NCERT Solutions for Class 7th Science Chapter 5 Changes Around Us: Physical and Chemical
Updated on 2026-09-19
About this chapter
A physical change alters only physical properties such as shape, size or state. No new substance is formed — folding paper, crushing chalk, melting ice. A chemical change forms one or more new substances through a chemical reaction , which can be written in short as a chemical equation . Blowing air into lime water, vinegar + baking soda, rusting and burning are chemical changes. Lime water turning milky is the standard test for carbon dioxide . Calcium hydroxide + Carbon dioxide → Calcium carbonate (insoluble, white) + Water. Combustion is a reaction with oxygen that gives out heat and/or light . It needs three things — the fire triangle : a combustible substance (fuel), oxygen , and enough heat to reach the ignition temperature . One process can involve both kinds of change . In a burnin
- Chapter opener
- Table 5.1: Some changes observed around us
- .1 A Substance May Change in Appearance but Remain the Same!
- .3.2 Combustion
- Is the Presence of Air Enough for Combustion?
- .5 Are Changes Permanent?
- Table 5.2: Can changes be reversed?
- .6 Are All Changes Desirable?
- .7.1 Weathering of rocks
- .7.2 Erosion
Quick revision
| Term | What it means | Where it comes in the chapter | Example from the book |
|---|---|---|---|
| Physical change | A change in which only physical properties (shape, size, state) change and no new substance is formed | Section 5.1, page 59 | Folding paper into objects; crushing chalk into powder; melting ice |
| Chemical change | A change in which one or more new substances are formed | Section 5.2, page 60 | Blowing air into lime water — a white insoluble substance settles |
| Chemical reaction | The process by which new substances are formed in a chemical change | Page 60 | Calcium hydroxide reacting with carbon dioxide |
| Chemical equation | The short form in which a chemical reaction is written | Page 61 | Vinegar + Baking soda → Carbon dioxide + Other substances |
| Lime water test | Lime water turns milky (cloudy) when carbon dioxide is passed through it | Activity 5.3 and 5.4, pages 60–61 | Used to identify the gas from vinegar and baking soda |
| Rusting | Formation of a new brown substance, iron oxide, on iron | Section 5.3.1, page 62 | Rusted iron nails, Fig. 5.4 |
| Combustion | A chemical reaction in which a substance reacts with oxygen and produces heat and/or light | Section 5.3.2, page 62 | Magnesium + Oxygen → Magnesium oxide + Heat + Light |
| Combustible substance | A substance that undergoes combustion; also called a fuel | Page 62 | Wood, paper, cotton, kerosene |
| Ignition temperature | The minimum temperature at which a substance catches fire | Activity 5.6, page 64 | Paper reaches it when sunrays are focused on it with a lens |
| Fire triangle | The three requirements for combustion — fuel, oxygen and heat | Fig. 5.8, page 64 | Covering a candle with a tumbler removes oxygen, so it goes out |
| Reversible / irreversible change | A change in which the original substance can / cannot be brought back | Section 5.5, page 66 | Melting ice — yes; making popcorn — no |
| Weathering | The physical and chemical changes that break down rocks and finally form soil | Section 5.7.1, page 67 | Black basalt turning red from iron oxide, Fig. 5.10b |
| Erosion | Carrying away of broken rock, soil and sediment by wind and flowing water — a physical change | Section 5.7.2, page 68 | Smooth river pebbles; fine sand on riverbeds |
Exercises
- Chapter opener — In-text Questions Page 57
- Table 5.1: Some changes observed around us — Activity 5.1: Let us think and reflect Page 58
- .1 A Substance May Change in Appearance but Remain the Same! — Activity 5.2: Let us create and discuss Page 595
- .2 A Substance May Change in Appearance and Not Remain the Same! — Activity 5.3: Let us explore Page 605
- .2 A Substance May Change in Appearance and Not Remain the Same! — Activity 5.4: Let us experiment Page 615
- .3.2 Combustion — In-text Questions Page 625
- .3.2 Combustion — Activity 5.5: Let us investigate Page 625
- .3.2 Combustion · Science and Society · Is the Presence of Air Enough for Combustion? — In-text Questions Page 635
- Is the Presence of Air Enough for Combustion? — Activity 5.6: Let us investigate Page 64
- .4 Can Physical and Chemical Changes Occur in the Same Process? — Activity 5.7: Think, pair, and share Page 655
- .5 Are Changes Permanent? — In-text Questions Page 665
- Table 5.2: Can changes be reversed? — Activity 5.8: Let us think Page 66
- .6 Are All Changes Desirable? — In-text Questions Page 665
- .7.1 Weathering of rocks — In-text Questions Page 675
- .7.2 Erosion — In-text Questions Page 685
- Chapter-end exercise, Questions 1–10 — Let Us Enhance Our Learning Page 69
- Projects and investigations at the end of the chapter — Exploratory Projects Page 71