A New Substance Is the Only Test That Actually Works
Learn the characteristics of a physical change, the evidence that proves a chemical change has happened, how the two compare on mass and reversibility, and which conditions bring a chemical change about.
Is a change chemical just because it cannot be undone?
No — and that is the trap in this chapter.
Tearing a page into fifty pieces cannot be undone, yet nothing new was made: every scrap is still paper. Dissolving salt in water looks completely irreversible until you evaporate the water and the salt crystals come back.
The only reliable test is whether a new substance with new properties has been formed. Reversibility, appearance and permanence are all clues that can mislead. This page covers the second part of the ICSE Class 8 Chemistry chapter on changes: what marks a physical change, what proves a chemical one, how they compare, and the conditions that make a chemical change happen.
Tearing a page into fifty pieces cannot be undone, yet nothing new was made: every scrap is still paper. Dissolving salt in water looks completely irreversible until you evaporate the water and the salt crystals come back.
The only reliable test is whether a new substance with new properties has been formed. Reversibility, appearance and permanence are all clues that can mislead. This page covers the second part of the ICSE Class 8 Chemistry chapter on changes: what marks a physical change, what proves a chemical one, how they compare, and the conditions that make a chemical change happen.
What are the characteristics of a physical change?
A physical change alters the state, shape, size or appearance of a substance without producing any new substance.
Its characteristics:
- No new substance is formed — the chemical composition is unchanged.
- It is usually reversible, and the original can be recovered.
- The mass remains the same.
- Any energy change is small, and is usually absorbed or given out as heat during a change of state.
- The substance keeps its own properties — the same melting point, density and colour on returning.
Examples, with what actually changed:
- Melting ice — state changes; still .
- Boiling water — state changes; the steam condenses back to the same water.
- Dissolving sugar or salt in water — the solute is dispersed, and evaporation returns it.
- Breaking a glass, tearing paper, cutting vegetables — size and shape change only.
- Stretching a rubber band, inflating a balloon, beating gold into foil — shape or size only.
- Magnetising an iron nail, glowing of an electric bulb filament — a property changes and then returns.
- Sublimation of camphor — state changes; cool the vapour and solid camphor is back.
The two that look chemical but are not. Dissolving a substance is physical, because the solute can be recovered. And the glowing of a bulb filament is physical — the tungsten is merely hot, and returns to exactly what it was on switching off.
The boundary case worth stating. A physical change need not be easy to reverse, only possible in principle. Tearing paper and grinding wheat into flour are both physical: you cannot reassemble them, but no new substance was created, and every particle is still paper or still wheat. This is why reversibility is a guide and composition is the test.
Its characteristics:
- No new substance is formed — the chemical composition is unchanged.
- It is usually reversible, and the original can be recovered.
- The mass remains the same.
- Any energy change is small, and is usually absorbed or given out as heat during a change of state.
- The substance keeps its own properties — the same melting point, density and colour on returning.
Examples, with what actually changed:
- Melting ice — state changes; still .
- Boiling water — state changes; the steam condenses back to the same water.
- Dissolving sugar or salt in water — the solute is dispersed, and evaporation returns it.
- Breaking a glass, tearing paper, cutting vegetables — size and shape change only.
- Stretching a rubber band, inflating a balloon, beating gold into foil — shape or size only.
- Magnetising an iron nail, glowing of an electric bulb filament — a property changes and then returns.
- Sublimation of camphor — state changes; cool the vapour and solid camphor is back.
The two that look chemical but are not. Dissolving a substance is physical, because the solute can be recovered. And the glowing of a bulb filament is physical — the tungsten is merely hot, and returns to exactly what it was on switching off.
The boundary case worth stating. A physical change need not be easy to reverse, only possible in principle. Tearing paper and grinding wheat into flour are both physical: you cannot reassemble them, but no new substance was created, and every particle is still paper or still wheat. This is why reversibility is a guide and composition is the test.
What evidence proves a chemical change has taken place?
A chemical change produces one or more new substances with different properties from the originals. It is also called a chemical reaction.
Since you cannot see composition directly, you look for observable signs — and one or more of these usually appear.
- Evolution of a gas. Drop zinc granules into dilute sulphuric acid and bubbles of hydrogen come off steadily. Add dilute acid to baking soda and carbon dioxide fizzes out.
- Change in colour. Put an iron nail into blue copper sulphate solution: the solution turns pale green and a brown deposit of copper appears on the nail. Neither colour existed before.
- Formation of a precipitate. Mix barium chloride solution with sodium sulphate solution and a white insoluble solid settles out. Lead nitrate with potassium iodide gives a yellow precipitate.
- Change in temperature. Add water to quicklime and the vessel becomes noticeably hot — heat is given out, so the change is exothermic. Some reactions absorb heat instead and are endothermic.
- Emission of light. A burning magnesium ribbon gives a dazzling white light along with its white ash.
- Change in smell. Milk turning sour, or food spoiling, gives off a smell that was not there before.
Its characteristics, to set against the physical list:
- A new substance is formed, with new properties.
- It is usually irreversible by simple means.
- Energy is absorbed or released, often in substantial amounts.
- The new substance does not retain the properties of the originals.
Why the properties point matters more than it sounds. Sodium is a soft metal that reacts violently with water; chlorine is a poisonous green gas. Combine them and you get common salt — safe, stable and eaten daily. Nothing about the product could be guessed from its ingredients, and that is the strongest possible evidence that something genuinely new exists.
One sign that can deceive. A colour change alone does not settle it. Heating a purple crystal of iodine gives violet vapour, which looks dramatic but is only sublimation — a physical change, since cooling returns the same crystals. So look for a new substance that stays new.
Since you cannot see composition directly, you look for observable signs — and one or more of these usually appear.
- Evolution of a gas. Drop zinc granules into dilute sulphuric acid and bubbles of hydrogen come off steadily. Add dilute acid to baking soda and carbon dioxide fizzes out.
- Change in colour. Put an iron nail into blue copper sulphate solution: the solution turns pale green and a brown deposit of copper appears on the nail. Neither colour existed before.
- Formation of a precipitate. Mix barium chloride solution with sodium sulphate solution and a white insoluble solid settles out. Lead nitrate with potassium iodide gives a yellow precipitate.
- Change in temperature. Add water to quicklime and the vessel becomes noticeably hot — heat is given out, so the change is exothermic. Some reactions absorb heat instead and are endothermic.
- Emission of light. A burning magnesium ribbon gives a dazzling white light along with its white ash.
- Change in smell. Milk turning sour, or food spoiling, gives off a smell that was not there before.
Its characteristics, to set against the physical list:
- A new substance is formed, with new properties.
- It is usually irreversible by simple means.
- Energy is absorbed or released, often in substantial amounts.
- The new substance does not retain the properties of the originals.
Why the properties point matters more than it sounds. Sodium is a soft metal that reacts violently with water; chlorine is a poisonous green gas. Combine them and you get common salt — safe, stable and eaten daily. Nothing about the product could be guessed from its ingredients, and that is the strongest possible evidence that something genuinely new exists.
One sign that can deceive. A colour change alone does not settle it. Heating a purple crystal of iodine gives violet vapour, which looks dramatic but is only sublimation — a physical change, since cooling returns the same crystals. So look for a new substance that stays new.
How do physical and chemical changes compare?
On four points, and it is worth holding them as contrasts rather than as two separate lists.
- New substance. Physical: none. Chemical: formed, with different properties.
- Reversibility. Physical: usually reversible by simple means. Chemical: usually not.
- Change in mass. Physical: no change. Chemical: the mass of the product differs from the mass of the starting substance, though the total mass of everything taking part is conserved.
- Energy change. Physical: small. Chemical: often large, absorbed or released as heat, light or electricity.
The mass point needs care, because it is where most marks are lost. Nothing is ever created or destroyed. But the product can easily be heavier or lighter than what you started with, because something from the surroundings joined in or escaped.
Worked example — burning magnesium in air. The reaction is
Taking the atomic masses of magnesium as and oxygen as :
So of shiny magnesium ribbon leaves of white ash. The ash is heavier than the ribbon — by exactly the mass of oxygen that combined with it.
And the reverse case. Burn a candle or a piece of paper and the residue weighs less than you started with, because carbon dioxide and water vapour have gone off into the air. Collect those gases and the total comes out unchanged.
Rusting gains mass for the same reason as magnesium. In
with iron as :
Rusted iron is heavier than the iron was, which is why a badly rusted object is not simply worn away.
So the correct statement is precise. Mass is conserved in every change, and the mass of the product may differ from the mass of the reactant you weighed. Both are true, and writing only one of them is what causes the confusion.
- New substance. Physical: none. Chemical: formed, with different properties.
- Reversibility. Physical: usually reversible by simple means. Chemical: usually not.
- Change in mass. Physical: no change. Chemical: the mass of the product differs from the mass of the starting substance, though the total mass of everything taking part is conserved.
- Energy change. Physical: small. Chemical: often large, absorbed or released as heat, light or electricity.
The mass point needs care, because it is where most marks are lost. Nothing is ever created or destroyed. But the product can easily be heavier or lighter than what you started with, because something from the surroundings joined in or escaped.
Worked example — burning magnesium in air. The reaction is
Taking the atomic masses of magnesium as and oxygen as :
So of shiny magnesium ribbon leaves of white ash. The ash is heavier than the ribbon — by exactly the mass of oxygen that combined with it.
And the reverse case. Burn a candle or a piece of paper and the residue weighs less than you started with, because carbon dioxide and water vapour have gone off into the air. Collect those gases and the total comes out unchanged.
Rusting gains mass for the same reason as magnesium. In
with iron as :
Rusted iron is heavier than the iron was, which is why a badly rusted object is not simply worn away.
So the correct statement is precise. Mass is conserved in every change, and the mass of the product may differ from the mass of the reactant you weighed. Both are true, and writing only one of them is what causes the confusion.
What conditions are needed to bring about a chemical change?
Five, and each can be matched to a specific reaction.
Heat. The commonest condition. Heating limestone, which is calcium carbonate, breaks it into quicklime and carbon dioxide:
Cooking food, burning fuel and burning a magnesium ribbon all need heat to start.
Light. Some reactions need light rather than heat. Photosynthesis combines carbon dioxide and water into glucose in the presence of sunlight and chlorophyll. Silver chloride, which is white, turns grey on exposure to sunlight as it decomposes — the reaction behind photographic film.
Electricity. Passing electricity through acidified water splits it into its elements:
This is electrolysis, and the same idea drives electroplating, where a metal coating is deposited from a solution.
Pressure. Some reactions proceed only under high pressure. Nitrogen and hydrogen combine to form ammonia under high pressure with heat and a catalyst — a reaction central to making fertiliser.
A catalyst. A catalyst speeds up a reaction without itself being used up. Manganese dioxide added to hydrogen peroxide makes it release oxygen rapidly, while the manganese dioxide can be recovered unchanged at the end. Enzymes are the body's catalysts, and they are what make digestion possible at body temperature.
A sixth condition, often listed separately — solution or mixing. Many reactions happen only when the substances are dissolved and their particles can meet. Dry barium chloride and dry sodium sulphate powders can sit together indefinitely; dissolve both and the white precipitate appears immediately.
What a catalyst does not do. It does not start a reaction that would otherwise never happen, and it is not consumed. Hydrogen peroxide decomposes on its own — slowly. The catalyst only changes the rate, which is why it can be filtered off and weighed afterwards, unchanged.
And why the conditions matter practically. Knowing that rusting needs both air and moisture tells you how to prevent it: paint, grease, galvanise or keep the metal dry. Every method of prevention is simply the removal of one necessary condition.
Heat. The commonest condition. Heating limestone, which is calcium carbonate, breaks it into quicklime and carbon dioxide:
Cooking food, burning fuel and burning a magnesium ribbon all need heat to start.
Light. Some reactions need light rather than heat. Photosynthesis combines carbon dioxide and water into glucose in the presence of sunlight and chlorophyll. Silver chloride, which is white, turns grey on exposure to sunlight as it decomposes — the reaction behind photographic film.
Electricity. Passing electricity through acidified water splits it into its elements:
This is electrolysis, and the same idea drives electroplating, where a metal coating is deposited from a solution.
Pressure. Some reactions proceed only under high pressure. Nitrogen and hydrogen combine to form ammonia under high pressure with heat and a catalyst — a reaction central to making fertiliser.
A catalyst. A catalyst speeds up a reaction without itself being used up. Manganese dioxide added to hydrogen peroxide makes it release oxygen rapidly, while the manganese dioxide can be recovered unchanged at the end. Enzymes are the body's catalysts, and they are what make digestion possible at body temperature.
A sixth condition, often listed separately — solution or mixing. Many reactions happen only when the substances are dissolved and their particles can meet. Dry barium chloride and dry sodium sulphate powders can sit together indefinitely; dissolve both and the white precipitate appears immediately.
What a catalyst does not do. It does not start a reaction that would otherwise never happen, and it is not consumed. Hydrogen peroxide decomposes on its own — slowly. The catalyst only changes the rate, which is why it can be filtered off and weighed afterwards, unchanged.
And why the conditions matter practically. Knowing that rusting needs both air and moisture tells you how to prevent it: paint, grease, galvanise or keep the metal dry. Every method of prevention is simply the removal of one necessary condition.
Exam tip
Exam tip: name the new substance, not just the change
To prove a change is chemical, name the new substance formed and its new property. Iron and oxygen form rust, which is brown, flaky and not magnetic like iron earns far more than it is a chemical change because it cannot be undone.
Never use irreversibility as your only argument. Tearing paper and grinding wheat are irreversible and physical.
Give at least two pieces of evidence when identifying a chemical change — gas evolution, colour change, precipitate, temperature change, light or smell.
Be exact about mass. Write that total mass is conserved, and that the product may weigh more or less than the substance weighed at the start — magnesium ash is heavier because oxygen joined; candle residue is lighter because gases escaped.
For the magnesium calculation, set out the masses line by line: , , total .
When listing conditions, pair each with a reaction: heat with limestone decomposing, light with photosynthesis or silver chloride, electricity with electrolysis of water, pressure with ammonia, a catalyst with manganese dioxide and hydrogen peroxide.
Say a catalyst changes the rate and is not consumed — both halves are needed.
And remember dissolving is physical and glowing of a bulb filament is physical. Both are common wrong answers.
Never use irreversibility as your only argument. Tearing paper and grinding wheat are irreversible and physical.
Give at least two pieces of evidence when identifying a chemical change — gas evolution, colour change, precipitate, temperature change, light or smell.
Be exact about mass. Write that total mass is conserved, and that the product may weigh more or less than the substance weighed at the start — magnesium ash is heavier because oxygen joined; candle residue is lighter because gases escaped.
For the magnesium calculation, set out the masses line by line: , , total .
When listing conditions, pair each with a reaction: heat with limestone decomposing, light with photosynthesis or silver chloride, electricity with electrolysis of water, pressure with ammonia, a catalyst with manganese dioxide and hydrogen peroxide.
Say a catalyst changes the rate and is not consumed — both halves are needed.
And remember dissolving is physical and glowing of a bulb filament is physical. Both are common wrong answers.
Did you know
Why is common salt safe when both its elements are dangerous?
Sodium is a soft metal that must be stored under oil, because it reacts fiercely with the moisture in air. Chlorine is a choking green gas. Neither is anything you would want near food.
Combine them chemically and the result is sodium chloride — ordinary table salt, stable enough to sit in an open bowl and necessary in every diet.
Nothing about salt could be predicted from its ingredients. It is not a soft metal, it is not a green gas, and it is not a mixture of the two in any sense you could separate with a sieve or a magnet. Its melting point, its appearance, its taste and its behaviour in water all belong to salt alone.
That is precisely what a chemical change means, and why a new substance with new properties is the definition rather than a description. In a physical change you can always point to the original substance still being there. Here you cannot point to the sodium or the chlorine at all.
Combine them chemically and the result is sodium chloride — ordinary table salt, stable enough to sit in an open bowl and necessary in every diet.
Nothing about salt could be predicted from its ingredients. It is not a soft metal, it is not a green gas, and it is not a mixture of the two in any sense you could separate with a sieve or a magnet. Its melting point, its appearance, its taste and its behaviour in water all belong to salt alone.
That is precisely what a chemical change means, and why a new substance with new properties is the definition rather than a description. In a physical change you can always point to the original substance still being there. Here you cannot point to the sodium or the chlorine at all.
Key takeaways
Physical and chemical changes: quick revision
- A physical change alters state, shape, size or appearance with no new substance, is usually reversible, keeps the mass unchanged, and involves only small energy changes.
- Examples: melting ice, boiling water, dissolving salt, tearing paper, stretching rubber, magnetising a nail, sublimation of camphor, glowing of a bulb filament.
- A chemical change forms a new substance with new properties, is usually irreversible by simple means, and involves a large energy change.
- Evidence: gas evolution (zinc with acid), colour change (iron nail in copper sulphate — pale green solution, brown deposit), precipitate (barium chloride with sodium sulphate — white), temperature change (quicklime with water), light (burning magnesium), smell (milk souring).
- Comparison: new substance — none against formed; reversibility — usually against usually not; mass — unchanged against product differing; energy — small against large.
- Total mass is always conserved, but the product may weigh more or less: turns of ribbon into of ash, and turns of iron into of rust. A burnt candle leaves less, because gases escaped.
- Conditions: heat (), light (photosynthesis, silver chloride darkening), electricity (electrolysis of water, electroplating), pressure (ammonia from nitrogen and hydrogen), catalyst (manganese dioxide with hydrogen peroxide, enzymes in digestion), and solution for reactions needing particles to meet.
- A catalyst changes only the rate and is not consumed.
- Irreversibility is not the test — tearing paper and grinding wheat are irreversible and physical. The test is a new substance.
- Sodium and chlorine forming common salt shows a product whose properties resemble neither ingredient.
Practise identifying changes from a mixed list, giving the new substance for each chemical one — that single habit turns this chapter from guesswork into a method.
- Examples: melting ice, boiling water, dissolving salt, tearing paper, stretching rubber, magnetising a nail, sublimation of camphor, glowing of a bulb filament.
- A chemical change forms a new substance with new properties, is usually irreversible by simple means, and involves a large energy change.
- Evidence: gas evolution (zinc with acid), colour change (iron nail in copper sulphate — pale green solution, brown deposit), precipitate (barium chloride with sodium sulphate — white), temperature change (quicklime with water), light (burning magnesium), smell (milk souring).
- Comparison: new substance — none against formed; reversibility — usually against usually not; mass — unchanged against product differing; energy — small against large.
- Total mass is always conserved, but the product may weigh more or less: turns of ribbon into of ash, and turns of iron into of rust. A burnt candle leaves less, because gases escaped.
- Conditions: heat (), light (photosynthesis, silver chloride darkening), electricity (electrolysis of water, electroplating), pressure (ammonia from nitrogen and hydrogen), catalyst (manganese dioxide with hydrogen peroxide, enzymes in digestion), and solution for reactions needing particles to meet.
- A catalyst changes only the rate and is not consumed.
- Irreversibility is not the test — tearing paper and grinding wheat are irreversible and physical. The test is a new substance.
- Sodium and chlorine forming common salt shows a product whose properties resemble neither ingredient.
Practise identifying changes from a mixed list, giving the new substance for each chemical one — that single habit turns this chapter from guesswork into a method.