physical causes include temperature changes, growing tree roots, and freezing of water in rock cracks. Chemical weathering occurs when water or chemicals in water react with rock minerals — for example, basalt (a black rock containing iron) develops a red layer of iron oxide when exposed to water or moist air for a long time.

Erosion is the process by which rock pebbles, soil, and sediments are broken down and moved from one location to another by natural forces like wind and flowing water. It is a physical change. Weathering is the broader term for both the physical and chemical breakdown of rocks in place; erosion specifically involves the transport of broken material. River rocks appear smoother due to constant erosion by flowing water, and the transported material eventually settles and hardens into new rocks.

Bioluminescence is the production of light (without heat) in living organisms through a chemical change. The chapter gives the example of fireflies, which emit light in late evenings through such a chemical process.

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