Changes Around Us: Physical and Chemical

About this chapter

Changes Around Us: Physical and Chemical is the fifth chapter of Class 7 Curiosity. Through Activity 5.1 the chapter asks students to record what changes when ice melts, vegetables are chopped, water boils, popcorn pops, paper is cut, beetroot is added to water, wood burns, clothes dry, dough is rolled into chapatis. From these everyday cases the chapter draws two definitions. A physical change is one in which only physical properties like shape, size and state alter and no new substance is formed; folding paper, deflating a balloon and crushing chalk are the worked examples. A chemical change is one in which one or more new substances are formed through a chemical reaction; blowing air into lime water (forming calcium carbonate) and adding baking soda to vinegar (releasing carbon dioxide) are the worked examples. rusting and combustion to the chemical-change list — burning magnesium ribbon gives magnesium oxide with heat and light. Activity 5.6 with a magnifying glass introduces ignition temperature, leading to the fire triangle of fuel, oxygen and heat. a burning candle involves both kinds at once: melting, solidifying and vaporising are physical, but burning of vapour is chemical. Sections 5.5–5.7 cover reversible vs irreversible changes, desirable vs undesirable changes, and the slow natural changes of weathering and erosion that ultimately form soil. Michael Faraday's Chemical History of a Candle and the bioluminescence Fascinating-Fact anchor the chapter culturally. CTET Paper 2 Science tests this chapter through classification items (which change is physical, which chemical), the lime-water-turning-milky reasoning chain, ignition-temperature definition, reversed/irreversible matching, and integrated weathering-versus-erosion problems. The four tests below — Practice 15 Q, Quiz 15 Q, Hard 15 Q, Mastery 30 Q — cover these ideas at exam depth.