Some Reactions Warm the Test Tube and Some Chill It
Learn the observable signs that a chemical reaction has taken place, the seven conditions a reaction may need before it will start, and how to tell an exothermic reaction from an endothermic one.
Why does one reaction heat a test tube while another cools it?
Because a reaction both breaks old bonds and makes new ones, and those two jobs are not equally expensive.
Breaking bonds absorbs energy; making them releases energy. If more comes out than went in, the surplus escapes as heat and the test tube warms — that is exothermic. If more goes in than comes out, the shortfall is drawn from the surroundings and the test tube cools — that is endothermic.
So a thermometer in a reaction mixture is telling you something real about the chemistry, not just about the water bath. This page covers the first part of the ICSE Class 8 Chemistry chapter on chemical reactions: how to spot one, what it needs to start, and which way its energy flows.
Breaking bonds absorbs energy; making them releases energy. If more comes out than went in, the surplus escapes as heat and the test tube warms — that is exothermic. If more goes in than comes out, the shortfall is drawn from the surroundings and the test tube cools — that is endothermic.
So a thermometer in a reaction mixture is telling you something real about the chemistry, not just about the water bath. This page covers the first part of the ICSE Class 8 Chemistry chapter on chemical reactions: how to spot one, what it needs to start, and which way its energy flows.
What evidence tells you a chemical reaction has taken place?
A reaction forms a new substance, and since composition cannot be seen directly, you look for observable signs. One or more of these appear.
Evolution of a gas. Bubbles come off steadily, not from boiling. Drop zinc into dilute sulphuric acid:
Adding dilute acid to baking soda releases carbon dioxide the same way.
Change in colour. Stand an iron nail in blue copper sulphate solution and the solution fades to pale green while a reddish-brown deposit of copper builds on the nail:
Formation of a precipitate. Mix lead nitrate with potassium iodide and a yellow insoluble solid appears at once:
Change in temperature. The mixture becomes hot or cold, as the next-but-one section explains in detail.
Change in state. A solid and a liquid may give a gas, or two solutions may give a solid.
Emission of light. Burning magnesium gives a dazzling white light along with its white ash.
Change in smell. Milk souring or food spoiling produces a smell that was not there before.
The one sign that can mislead. A colour change alone is not conclusive. Heating iodine gives a striking violet vapour and no reaction has occurred — that is sublimation, a physical change, and cooling returns the same crystals. So look for a change that persists, and if possible for a second sign alongside the first.
The safest test of all. Ask whether the product has different properties from what you started with. The copper on the nail is not iron and not copper sulphate; it is a red-brown metal that neither of the starting substances resembled.
Evolution of a gas. Bubbles come off steadily, not from boiling. Drop zinc into dilute sulphuric acid:
Adding dilute acid to baking soda releases carbon dioxide the same way.
Change in colour. Stand an iron nail in blue copper sulphate solution and the solution fades to pale green while a reddish-brown deposit of copper builds on the nail:
Formation of a precipitate. Mix lead nitrate with potassium iodide and a yellow insoluble solid appears at once:
Change in temperature. The mixture becomes hot or cold, as the next-but-one section explains in detail.
Change in state. A solid and a liquid may give a gas, or two solutions may give a solid.
Emission of light. Burning magnesium gives a dazzling white light along with its white ash.
Change in smell. Milk souring or food spoiling produces a smell that was not there before.
The one sign that can mislead. A colour change alone is not conclusive. Heating iodine gives a striking violet vapour and no reaction has occurred — that is sublimation, a physical change, and cooling returns the same crystals. So look for a change that persists, and if possible for a second sign alongside the first.
The safest test of all. Ask whether the product has different properties from what you started with. The copper on the nail is not iron and not copper sulphate; it is a red-brown metal that neither of the starting substances resembled.
What conditions does a reaction need before it will start?
Seven, and a reaction that seems not to happen is usually missing one of them.
Close contact. The particles must actually meet. Dry barium chloride and dry sodium sulphate powders can sit mixed in a bottle indefinitely and do nothing; grinding them together, or better still dissolving them, brings the particles into contact and the reaction is immediate. Powdering a solid speeds a reaction for the same reason — more surface is exposed.
Heat. The commonest condition. Limestone will not decompose at room temperature but does on strong heating:
Burning fuel, cooking food and igniting a magnesium ribbon all need heat to begin.
Light. Some reactions need light rather than heat. Photosynthesis needs sunlight and chlorophyll:
White silver chloride turns grey in sunlight as it decomposes, which is why it is stored in dark bottles.
Electricity. Passing a current through acidified water splits it:
Electroplating works the same way.
Pressure. Nitrogen and hydrogen combine to give ammonia only under high pressure, with heat and a catalyst.
Solvent. Many reactions proceed only in solution, because dissolving separates the particles and lets them move and meet freely. This is why so many test-tube reactions begin with dissolve in water.
Catalyst. A catalyst changes the rate of a reaction without being consumed. Manganese dioxide added to hydrogen peroxide releases oxygen rapidly and can be recovered unchanged afterwards. Enzymes are the body's catalysts, letting digestion proceed at body temperature.
What a catalyst cannot do. It cannot start a reaction that would otherwise never occur, and it is never used up. Hydrogen peroxide decomposes on its own — just very slowly — and the catalyst only alters how fast.
Why conditions are practically useful. Rusting needs air and moisture together. Remove either and rusting stops, which is why painting, greasing, galvanising and keeping metal dry all work. Every method of preventing a reaction is the removal of a necessary condition.
Close contact. The particles must actually meet. Dry barium chloride and dry sodium sulphate powders can sit mixed in a bottle indefinitely and do nothing; grinding them together, or better still dissolving them, brings the particles into contact and the reaction is immediate. Powdering a solid speeds a reaction for the same reason — more surface is exposed.
Heat. The commonest condition. Limestone will not decompose at room temperature but does on strong heating:
Burning fuel, cooking food and igniting a magnesium ribbon all need heat to begin.
Light. Some reactions need light rather than heat. Photosynthesis needs sunlight and chlorophyll:
White silver chloride turns grey in sunlight as it decomposes, which is why it is stored in dark bottles.
Electricity. Passing a current through acidified water splits it:
Electroplating works the same way.
Pressure. Nitrogen and hydrogen combine to give ammonia only under high pressure, with heat and a catalyst.
Solvent. Many reactions proceed only in solution, because dissolving separates the particles and lets them move and meet freely. This is why so many test-tube reactions begin with dissolve in water.
Catalyst. A catalyst changes the rate of a reaction without being consumed. Manganese dioxide added to hydrogen peroxide releases oxygen rapidly and can be recovered unchanged afterwards. Enzymes are the body's catalysts, letting digestion proceed at body temperature.
What a catalyst cannot do. It cannot start a reaction that would otherwise never occur, and it is never used up. Hydrogen peroxide decomposes on its own — just very slowly — and the catalyst only alters how fast.
Why conditions are practically useful. Rusting needs air and moisture together. Remove either and rusting stops, which is why painting, greasing, galvanising and keeping metal dry all work. Every method of preventing a reaction is the removal of a necessary condition.
How do you classify a reaction as exothermic or endothermic?
By whether heat is released to the surroundings or absorbed from them.
Exothermic reactions give out heat, so the mixture and its container become warm. The heat is written as a product.
- Quicklime and water — the vessel becomes distinctly hot:
- Burning of any fuel — charcoal, wood, cooking gas:
- Burning magnesium — heat and light:
- Neutralisation of an acid by an alkali:
- Respiration in the body, which is why we stay warm; rusting, which is exothermic but far too slow to feel; and dissolving sodium hydroxide in water, which heats the beaker noticeably.
Endothermic reactions absorb heat, so the mixture becomes cold or needs heating continuously to proceed. The heat is written as a reactant.
- Decomposition of limestone, which stops the moment you remove the flame:
- Photosynthesis, which absorbs energy from sunlight.
- Electrolysis of water, which absorbs electrical energy.
- Dissolving ammonium chloride in water — the beaker becomes noticeably cold to touch, which is the easiest endothermic change to demonstrate.
- Cooking food, which needs a continuous supply of heat.
How to tell them apart in the laboratory. Touch the container, or better, put a thermometer in. A rise in temperature means exothermic; a fall means endothermic. That is the whole test.
The distinction students get wrong. Needs heat to start is not the same as endothermic. Burning magnesium needs a flame to begin, yet once lit it pours out far more heat than it took — it is firmly exothermic. The question is not what starts the reaction but whether, once running, it gives out or takes in energy overall. A reaction that continues on its own after being lit is exothermic by that very fact.
Exothermic reactions give out heat, so the mixture and its container become warm. The heat is written as a product.
- Quicklime and water — the vessel becomes distinctly hot:
- Burning of any fuel — charcoal, wood, cooking gas:
- Burning magnesium — heat and light:
- Neutralisation of an acid by an alkali:
- Respiration in the body, which is why we stay warm; rusting, which is exothermic but far too slow to feel; and dissolving sodium hydroxide in water, which heats the beaker noticeably.
Endothermic reactions absorb heat, so the mixture becomes cold or needs heating continuously to proceed. The heat is written as a reactant.
- Decomposition of limestone, which stops the moment you remove the flame:
- Photosynthesis, which absorbs energy from sunlight.
- Electrolysis of water, which absorbs electrical energy.
- Dissolving ammonium chloride in water — the beaker becomes noticeably cold to touch, which is the easiest endothermic change to demonstrate.
- Cooking food, which needs a continuous supply of heat.
How to tell them apart in the laboratory. Touch the container, or better, put a thermometer in. A rise in temperature means exothermic; a fall means endothermic. That is the whole test.
The distinction students get wrong. Needs heat to start is not the same as endothermic. Burning magnesium needs a flame to begin, yet once lit it pours out far more heat than it took — it is firmly exothermic. The question is not what starts the reaction but whether, once running, it gives out or takes in energy overall. A reaction that continues on its own after being lit is exothermic by that very fact.
Exam tip
Exam tip: quote heat on the correct side of the arrow
Write heat as a product for an exothermic reaction and as a reactant for an endothermic one. against . Which side it sits on is the classification.
Never confuse needs heat to start with endothermic. Burning magnesium needs a flame and is exothermic, because it releases far more than it took.
When giving evidence of a reaction, name the observation and the substance: a yellow precipitate of lead iodide, not just a precipitate.
Give at least two signs where you can, and avoid relying on colour change alone — iodine sublimation looks dramatic and is physical.
For conditions, pair each with a reaction: close contact with the dry-powders case, heat with limestone, light with photosynthesis or silver chloride, electricity with electrolysis, pressure with ammonia, solvent with test-tube reactions, catalyst with manganese dioxide.
Say a catalyst changes the rate and is not consumed — both halves are needed for the mark.
Write every equation balanced, and add state symbols if the question asks.
And remember the laboratory test for the classification: a thermometer. Temperature up means exothermic, down means endothermic.
Never confuse needs heat to start with endothermic. Burning magnesium needs a flame and is exothermic, because it releases far more than it took.
When giving evidence of a reaction, name the observation and the substance: a yellow precipitate of lead iodide, not just a precipitate.
Give at least two signs where you can, and avoid relying on colour change alone — iodine sublimation looks dramatic and is physical.
For conditions, pair each with a reaction: close contact with the dry-powders case, heat with limestone, light with photosynthesis or silver chloride, electricity with electrolysis, pressure with ammonia, solvent with test-tube reactions, catalyst with manganese dioxide.
Say a catalyst changes the rate and is not consumed — both halves are needed for the mark.
Write every equation balanced, and add state symbols if the question asks.
And remember the laboratory test for the classification: a thermometer. Temperature up means exothermic, down means endothermic.
Did you know
Why does an instant cold pack get cold with no fridge involved?
A chemical cold pack contains a salt and a separate pouch of water. Squeeze it to burst the inner pouch and the salt dissolves — and within seconds the pack is cold enough to soothe a swelling.
Nothing was refrigerated and nothing was taken away. The dissolving simply needs more energy to pull the salt's particles apart than it releases when they settle among the water molecules, and the shortfall is drawn from the surroundings. What it draws from first is the water in the pack, and then whatever the pack is pressed against.
Ammonium chloride and ammonium nitrate both behave this way, which is why the same effect can be shown with a beaker, a thermometer and a spoonful of ammonium chloride.
The reverse pack exists too, using a salt whose dissolving is exothermic. Same device, same squeeze, opposite direction of heat flow — and the only thing that changed was which of the two energy costs happened to be larger.
Nothing was refrigerated and nothing was taken away. The dissolving simply needs more energy to pull the salt's particles apart than it releases when they settle among the water molecules, and the shortfall is drawn from the surroundings. What it draws from first is the water in the pack, and then whatever the pack is pressed against.
Ammonium chloride and ammonium nitrate both behave this way, which is why the same effect can be shown with a beaker, a thermometer and a spoonful of ammonium chloride.
The reverse pack exists too, using a salt whose dissolving is exothermic. Same device, same squeeze, opposite direction of heat flow — and the only thing that changed was which of the two energy costs happened to be larger.
Key takeaways
Reactions, conditions and energy change: quick revision
- A chemical reaction forms a new substance; look for observable evidence.
- Signs: gas evolution (), colour change (iron nail in copper sulphate — pale green solution, red-brown copper), precipitate (), temperature change, change of state, light, smell.
- A colour change alone can mislead — iodine subliming is physical. Prefer two signs, or check for new properties.
- Seven conditions: close contact (dry powders do nothing; dissolving or grinding starts it), heat (), light (photosynthesis, silver chloride darkening), electricity (), pressure (ammonia), solvent (most test-tube reactions), catalyst (manganese dioxide with hydrogen peroxide, enzymes).
- A catalyst changes only the rate and is not consumed.
- Preventing a reaction means removing a condition — painting or greasing iron removes air and moisture.
- Exothermic reactions release heat, so write heat as a product: quicklime and water, all combustion, burning magnesium (heat and light), neutralisation, respiration, rusting, dissolving sodium hydroxide.
- Endothermic reactions absorb heat, so write heat as a reactant: limestone decomposing, photosynthesis, electrolysis, dissolving ammonium chloride (the beaker turns cold), cooking.
- The laboratory test is a thermometer — a rise means exothermic, a fall endothermic.
- Needs heat to start is not the same as endothermic; burning magnesium needs a flame and is exothermic.
Sort a list of reactions into exothermic and endothermic and write heat on the correct side of each arrow — that placement is what the question is really checking.
- Signs: gas evolution (), colour change (iron nail in copper sulphate — pale green solution, red-brown copper), precipitate (), temperature change, change of state, light, smell.
- A colour change alone can mislead — iodine subliming is physical. Prefer two signs, or check for new properties.
- Seven conditions: close contact (dry powders do nothing; dissolving or grinding starts it), heat (), light (photosynthesis, silver chloride darkening), electricity (), pressure (ammonia), solvent (most test-tube reactions), catalyst (manganese dioxide with hydrogen peroxide, enzymes).
- A catalyst changes only the rate and is not consumed.
- Preventing a reaction means removing a condition — painting or greasing iron removes air and moisture.
- Exothermic reactions release heat, so write heat as a product: quicklime and water, all combustion, burning magnesium (heat and light), neutralisation, respiration, rusting, dissolving sodium hydroxide.
- Endothermic reactions absorb heat, so write heat as a reactant: limestone decomposing, photosynthesis, electrolysis, dissolving ammonium chloride (the beaker turns cold), cooking.
- The laboratory test is a thermometer — a rise means exothermic, a fall endothermic.
- Needs heat to start is not the same as endothermic; burning magnesium needs a flame and is exothermic.
Sort a list of reactions into exothermic and endothermic and write heat on the correct side of each arrow — that placement is what the question is really checking.