CLASS 7 SCIENCE · CHAPTER 5 · STUDY NOTES
Changes Around Us: Physical and Chemical
Explain what changes, look for new substances and separate physical/chemical classification from reversibility.
01 / EXPLORE & UNDERSTAND
First describe what changed
An observation comes before a classification.
1. Name the visible change
Ice becomes liquid, a sheet becomes folded, and a banana may develop brown spots. Describe the size, shape, state, colour or smell that changed. This is more useful than simply writing “something happened”.
2. Then ask a different question
Did the original substance remain, or did a new substance form? Appearance alone may not settle this. We use what we know about the process and suitable evidence. Never taste or directly smell unknown substances as a test.
Your turn
Question
“The ice cube became liquid.” Is that an observation or already proof of a new substance?
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02 / EXPLORE & UNDERSTAND
A new shape is not a new substance
Paper can become a boat and still be paper.
Different forms, the same paper
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1. Follow what stays the same
Folding paper changes its shape. Crushing chalk changes the size of the pieces. Ice melting changes water’s state. In these examples, a new substance is not formed, so we call the changes physical.
2. Do not confuse damage with a reaction
A balloon bursting leaves torn rubber. It may be impossible to restore the same balloon easily, but no new substance is needed to explain the tearing. Whether the original shape can return is a separate question.
Your turn
Question
A chalk stick is crushed into powder. Why is this physical even if the stick cannot easily be rebuilt?
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03 / EXPLORE & UNDERSTAND
Look for a new substance
Cloudiness can be evidence when we know what formed.
1. Compare water with lime water
In the source demonstration, exhaled air makes bubbles in ordinary water. In lime water, carbon dioxide reacts with calcium hydroxide and forms insoluble calcium carbonate. Tiny solid particles make the liquid look milky.
2. Read the word equation
Calcium hydroxide + carbon dioxide → calcium carbonate + water. The arrow means “reacts to form”, not a direction of movement. Calcium carbonate is a new substance, so this is a chemical change. This demonstration belongs under teacher guidance.
Your turn
Question
Why are the bubbles alone less convincing than identifying a new white solid?
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04 / EXPLORE & UNDERSTAND
A reaction can make a gas
Test the gas instead of guessing from fizzing.
Follow the gas through the tube
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1. Follow the gas from one container to the next
Vinegar and baking soda react and release carbon dioxide. The source apparatus carries the gas along a tube into fresh lime water. The lime water turns milky, supporting the identification of carbon dioxide.
2. Separate making a gas from releasing trapped air
Fizzing is a clue, not an automatic answer. Air can also escape when a material is put in water, and water boils into water vapour. Here, the known reactants and carbon dioxide test together explain a chemical change.
Your turn
Question
Why send the gas into lime water rather than simply calling every bubble carbon dioxide?
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05 / EXPLORE & UNDERSTAND
Rust is a new material
Rusting changes more than the surface colour.
1. Connect with the nail investigation
Moist air allows iron to form rust over time. The reddish-brown deposit has different properties from the original metal. This formation of new material makes rusting a chemical change.
2. Do not use speed as the test
Rusting is often slow, while burning magnesium can be quick. Both form new substances. A slow change can be chemical, and a quick change such as breaking chalk can be physical.
Your turn
Question
Does a change have to happen quickly to be chemical? Give a reason.
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06 / EXPLORE & UNDERSTAND
Combustion forms new substances and releases energy
Burning is a chemical reaction, not just becoming hot.
An open candle and a covered candle
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1. Compare before and after
Magnesium ribbon reacts with oxygen to form white magnesium oxide. Burning wood produces gases and other products, with ash left behind. The new substances distinguish burning from simply warming an unchanged material.
2. Explain the covered candle
A candle under a glass goes out after a while because fresh oxygen cannot keep entering. The outside candle has a continuing air supply. The covered flame goes out when conditions no longer support burning; this does not mean every bit of oxygen has disappeared.
Your turn
Question
Why does a closed glass not let a candle burn indefinitely?
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07 / EXPLORE & UNDERSTAND
Fire needs three things together
Fuel in air does not normally catch fire by itself.
Three requirements for burning
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1. Read the three requirements
The chapter’s fire triangle links fuel, oxygen and enough heat to reach ignition temperature. Paper on a desk has fuel and oxygen, but usually lacks the required heat. A flame is one possible heat source, not the only one.
2. Explain focused sunlight
A lens can concentrate sunlight into a small area and raise its temperature. The textbook uses this to explain ignition without a starting flame. Never use a lens to start a fire or focus sunlight towards eyes. The scientific point is concentrated heating.
Your turn
Question
Why does paper in a room not burn just because air is around it?
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08 / EXPLORE & UNDERSTAND
One candle has physical and chemical changes
Follow the wax through each stage.
Follow the wax—not just the flame
- Wax melts: physical
Solid wax becomes liquid in the warm pool. It is still wax.
- The wick carries it upward
Liquid travels through the wick. Near the flame, it becomes vapour—a physical change.
- Wax vapour burns: chemical
The vapour reacts with oxygen. New substances form, releasing heat and light.
1. First the wax changes state
Heat melts solid wax. Liquid wax travels up the wick and vaporises near the flame. Wax that flows away can cool and become solid again. Melting, vaporising and solidifying are physical changes.
2. Then the vapour burns
Wax vapour reacts with oxygen, forming new substances and releasing heat and light. This part is chemical. So “burning a candle” names a whole process containing more than one kind of change.
A little deeper: Why Faraday used a candle to teach science
The chapter describes Michael Faraday’s nineteenth-century candle lectures. A familiar object can reveal several connected ideas: states of matter, movement of liquid, vaporisation, reaction and energy transfer. Look for those links rather than memorising only “candle = both”.
Your turn
Question
Is wax melting the same change as wax vapour burning?
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09 / EXPLORE & UNDERSTAND
Can it be reversed? That is a separate question
Physical does not always mean easy to undo.
1. Compare melting with cutting
Melted water can freeze again, so the state change can be reversed. A cut sheet is still paper, but simply undoing the cut does not restore the original sheet. Both changes are physical despite their different reversibility.
2. State the conditions
Water boiled away from an open pan is difficult to recover from the whole room. In a setup that collects the vapour and cools it, liquid water can be recovered. Explain what system you mean when discussing reversal.
| Change | New substance? | Easy to restore? |
|---|---|---|
| Ice melts | No | Yes, by freezing |
| Paper is cut | No | Not the original sheet by simply undoing |
| Wax vapour burns | Yes | Not by simply cooling the products |
Your turn
Question
A physical change cannot easily be undone. Does that make it chemical?
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10 / EXPLORE & UNDERSTAND
Useful or unwanted depends on the situation
The same process can help in one place and cause trouble in another.
1. Compare food storage with composting
Food decomposing in a lunch box is unwanted. Food waste decomposing in a managed compost system can help produce compost. The process is not labelled good or bad without considering its place and purpose.
2. Consider wider effects
Fuel combustion may provide useful transport while also releasing carbon dioxide. Paint drying can involve evaporation of substances into air. A useful immediate result does not mean there are no environmental effects.
Your turn
Question
Why can decomposition be undesirable in a kitchen but desirable in composting?
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11 / EXPLORE & UNDERSTAND
Rocks change where they are
Weathering can be physical, chemical or both.
Rock fragments at a cliff
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A red layer records chemical alteration
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1. Follow physical breaking
Water entering a crack can freeze and expand, widening the crack. Growing roots and temperature changes can also help break rock. The pieces may become smaller without forming a new substance in that particular step.
2. Then follow chemical alteration
Water and substances dissolved in it can react with rock minerals. NCERT shows a red layer formed from chemical changes involving iron in rock. Weathered mineral particles contribute to soil, together with other materials such as organic matter.
Your turn
Question
Is a rock breaking into smaller pieces always proof of a chemical change?
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12 / EXPLORE & UNDERSTAND
Erosion moves the pieces; deposition leaves them
Breakdown, movement and settling are related but different.
1. Trace the journey
Weathering can loosen fragments. Flowing water or wind can carry them away: erosion and transport. Rubbing and collisions during movement may make pebbles smoother and smaller. These changes do not necessarily form new substances.
2. Notice where material settles
When water or wind loses enough carrying power, particles settle: deposition. Over long periods, buried sediments may be compacted and cemented into sedimentary rock. Not every deposited particle immediately becomes rock.
Your turn
Question
Rainwater carries loose soil downhill and leaves it in a quiet pool. Name the movement and the settling.
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13 / EXPLORE & UNDERSTAND
Living things also make chemical changes
A chemical reaction does not always have a flame.
1. Look at firefly light
Fireflies make light through chemical reactions in their bodies. They do not burn like a candle flame. This light is produced with very little heat, which is why “a chemical change must feel hot” is not a useful rule.
2. Connect yeast with rising dough
In suitable conditions, yeast uses sugar and releases carbon dioxide. Gas trapped in dough helps it expand. The gas production involves chemical changes; stretching a balloon or expanding dough because of trapped gas also involves a physical change of shape.
Your turn
Question
Can one living process include a chemical reaction and a visible physical expansion?
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Take these ideas with you
- Physical changes do not form new substances; chemical changes do.
- Bubbles or colour changes are clues: explain the evidence.
- Combustion needs fuel, oxygen and sufficient heating.
- A candle combines physical state changes with chemical burning.
- Reversible/irreversible is a separate classification.
- Weathering alters or breaks rock; erosion moves it; deposition settles it.
Use your understanding in a new situation
Question
A student calls every irreversible change chemical. Use one counterexample.
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Explain two different changes at a candle without using only the word “both”.
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