- A change is any difference in a substance or object — in shape, size, state, colour, smell or composition. Changes happen around us all the time.
- Changes are grouped as physical changes and chemical changes; they can also be described as reversible or irreversible, and fast or slow.
- Physical change: only physical properties (shape, size, state) change; no new substance forms and it is usually reversible. Example: melting of ice, tearing of paper.
- Chemical change (chemical reaction): one or more new substances with new properties are formed; it is usually irreversible. Example: burning of paper, rusting of iron, cooking food.
- Signs of a chemical change: change in colour, evolution of a gas, change in smell or taste, formation of a precipitate, change in temperature, emission of light.
- Rusting of iron needs both air (oxygen) and moisture (water); it can be prevented by painting, oiling, greasing, galvanisation and using stainless steel.
- Many useful processes — crystallisation, photosynthesis, respiration, ripening of fruit, curdling of milk — are everyday changes around us.
- Weightage: ~6 marks/year — 1-mark identification questions, 2-mark difference/reason questions, 3-mark activity-based or rusting questions.
1. What is a Change?
Everything around us is changing constantly. A seed grows into a plant, milk turns into curd, an ice cube melts in your glass, a slice of apple turns brown, and a piece of paper tears when you pull it. Each of these is a change. In science, a change is any process in which the properties, state or composition of a substance becomes different from what it was before.
To understand changes better, scientists classify them in three useful ways:
- Physical change versus chemical change (based on whether a new substance forms).
- Reversible change versus irreversible change (based on whether the original substance can be recovered).
- Slow change versus fast change (based on how quickly the change occurs).
The most important classification for this chapter is the first one: physical versus chemical.
2. Physical Changes
A physical change is a change in which only the physical properties of a substance — such as shape, size, state (solid, liquid, gas), colour or appearance — are altered. No new substance is formed, and the chemical composition of the substance stays the same.
2.1 Examples of physical changes
- Melting of ice — solid ice becomes liquid water, but it is still water (H2O). Freezing it again gives back ice.
- Boiling of water — liquid water becomes steam, still water. Cooling the steam gives back water.
- Tearing or folding of paper — the shape changes but it is still paper.
- Dissolving sugar or salt in water — the solid spreads through the water but can be recovered by evaporation.
- Stretching a rubber band, breaking a piece of chalk, making a clay toy, beating a metal into a sheet.
- Magnetising an iron strip, glowing of an electric bulb filament.
Dissolve a spoon of common salt in a beaker of water. The salt disappears, but you can taste that it is still there. Now heat the solution gently until all the water evaporates. White salt is left behind on the dish. The salt was never destroyed — it only spread among the water particles and was recovered unchanged. This proves that dissolving salt is a physical change (reversible, no new substance).
Note that a physical change does not always have to be reversible in practice. Cutting a log of wood into pieces is a physical change (still wood), but you cannot easily join the pieces back into the original log. The key test is: has a new substance formed? If not, it is physical.
3. Chemical Changes
A chemical change, also called a chemical reaction, is a change in which one or more new substances are formed. The new substances have different properties from the original substances. Chemical changes are usually irreversible — you cannot easily get back the starting materials.
3.1 Examples of chemical changes
- Burning of paper, wood or a candle — new substances (ash, carbon dioxide, water vapour) form; cannot be reversed.
- Rusting of iron — reddish-brown rust (iron oxide) forms; quite different from shiny iron.
- Cooking food, baking a cake — new substances with new taste, smell and texture are formed.
- Souring of milk / curdling into curd — bacteria turn milk into curd with a sour taste.
- Digestion of food, respiration, photosynthesis — chemical reactions inside living things.
- Burning of a magnesium ribbon — produces a bright white light and white ash (magnesium oxide).
- Ripening of fruit, fermentation of dough, bursting of a cracker.
Hold a small piece of magnesium ribbon with tongs and bring it to a flame (teacher demonstration; do not look directly at the bright light). The ribbon burns with a brilliant white flame and leaves behind a white powdery ash. The shiny silver-grey metal has turned into white magnesium oxide — a completely new substance. This is a chemical change. If the ash is dissolved in water and tested with red litmus, the litmus turns blue, showing a new basic substance has formed.
4. How to Recognise a Chemical Change
During a chemical change, several signs may appear. Observing one or more of these tells us a new substance is being formed:
- Change in colour — e.g. shiny iron turning to brown rust; cut apple turning brown.
- Evolution of a gas — bubbles seen when an antacid tablet is dropped in water, or when baking soda meets vinegar.
- Change in smell or taste — milk turning sour; food going stale.
- Formation of a precipitate — an insoluble solid appearing when two solutions are mixed.
- Change in temperature — heat given out (burning) or absorbed.
- Emission of light — burning magnesium gives off bright light.
Take a little baking soda (sodium hydrogencarbonate) in a glass and add a spoon of vinegar (dilute acetic acid). Brisk bubbling occurs and a gas is released. Pass this gas through limewater — the limewater turns milky, confirming the gas is carbon dioxide (CO2). A new substance (a gas) has formed, so this is a chemical change. The temperature of the mixture also falls slightly, another sign of a chemical reaction.
5. Physical Change vs Chemical Change
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance? | No new substance formed | One or more new substances formed |
| Reversible? | Usually reversible | Usually irreversible |
| Composition | Stays the same | Changes |
| Energy change | Small or none | Often large (heat/light given out or absorbed) |
| Example | Melting of ice, tearing paper | Burning of paper, rusting of iron |
A tricky case — the burning candle. When a candle burns, two changes happen together. The wax melting near the flame is a physical change (solid wax to liquid wax, still wax). The wax vapour burning in the flame to give carbon dioxide, water vapour, heat and light is a chemical change. So a single object can show both kinds of change at once.
6. Reversible and Irreversible Changes
A change is reversible if the original substance can be brought back. A change is irreversible if the original cannot be recovered.
- Reversible: melting and freezing of water, dissolving and recovering salt, stretching a rubber band, inflating and deflating a balloon, magnetising an iron nail.
- Irreversible: burning of wood, cooking of an egg, ripening of a mango, setting of curd, rusting of iron.
Most physical changes are reversible and most chemical changes are irreversible, but this is a guideline, not a strict rule. For example, melting wax (physical) is reversible, while cutting a tree into planks (physical) is not practically reversible.
7. Rusting of Iron
Rusting is a very common and damaging chemical change. When iron objects are left in the open, a reddish-brown flaky layer of rust (iron oxide) slowly forms on the surface. Rust is brittle and weakens the iron, which is why bridges, railings, tools and vehicles get damaged over time.
Take three test tubes, each with a clean iron nail. (1) In the first, add ordinary tap water (nail exposed to air and water). (2) In the second, add boiled (cooled) water and a layer of oil on top to keep air out (water but no air). (3) In the third, put dry nails with some drying agent and a stopper to keep moisture out (air but no water). After a few days, only the nail in the first tube rusts. This proves that rusting needs both air and water together.
7.1 Conditions that speed up rusting
Rusting is faster in humid (moist) climates and in coastal areas, because the salty sea air and high humidity provide plenty of moisture. Acidic surroundings also speed up rusting.
7.2 Preventing rusting
- Painting — a coat of paint keeps air and water away from the metal (used on gates, railings, vehicles).
- Oiling or greasing — a thin layer of oil prevents contact with moisture (used on tools, machine parts).
- Galvanisation — coating iron with a layer of zinc; used for pipes, buckets and roofing sheets.
- Chromium plating / tin plating — a shiny protective metal layer.
- Using stainless steel — an alloy of iron with chromium and nickel that does not rust; used for cutlery and utensils.
8. Crystallisation and Other Everyday Changes
8.1 Crystallisation
Crystallisation is the process of obtaining pure crystals of a substance from its solution. It is a physical change and is one of the best ways to get large, pure crystals of salts like copper sulphate.
Dissolve copper sulphate in hot water until no more will dissolve (a saturated solution). Add a few drops of dilute sulphuric acid to keep the solution clear, then filter it. Leave the clear solution undisturbed to cool slowly. As the water cools and slowly evaporates, beautiful blue crystals of copper sulphate form. Slow cooling gives larger, well-shaped crystals. Crystallisation gives a purer product than simple evaporation, because impurities stay behind in the solution.
8.2 Changes that help and harm us
Some chemical changes are very useful: cooking makes food digestible and tasty; photosynthesis in plants makes our food and oxygen; respiration releases energy in our bodies; fermentation helps make curd, bread and idli batter. Other changes are harmful: rusting destroys iron; spoiling of food wastes resources; burning of fuels pollutes the air.
Understanding which changes are physical and which are chemical helps us use the good ones and control or prevent the bad ones.
- Melting of ice
- Tearing of paper
- Burning of paper
- Dissolving sugar in water
- Rusting of iron
- Melting of wax
- Cooking of food
- Souring of milk
- Only air
- Only water
- Both air and water
- Neither air nor water
- Copper
- Zinc
- Aluminium
- Silver
- Turns blue
- Turns milky
- Turns red
- Remains unchanged
- Change in colour
- Evolution of a gas
- Change in shape only
- Change in temperature
- A black powder
- A white ash (magnesium oxide)
- A blue liquid
- No residue at all
- Burning of wood
- Setting of curd
- Melting and freezing of water
- Ripening of a mango
- Iron sulphate
- Iron oxide
- Iron chloride
- Iron carbonate
- Filtration
- Evaporation
- Crystallisation
- Distillation
- Chemical change
- Physical change
- Irreversible change
- Reaction producing CO2
- Wiping it with a dry cloth once
- Applying a coat of paint
- Keeping it in sunlight
- Rubbing it with sandpaper
- Only the colour changes
- A new substance with a sour taste is formed
- It can be easily reversed
- No energy change occurs
- Chemical change
- Irreversible change
- Physical and reversible change
- Change that destroys the salt
- Melting of ice
- Photosynthesis in plants
- Breaking of chalk
- Folding of cloth
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