Educational Galaxy

Chapter 5: Changes Around Us: Physical and Chemical — EG's own notes, in simple words

Book: Curiosity (CBSE, Class 7)  |  Chapter 5  |  Medium: English

Diagram (labels are printed on the image): Melting ice, a physical change, beside a rusty nail, a chemical change. A general picture; it is not a scene from the book.

Burning and melting as examples of changes. A general picture, not a book scene.

What the chapter is about

Changes happen all around us, and they can be sorted into two kinds. In a physical change, no new substance is formed. Melting ice, tearing paper, dissolving salt in water and breaking a chalk stick are physical changes. The substance may change shape, size or state, but it stays the same substance, and many physical changes can be reversed.

In a chemical change, one or more new substances are formed, with different properties. Burning paper gives ash and smoke, rusting iron gives a brown coating, cooking food changes its taste and smell, and milk turning sour forms new substances. Chemical changes are usually hard to reverse, and they often show signs like a colour change, a gas being given off, heat or light, or a new smell.

Some changes look small but are chemical. When an iron nail is left in water, a brown layer of rust forms on it. Rusting is slow, but it weakens iron gates and tools, so painting or oiling iron protects it from air and water. Learning to spot the signs of a chemical change helps us tell the two kinds apart in exams and in daily life.

5.1 A substance may change in appearance but remain the same

Ice melts to water, water boils to steam, a paper is torn or folded, vegetables are chopped, sugar dissolves in water, and wet clothes dry. In every case the substance itself is unchanged: water is still water whether it is ice, liquid or steam, and torn paper is still paper. These are physical changes — changes in size, shape, state or appearance, with no new substance formed, and often reversible.

5.2 A substance may change in appearance and not remain the same

Milk curdles, a banana ripens and gets brown spots and a strong smell, wood burns to ash, an iron nail rusts, and food cooks. Here a new substance appears with different properties, and the change usually cannot be undone by simply reversing the steps. These are chemical changes. The chapter's test for them is simple: has something new been formed? If yes, the change is chemical, whatever it looks like.

5.3 Some other processes involving chemical changes

Rusting is a chemical change: iron plus moist air slowly becomes iron oxide, a new substance. Combustion is another: burning magnesium ribbon gives a white ash of magnesium oxide, and burning wood gives ash, smoke and gases. Blowing exhaled air through lime water turns it milky, because exhaled air contains carbon dioxide, which reacts with lime water to form a new substance. That milky test is the chapter's detector for carbon dioxide.

5.4 Can physical and chemical changes occur in the same process?

Yes — burning a candle is the chapter's example. The wax near the flame melts (a physical change: solid wax becomes liquid wax, still wax) and the melted wax then burns (a chemical change: it forms new substances). One process, two kinds of change at once. The same is true of cooking: chopping is physical, but the food itself changes chemically as it cooks.

5.5–5.7 Are changes permanent, desirable, or slow and natural?

Not every physical change can be undone, and not every chemical change is permanent in its effects — the chapter warns against equating physical with reversible and chemical with irreversible. Some changes are desirable: rotting food waste becomes useful compost. Others are harmful: burning too much fuel adds gases that damage the environment over years. And some changes are slow and natural: weathering breaks rocks by temperature changes, growing tree roots, freezing water in cracks and chemical attack by water, forming soil over a long time; erosion then carries the broken material away by wind and flowing water, smoothing river rocks and, in landslides, causing damage.

What you should learn

Learn the test: if a new substance forms, the change is chemical; if only shape, size or state changes, it is physical. Look for signs like colour change, gas, heat or smell.

• Tell physical changes from chemical changes by asking: has a new substance formed?
• Use the lime-water test to detect carbon dioxide.
• Explain weathering, erosion, composting and why burning a candle is both kinds of change.

Chart / Table: Physical versus chemical changes

Table: Physical versus chemical changes

Steps: testing gases with lime water

1. Take two tumblers: one with tap water, one with lime water, each about one-fourth full.
2. Blow exhaled air into each through separate straws — never suck the liquid up the straw.
3. Compare: the lime water turns milky while the tap water does not, showing carbon dioxide in exhaled air.

Practice set — try it yourself

1. Is melting ice a physical or chemical change? Why?
   Hint: Think about whether a new substance forms.
2. Why is burning paper a chemical change?
   Hint: Think about ash and smoke.
3. Why is dissolving salt in water physical?
   Hint: Think about getting the salt back.
4. What two things does iron need in order to rust?
   Hint: Think about air and water.
5. Give one sign that a chemical change has happened.
   Hint: Think about colour, gas, heat or smell.

Questions with Answers

1. What is the difference between a physical and a chemical change?
   In a physical change no new substance is formed; only shape, size or state changes, as in melting ice. In a chemical change new substances with different properties are formed, as in burning paper, which gives ash and smoke.
2. Is rusting a physical or chemical change? Explain.
   Rusting is a chemical change because a new substance, rust, forms on iron when it reacts with air and water. The rust has different properties from iron and cannot easily be turned back into iron.
3. Why is cooking food a chemical change?
   Cooking changes the food's taste, smell, colour and texture because new substances form. Cooked food cannot be changed back into raw food, which shows the change is chemical.
4. A student dissolves sugar in water and says a new substance formed. Is she right?
   No. The sugar is still present in the solution and can be recovered by evaporating the water. No new substance formed, so dissolving is a physical change.
5. How can rusting of an iron gate be prevented?
   Rusting needs air and water together. Painting, oiling or galvanising the gate covers the iron so air and water cannot reach it, which slows or prevents rusting.

More questions with answers — extension

1. Classify as physical or chemical, with a reason each: (i) melting ice (ii) burning wood (iii) chopping vegetables (iv) curdling milk (v) dissolving sugar (vi) rusting.
   (i) Physical — only the state changes, water stays water. (ii) Chemical — ash and gases, new substances, are formed. (iii) Physical — only size and shape change. (iv) Chemical — a new substance (curd) forms and milk cannot simply be restored. (v) Physical — sugar can be recovered by evaporating the water. (vi) Chemical — iron oxide, a new substance, forms.
2. Are water-to-ice and water-to-steam changes physical or chemical? Explain.
   Physical. In both, only the state changes; the substance remains water, and the change can be reversed by heating or cooling.
3. Is curdling of milk a physical or chemical change? Justify.
   Chemical. Milk changes into curd, a substance with different taste, texture and properties, and the milk cannot be got back by a simple reversal.
4. Four experiments mix liquids with lime water: (a) vinegar + baking soda (b) lemon juice + vinegar (c) vinegar + common salt (d) lemon juice + baking soda. In which cases does lime water turn milky, and why?
   In (a) and (d) only. Both pairs react to release carbon dioxide gas, which turns lime water milky. In (b) and (c) no carbon dioxide is released, so the lime water stays clear.
5. Sort into A (physical only), B (chemical only) and C (both): burning a candle, tearing paper, rusting, curdling milk, ripening fruit, melting ice, folding clothes, burning magnesium, mixing baking soda with vinegar.
   A: tearing paper, melting ice, folding clothes. B: rusting, curdling milk, ripening fruit, burning magnesium, mixing baking soda with vinegar. C: burning a candle — the wax melts (physical) and burns (chemical).
6. A man chops vegetables and peels potatoes (physical or chemical?), collects peels in a clay pot where they decompose into compost (physical or chemical?), then plants seeds in the compost and they germinate and flower (physical or chemical?). Classify each stage.
   Chopping and peeling: physical — only size and shape change. Decomposition into compost: chemical — bacteria and fungi change the waste into new substances. Germination and flowering: chemical — the seed's stored food is used to build a new plant, a change that cannot be undone.
7. Yeast, sugar and water are mixed in a bottle with a balloon on its mouth. After an hour the balloon fills. When the balloon's gas is shaken with lime water, the lime water turns milky. What does this show, and which changes in the experiment are physical and which are chemical?
   It shows the yeast released carbon dioxide: the gas fills the balloon (a physical movement of gas) and turns lime water milky (a chemical test for carbon dioxide). Dissolving the sugar is physical; the yeast's action on sugar, producing gas and new substances, is chemical.
8. A chameleon changes colour to blend in and when it senses danger. Is this change reversible? What kind of change is it, in the chapter's terms?
   It is reversible — the chameleon changes back when the situation changes — so it is not like burning or rusting, which make permanent new substances. The chapter asks you to explore this: colour change alone does not prove a chemical change; what matters is whether a new substance has formed.
9. How do weathering and erosion together make soil and move it? Give one human consequence of erosion mentioned in the chapter.
   Weathering breaks rocks in place — by temperature swings, roots, freezing water and chemical attack — and over time the broken rock becomes soil. Erosion then carries that material away by wind and flowing water. In hilly regions, erosion during landslides breaks rocks and damages life and property, which is why the chapter asks what steps can reduce landslides and rock erosion.
10. A message written in lemon juice dries invisible. Warming the paper over a flame (with an adult) turns the letters dark brown. Can this change be reversed? What kind of change is it?
    It cannot be reversed by cooling or unwarming — the paper's substance has changed where it was heated, turning brown. It is a chemical change: heat has made new, brown substances in the paper, not merely changed its shape or state.

How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.