Natural, man-made, useful and harmful changes
The milk you forgot to boil turns sour by evening. A mango on the tree turns from green to yellow. You sharpen a pencil, cook rice, or tie a toran. Changes are happening everywhere, some by themselves and some because we do them.
A change (बदल) is any alteration in a substance or situation. Changes can be natural or man-made, and useful or harmful.
In the figure : In the figure natural changes (a fruit falling, rusting of iron, raining) happen by themselves. Man-made changes (lighting a bulb, cutting vegetables, making a toran) are done by people.
Rule 1: Natural and man-made — Changes like the ripening of a fruit and the spoiling of milk occur naturally. Changes can also be done by people.
- Easy : Natural: a leaf turning yellow and falling.
- Medium : Man-made: building a sandcastle, lighting a lamp.
- Exam-type : Rusting happens by itself, so it is natural; painting a gate is done by people, so it is man-made.
In the figure : In the figure useful changes are useful to us (sharpening a pencil, baking bread, cooking food). Harmful changes are not useful or do us harm (rusting of iron, spoiling of milk, rotting of grain).
Rule 2: Useful and harmful — Changes like sharpening a pencil, baking bread and cooking food are useful. Changes that are not useful, or harm us, are harmful.
- Easy : Cooking food: useful.
- Medium : Rotting of grain in a damp store: harmful.
- Exam-type : The same rain can be useful for a farmer and harmful for a house with a leaking roof. It depends on the situation.
Natural = by itself; man-made = done by people. Useful = benefits us; harmful = damages.
Thinking all natural changes are useful. Many are harmful, like floods.
Memory clue: "N-M" = Nature or Man; "U-H" = Use or Harm: two separate questions for every change.
Spot the difference
| Type | Meaning | Example |
|---|---|---|
| Natural | Happens by itself | Fruit ripening |
| Man-made | Done by people | Making a toran |
| Useful | Helps us | Baking bread |
| Harmful | Damages | Spoiling of milk |
Practice questions
- 1What is a natural change?
- 2Give two examples of natural changes.
- 3Give two examples of man-made changes.
- 4What is a useful change?
- 5Name two harmful changes.
- 6Classify "cooking rice" as natural or man-made, and useful or harmful.
- 7Is the rusting of an iron gate natural or man-made, useful or harmful?
- 8Heavy rain is useful for a farmer's field but harmful when it floods a house. What does this show about classifying changes?
Show answers (try them yourself first)
- 1A change that happens by itself.
- 2Ripening of a fruit and spoiling of milk.
- 3Cutting vegetables and making a toran.
- 4A change that is useful to us.
- 5Rusting of iron and rotting of grain.
- 6Man-made and useful.
- 7Natural and harmful.
- 8The same change can be useful in one situation and harmful in another.
Fast, slow, reversible and irreversible changes
A balloon bursts with a bang in a second, but a mango on the tree takes weeks to ripen. Ice cream melts and can be frozen again, but a burnt roti cannot become flour again.
A fast change (जलद बदल) takes a short time and a slow change (मंद बदल) takes a long time. A reversible change (उलटवता येणारा बदल) can be undone; an irreversible change (न उलटवता येणारा बदल) cannot.
In the figure : In the figure fast changes take a short time (a balloon bursting) and slow changes take a long time (a fruit ripening, an iron gate rusting). The times here are sample times.
Rule 1: Fast and slow — Changes in a short period, like the bursting of a balloon, are fast. Changes taking a long time, like the ripening of a fruit, are slow.
- Easy : Fast: a firecracker bursting.
- Medium : Slow: a stone wall getting mossy in the monsoon.
- Exam-type : Milk souring takes hours (slow); a balloon bursting takes less than a second (fast).
In the figure : In the figure P: melted wax can be changed back to solid wax again and again (reversible). Q and R: a ripe mango cannot become raw again and wood cannot be obtained from its ash (these changes cannot be reversed).
Rule 2: Reversible and irreversible — A reversible change can occur in the forward and reverse direction again and again, e.g. melting wax and getting solid wax back. A ripe mango cannot become raw again, and wood cannot be obtained from the ash formed on burning it: irreversible.
- Easy : Ice melting and freezing again: reversible.
- Medium : Frying a papad: irreversible.
- Exam-type : Activity: a piece of a glass bangle held in a candle flame becomes soft in the middle and can be bent into a link; links form a toran.
Fast = short time. Slow = long time. Reversible = can be undone. Irreversible = cannot.
Thinking slow changes are always irreversible. Melting of ice is slow sometimes but reversible.
Memory clue: "Re-versible = can Return"; burnt, cooked and ripened things do not return.
Spot the difference
| Pair | Quick one | Slow one |
|---|---|---|
| Time | Fast: short | Slow: long |
| Undo | Reversible: possible | Irreversible: not possible |
| Example | Melting wax | Burning wood |
Practice questions
- 1What is a fast change?
- 2What is a slow change?
- 3What is a reversible change?
- 4Give one example of a reversible change.
- 5Give one example of an irreversible change.
- 6Classify the bursting of a balloon as fast or slow.
- 7Classify "milk turning into curd" as fast or slow, and reversible or irreversible.
- 8Reena says ice melting can never be reversed because water cannot be seen turning back. Correct her.
Show answers (try them yourself first)
- 1A change that takes a short time.
- 2A change that takes a long time.
- 3A change that can occur again and again in the forward and reverse direction.
- 4Melting wax and getting solid wax back.
- 5Ripening of a mango (or burning of wood).
- 6Fast
- 7Slow and irreversible.
- 8Water freezes back into ice, so melting is reversible.
Periodic and non-periodic changes
The sun rises every day after about 24 hours, and Diwali comes every year. But no one can tell the exact hour of tomorrow's rain shower. Some changes keep a fixed schedule; some do not.
A periodic change (आवर्ती बदल) repeats after a definite interval of time. A non-periodic change (अनावर्ती बदल) does not repeat at fixed intervals.
In the figure : In the figure P: the sunrise recurs after the same gap of 24 hours (a periodic change). Q: rain showers do not recur after a fixed gap (a non-periodic change). The times are sample times.
Rule 1: Periodic and non-periodic — A change that occurs again and again after a definite interval is periodic. If we cannot say for sure when it will recur, or the interval is not fixed even when it recurs, the change is non-periodic.
- Easy : Periodic: a clock pendulum swinging.
- Medium : Periodic: the moon's phases repeat after a fixed period.
- Exam-type : Non-periodic: an earthquake or a sudden storm; we cannot say when it will come again.
In the figure : In the figure the same change can be classified in many ways: the ripening of a mango is a natural, slow and irreversible change.
Rule 2: Many ways to classify — While classifying changes we take only one criterion at a time, but many different criteria can be applied to the same change: natural/man-made, useful/harmful, fast/slow, reversible/irreversible, periodic/non-periodic.
- Easy : A day is 24 hours long, so day-night repeats after 24 hours.
- Medium : Cooking an egg is man-made, useful, fast and irreversible.
- Exam-type : A year has 365 days, and the seasons repeat every year: periodic.
Periodic = fixed gap. Non-periodic = no fixed gap. One change can be classified in many ways.
Mixing the criteria. Use one criterion at a time while classifying.
Memory clue: "Periodic = Predictable gap" (P for P).
Spot the difference
| Type | Gap | Example |
|---|---|---|
| Periodic | Fixed | Sunrise, seasons |
| Non-periodic | Not fixed | Rain shower, earthquake |
Practice questions
- 1What is a periodic change?
- 2What is a non-periodic change?
- 3Give one example of a periodic change.
- 4Give one example of a non-periodic change.
- 5After how many hours does sunrise recur?
- 6Classify the ripening of a mango in as many ways as you can.
- 7Classify "cooking an egg" as natural/man-made, useful/harmful, fast/slow, reversible/irreversible.
- 8Is an earthquake periodic or non-periodic? Give a reason.
Show answers (try them yourself first)
- 1A change that repeats after a definite interval of time.
- 2A change whose recurrence time is not fixed or not known.
- 3Sunrise
- 4Rain showers
- 524 hours
- 6Natural, slow, irreversible.
- 7Man-made, useful, fast, irreversible.
- 8Non-periodic, because we cannot say for sure when it will occur again.
Physical and chemical changes
Water becomes ice in the freezer, and it is still water. But when you toast bread or burn sugar in a pan, you get something new that tastes and looks different. One change keeps the same substance; the other makes a new one.
A physical change (भौतिक बदल) forms no new substance. A chemical change (रासायनिक बदल) forms a new substance with new properties.
In the figure : In the figure melting and boiling are physical changes: ice, water and steam are the same substance and no new substance is formed.
Rule 1: Physical change — The properties and composition of the original substance remain unchanged. Dissolving, boiling and melting are physical changes.
- Easy : Ice melting to water: physical.
- Medium : Sugar dissolving in water: physical.
- Exam-type : Boiling water gives steam: the same substance in a different state, so physical.
In the figure : In the figure of the sugar activity: sugar is heated in an evaporating dish (A) on a tripod stand (B) over a burner (C). Stop heating when a blackish substance is seen at the bottom.
Rule 2: Chemical change — One substance is transformed into another with new and different properties. Sugar heated in an evaporating dish turns blackish, a new substance.
- Easy : Sugar heated until it turns black: chemical.
- Medium : Wood burning to ash: chemical.
- Exam-type : Magnesium ribbon burns with a bright flame and leaves a white powder: a chemical change, because the powder is a new substance.
In the figure : In the figure of evaporation the heat of the sun turns the water of seawater into vapour (A) and salt is left behind in the pan (C).
Rule 3: Evaporation — Evaporation is the formation of vapour from a liquid. Drying of clothes and formation of salt from seawater are due to it.
- Easy : Wet clothes dry by evaporation.
- Medium : Sea water evaporates and leaves salt: a physical change.
- Exam-type : Salt can be dissolved again in water, so making salt by evaporation is reversible.
In the figure : In the figure physical changes are melting of ice, boiling of water and dissolving sugar. Chemical changes are rusting of iron, wood burning to ash and sugar turning black.
Physical and chemical changes
In the app you can move this figure with your finger.
Open in the appPhysical: same substance, no new substance. Chemical: new substance, new properties.
Thinking all irreversible changes are chemical. Some physical changes are hard to reverse.
Memory clue: "Chemical = Creates a new thing"; physical keeps the same thing.
Spot the difference
| Point | Physical change | Chemical change |
|---|---|---|
| New substance? | No | Yes |
| Properties | Same | New and different |
| Examples | Melting, boiling, dissolving | Rusting, burning, souring |
Practice questions
- 1What is a physical change?
- 2What is a chemical change?
- 3Give two examples of physical changes.
- 4Give two examples of chemical changes.
- 5What is evaporation?
- 6How is salt obtained from sea water?
- 7Classify as physical or chemical: dissolving sugar, burning sugar, melting wax, rusting.
- 8Rina says melting of ice is a chemical change because the ice disappears. Correct her.
Show answers (try them yourself first)
- 1A change in which no new substance is formed.
- 2A change in which a new substance with new properties is formed.
- 3Melting of ice and boiling of water.
- 4Rusting of iron and burning of wood.
- 5Formation of vapour from a liquid.
- 6By evaporation of the water by the heat of the sun.
- 7Physical: dissolving sugar, melting wax. Chemical: burning sugar, rusting.
- 8It is physical; ice and water are the same substance and no new substance forms.
Corrosion and its prevention
An iron gate left outside in the rain slowly gets a reddish-brown coat and flakes off. Old brass vessels turn greenish. The metal is slowly being eaten away, and farmers and shopkeepers know the loss.
Corrosion (क्षरण) is the slow damage of metals by oxygen, moisture and chemical vapours in the air.
In the figure : In the figure a reddish brown layer forms on iron (P) and a greenish layer forms on copper (Q). The metal becomes weak.
Rule 1: What corrosion does — When an iron article rusts, a reddish brown layer forms; a greenish layer forms on copper. The articles become weak.
- Easy : Rust on iron is reddish brown.
- Medium : The green layer on copper vessels is also corrosion.
- Exam-type : Rusting is a chemical change, because a new substance (rust) is formed.
In the figure : In the figure corrosion is caused by oxygen, moisture and vapours of chemicals in the air.
Rule 2: Causes — Corrosion is caused by oxygen, moisture and vapours of chemicals in the air.
- Easy : An iron nail left in damp air rusts.
- Medium : A tool kept dry and oiled rusts more slowly.
- Exam-type : Coastal towns have damp, salty air, so iron things rust faster there.
In the figure : In the figure prevention: galvanization is a thin coat of zinc on iron (P), tinning is a coat of tin on copper and brass (Q), and powder coating gives coats of various colours on metals like iron and aluminium (R).
Rule 3: Prevention — A thin coat of zinc on iron articles is galvanization. Copper and brass articles are coated with tin: tinning. In powder coating, coats of various colours are applied on metals like iron and aluminium.
- Easy : Galvanization: zinc coat on iron.
- Medium : Tinning: tin coat on copper and brass vessels.
- Exam-type : Painting a gate keeps air and moisture away from the metal, just like a coat of zinc does.
Corrosion needs oxygen and moisture. Prevent it by covering the metal: zinc, tin, or powder coat.
Thinking only iron corrodes. Copper also does, with a green layer.
Memory clue: "Z-T-P": Zinc for iron (galvanizing), Tin for copper and brass, Powder coat for iron and aluminium.
Spot the difference
| Method | Coat | On |
|---|---|---|
| Galvanization | Zinc | Iron |
| Tinning | Tin | Copper and brass |
| Powder coating | Coloured powder | Iron, aluminium |
Practice questions
- 1What is corrosion?
- 2What colour is the layer on corroded iron?
- 3What colour is the layer on corroded copper?
- 4Name the three causes of corrosion.
- 5What is galvanization?
- 6What is tinning and on which metals is it done?
- 7Why does a painted iron gate rust more slowly?
- 8Is rusting a physical or a chemical change? Give a reason.
Show answers (try them yourself first)
- 1Slow damage of metals by oxygen, moisture and chemical vapours.
- 2Reddish brown
- 3Greenish
- 4Oxygen, moisture and vapours of chemicals in the air.
- 5A thin coat of zinc on iron articles.
- 6Coating with tin; on copper and brass.
- 7The coat keeps air and moisture away from the metal.
- 8Chemical, because a new substance (rust) is formed.
Quick revision
- Changes can be natural/man-made, useful/harmful, fast/slow, reversible/irreversible, periodic/non-periodic.
- Physical change: no new substance; chemical change: a new substance with new properties.
- Corrosion weakens metals; galvanization, tinning and powder coating prevent it.