Changes Around Us: Physical and Chemical – Class 7 Science Chapter 5 Notes & NCERT PDF

Introduction

The chapter opens with students describing things they noticed: an ice cube placed half an hour ago has become water, a rose bud seen yesterday is a flower today, a bottle of cold water is not cold anymore, and a banana has more brown spots and a strong smell today. What kinds of changes are these, and can we sort them into groups?

In Activity 5.1 we list everyday changes — melting ice cubes, chopping vegetables, boiling water, making popcorn, cutting paper, adding beetroot extract to water, burning wood, drying wet clothes, making dough balls and rolling them into chapatis. Changes may be in size, shape, smell or other properties, and we notice them with our senses of sight, smell, touch, hearing and taste.

Key Concepts

5.1 A substance may change in appearance but remain the same

In Activity 5.2 we fold paper into objects, let air out of a balloon (and prick another with a pin), and crush chalk into powder. The paper, the rubber of the balloon and the chalk remain the same material even though their shape or size changes. Water changing between solid, liquid and gas is also like this.

Physical change: a change in which only physical properties like shape, size and state change, and no new substance is formed.

5.2 A substance may change and not remain the same

In Activity 5.3 we blow our breath through a straw into tumbler A (tap water) and tumbler B (lime water). Tap water only bubbles. Lime water turns milky, and later a white substance settles at the bottom.

Chemical change: a change in which one or more new substances are formed. New substances form through a chemical reaction, which can be written as a chemical equation.

Carbon dioxide from exhaled air reacts with lime water:

Lime water turning milky is the test for carbon dioxide.

In Activity 5.4, a pinch of baking soda (sodium hydrogen carbonate) added to vinegar or lemon juice fizzes. When the gas is passed through fresh lime water, the lime water turns milky — so the gas is carbon dioxide:

5.3 Other processes involving chemical changes

Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Substances that burn — wood, paper, cotton, kerosene — are combustible substances.

Activity 5.5: of two burning candles, the one covered with a glass tumbler goes out because it has no continuous supply of air. Lime water added to the petri dish turns milky, showing carbon dioxide formed from the carbon in wax and oxygen in air. Oxygen is required for combustion.

Activity 5.6: paper held with tongs catches fire from a matchstick at once; paper under a magnifying glass focusing sunlight first smokes, then burns. Heating raised its temperature until it caught fire.

Ignition temperature: the minimum temperature at which a substance catches fire.

The fire triangle — three things needed for combustion:

5.4 Physical and chemical changes in the same process

In a burning candle, wax melts, is carried up the wick, and evaporates (physical changes); the wax vapour burns to make the flame (chemical change). So burning a candle involves both. Michael Faraday used the candle in his lectures The Chemical History of a Candle.

5.5 Are changes permanent?

Some changes can be reversed — melting ice can be refrozen, boiled water's vapour can be condensed. Others cannot — chopped vegetables and popcorn cannot go back to their original form.

5.6 Are all changes desirable?

5.7 Slow natural changes

Activities in the Chapter

Key Terms

Real-life Connections

Common Misconceptions

Keep the Curiosity Alive (exercise hints)