Changes Around Us: Physical and Chemical — Class 7 Science
Curiosity · Chapter 5
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- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Physical change
- A change in which only physical properties such as shape, size, or state alter and no new substance is formed.
- Chemical change
- A change in which one or more new substances are formed through a chemical reaction.
- Combustion
- A chemical reaction in which a substance reacts with oxygen and produces heat and/or light.
- Ignition temperature
- The lowest temperature at which a substance catches fire.
- Combustible substance
- A substance, also called fuel, that can undergo combustion, such as wood, paper, or kerosene.
- Weathering
- The collective physical and chemical changes that break rocks into smaller pieces, eventually forming soil.
- Erosion
- The process by which broken rock, soil, and sediments are moved from one place to another by wind and flowing water.
Extra questions & answers
What is a physical change? Give an example.
A physical change alters only physical properties like shape, size, or state, with no new substance formed; for example, melting ice or folding paper.
What is a chemical change? Give an example.
A chemical change forms one or more new substances through a chemical reaction; for example, baking soda reacting with vinegar to release carbon dioxide.
Why does lime water turn milky when you blow air into it?
Carbon dioxide from the exhaled air reacts with lime water to form a new white, insoluble substance, calcium carbonate, which makes it milky.
What is combustion?
Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light.
What three things are required for combustion (the fire triangle)?
A combustible substance (fuel), oxygen, and heat that allows the fuel to reach its ignition temperature.
What is ignition temperature?
Ignition temperature is the lowest temperature at which a substance catches fire.
Why is the burning of a candle both a physical and a chemical change?
The melting, solidifying, and evaporation of wax are physical changes, while the burning of the wax vapour is a chemical change.
Give one change that can be reversed and one that cannot.
Melting ice can be reversed by refreezing, but making popcorn from corn cannot be reversed.
What is weathering?
Weathering is the collective physical and chemical changes that break large rocks into smaller pieces and eventually form soil.
How does erosion differ from weathering?
Erosion is the moving of broken rock, soil, and sediments from one place to another by natural forces like wind and flowing water, whereas weathering is the breaking down itself.
Let Us Enhance Our Learning
1. Which of the following statements are the characteristics of a physical change? (i) The state of the substance may or may not change. (ii) A substance with different properties is formed. (iii) No new substance is formed. (iv) The substance undergoes a chemical reaction. (a) (i) and (ii) (b) (ii) and (iii) (c) (i) and (iii) (d) (iii) and (iv)
The correct option is (c) (i) and (iii). In a physical change only physical properties like shape, size or state may change and no new substance is formed. Statement (ii) and (iv) describe a chemical change where a new substance with different properties is formed by a chemical reaction.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (i) Stitching cloth to a shirt
Reversible to some extent. Stitching can be reversed by opening the stitches with a stitch ripper to get back the original cloth pieces, though the needle holes may remain. So it is considered reversible.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (ii) Twisting of straight string
Reversible. A twisted string can be untwisted by rotating it in the opposite direction to get back the straight string. It is a physical change, no new substance is formed.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (iii) Making idlis from a batter
Irreversible. Batter on steaming undergoes cooking, a chemical change. The cooked idli cannot be changed back into the original liquid batter.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (iv) Dissolving sugar in water
Reversible. Sugar can be recovered by evaporating the water and then condensing the water vapour. Both sugar and water remain chemically the same, it is a physical change.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (v) Drawing water from a well
Reversible. Water taken out can be poured back into the well to restore the original situation. It is a physical change of position.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (vi) Ripening of fruits
Irreversible. Ripening involves chemical changes that produce new colours, flavours and aromas. A ripe fruit cannot be made unripe again.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (vii) Boiling water in an open pan
Reversible if the water vapour is collected and condensed. In an open pan the steam escapes into air, so practically it seems irreversible, but chemically it is a reversible physical change
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (viii) Rolling up a mat
Reversible. A rolled mat can be unrolled to get the flat mat back. It is a physical change in shape.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (ix) Grinding wheat grains to flour
Irreversible. Flour cannot be joined back to form the original whole wheat grains. The size and texture has changed permanently, though chemically wheat remains wheat.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (x) Forming of soil from rocks
Irreversible. Weathering and erosion break rocks into soil over thousands of years by physical and chemical changes. Soil cannot be reassembled into the original solid rock under normal conditions.
3. State whether the following statements are True or False. In case a statement is False, write the correct statement. (i) Melting of wax is necessary for burning a candle. (True/False)
True. The solid wax must first melt, rise up the wick and vaporise before its vapour can burn with the flame.
3. State whether the following statements are True or False. In case a statement is False, write the correct statement. (ii) Collecting water vapour by condensing involves a chemical change. (True/False)
False. Condensing water vapour to liquid water is a physical change. No new substance is formed, only the state changes from gas to liquid,
3. State whether the following statements are True or False. In case a statement is False, write the correct statement. (iii) The process of converting leaves into compost is a chemical change. (True/False)
True. Decomposition by bacteria and fungi produces new substances like humus,
3. State whether the following statements are True or False. In case a statement is False, write the correct statement. (iv) Mixing baking soda with lemon juice is a chemical change. (True/False)
True. Baking soda
4. Fill in the blanks in the following statements: (i) Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to ________, and this is a ________ change.
The brown deposits are due to rusting / formation of iron oxide, and this is a chemical change. Iron reacts with oxygen and moisture to form a new substance.
4. Fill in the blanks in the following statements: (ii) Folding a handkerchief is a _________________ change and can be ________.
Folding a handkerchief is a physical change and can be reversed by unfolding it.
4. Fill in the blanks in the following statements: (iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called _________, and this is a __________ change.
A chemical process in which a substance reacts with oxygen with evolution of heat is called combustion, and this is a chemical change.
4. Fill in the blanks in the following statements: (iv) Magnesium, when burnt in air, produces a substance called __________. The substance formed is _________ in nature. Burning of magnesium is a _________ change.
Magnesium, when burnt in air, produces a substance called magnesium oxide
5. Are the changes of water to ice and water to steam, physical or chemical? Explain.
Both are physical changes. Water to ice
6. Is curdling of milk a physical or chemical change? Justify your statement.
It is a chemical change. Milk bacteria convert lactose into lactic acid which coagulates milk proteins to form curd. A new substance with different taste, texture and properties is formed and the change cannot be reversed to get back the original milk.
7. Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
It is both physical and chemical. Physical weathering breaks big rocks into smaller pieces by temperature changes, wind and water erosion without changing composition. Chemical weathering by water and dissolved
8. Read the following story titled ‘Eco-friendly Prithvi’, and tick the most appropriate option(s) given in the brackets. Provide a suitable title of your choice for the story. Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes/chemical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change/chemical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (physical change/chemical change). He decides to plant seeds in the compost and water them regularly. After a few days, he notices that the seeds begin to germinate and small plants start to grow, eventually blooming into colourful flowers (physical change/chemical change). His efforts are appreciated by all his family members.
Suitable title: From Kitchen Waste to Flower Garden or The Compost Circle. Correct choices: He chops vegetables, peels potatoes, and cuts fruits — physical changes (only size and shape change). He collects the seeds, fruits and vegetable peels into a clay pot — physical change (just mixing). The fruits, vegetable peels and other materials begin to decompose due to bacteria and fungi, forming compost — chemical change (new substances formed). Seeds germinate and grow into flowering plants — chemical change (growth involves new substances and cannot be reversed to seeds).
9. Some changes are given here. Write physical changes in the area marked ‘A’ and chemical changes in the area marked ‘B’. Enter the changes which are both physical and chemical in the area marked ‘C’. Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium and Mixing baking soda with vinegar.
A - only physical changes: Tearing of paper, Melting of ice
10. The experiments shown in Fig. 5.11a, b, c, and d were performed. Find out in which case(s) did lime water turn milky and why? Lime water with (a) Vinegar and baking soda (b) Vinegar and common salt (c) Lemon juice and vinegar (d) Lemon juice and baking soda
Lime water turns milky in (a) Vinegar and baking soda and in (d) Lemon juice and baking soda. In both, the acid (acetic acid in vinegar, citric acid in lemon juice) reacts with sodium hydrogen carbonate
Exploratory Projects
Write a message on a piece of paper using lemon juice as ink and let it dry. The message will become invisible. Now use a warm iron over the paper (or hold the paper over the flame of a candle, taking care that it does not catch fire). The invisible letters turn dark brown as the paper gets warm. Can any of these changes be reversed?
Writing with lemon juice and letting it dry is a physical change — the invisible message can be moistened again. But when the paper is heated, the citric acid in the dried juice chars (turns brown). This browning on heating is a chemical change, and it cannot be easily reversed, which is why the hidden message becomes visible.
We hear a lot of news about landslides and breaking of rocks in hilly regions these days, causing a lot of damage to life and property. Discuss what steps we can take to reduce landslides and rock erosion.
Steps include planting trees and vegetation to bind soil with roots, building retaining walls and terracing on slopes, providing proper drainage to prevent water accumulation in cracks, avoiding uncontrolled construction and mining, and using afforestation and contour ploughing to reduce wind and water erosion.
Observe the activities going on in the kitchen and note any changes that can be reversed. Are these physical or chemical changes?
Examples: Melting butter, dissolving salt or sugar in water, boiling water, chopping vegetables, folding chapati, rolling dough balls - these are physical and reversible by cooling, evaporating, unfolding. Cooking rice, baking chapati, curdling milk, burning toast - these are chemical and irreversible because new substances are formed.
Yeast is added while baking bread to make it fluffy and soft. How does yeast work? Try and find out! Take a small bottle, some sugar, fresh yeast, water, and a balloon. Make a sugar solution in the bottle by mixing two teaspoons of sugar and a small amount of water. Now add a spoonful of yeast and cover the mouth of the bottle with a balloon. Leave it undisturbed for about an hour. What do you observe? Carefully take off the balloon, holding its mouth tightly closed and attach it to another small bottle containing freshly prepared lime water. Shake the bottle so that the contents of the balloon get mixed with lime water. What do you observe? What can you conclude from this experiment? Identify all the changes occurring in the experiment and state which of them are physical and chemical changes.
Observation 1: After an hour the balloon inflates, showing gas formation. Observation 2: The gas turns freshly prepared lime water milky, proving it is carbon dioxide
Chameleons (Girgits) change colour to blend in with their surroundings and also when they are angry or sense danger (Fig. 5.12). Is this a change that can be reversed? Explore from the internet or from your school library.
Yes, it is a reversible physical-chemical change for the animal. Chameleons have special colour cells in skin that expand or contract to show different pigments. They can change back to their original colour when the surroundings change or the threat passes. It is a temporary, reversible biological change, not a permanent chemical transformation.