What is a chemical reaction? A chemical reaction happens when one or more substances, called reactants, change into one or more new substances, called products. Bonds between atoms break and new bonds form, so the atoms are rearranged into new combinations. No atoms are created or destroyed, but the new substances have different properties from the ones you started with.
This article explains the definition, what really happens to atoms, and why mass is conserved. It then shows how to spot the signs of a reaction, how a chemical change differs from a physical change, the main types of chemical reaction with balanced equations, and the everyday examples that GCSE courses love to ask about.
What is a chemical reaction? A clear definition
The International Union of Pure and Applied Chemistry (IUPAC) defines a chemical reaction as “a process that results in the interconversion of chemical species”. In plain English: something goes in, something different comes out. The key idea is that new substances are formed.
The AQA GCSE Chemistry specification puts it in a way that is easy to remember. Chemical reactions always involve the formation of one or more new substances, and often involve a detectable energy change. Compounds are formed from elements by chemical reactions, and compounds can only be separated back into their elements by chemical reactions.
Compare that with the physical processes you meet in a lab: filtration, crystallisation, distillation and chromatography. These separate mixtures, but they do not involve chemical reactions, because no new substance is made. In a mixture, each substance keeps its own chemical properties.
Reactants and products
The starting substances in a reaction are the reactants. The new substances that are made are the products. You can describe a reaction in words (a word equation) or with chemical symbols (a symbol equation). Take hydrogen burning in oxygen to make water:
hydrogen + oxygen → water
2H₂ + O₂ → 2H₂O
Hydrogen and oxygen are both gases that exist as pairs of atoms (H₂ and O₂). The product, water, is a liquid with completely different properties. That change in properties is why we call it a chemical reaction rather than a mixing of gases.
What happens to atoms in a chemical reaction?
Every substance is built from atoms, and the atoms in molecules are held together by bonds. If you want a refresher on what atoms are made of, see our guide to atoms and their structure. In a chemical reaction the atoms themselves stay exactly the same. What changes is how they are joined together.
It happens in three steps:
- Collision. Reactions only happen when particles collide with enough energy. The minimum energy needed is called the activation energy.
- Bonds break. Energy must be supplied to break the bonds in the reactants.
- New bonds form. Energy is released when the bonds in the products form, and the atoms are now arranged as new substances.
Our notes on covalent bonds explain how those shared pairs of electrons hold atoms in molecules such as H₂, O₂ and H₂O. Whether a reaction warms or cools its surroundings depends on the balance between the energy taken in to break bonds and the energy given out as new ones form. We return to that in the section on exothermic and endothermic reactions below.
Count the atoms in the water-making equation and you see the idea at work. On the left, 2H₂ gives 4 hydrogen atoms and O₂ gives 2 oxygen atoms. On the right, 2H₂O contains 4 hydrogen atoms and 2 oxygen atoms. The same atoms are there before and after; they have simply been regrouped.
The law of conservation of mass
This regrouping leads to one of the most important ideas in chemistry. The AQA specification states it like this: the law of conservation of mass says that no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.
The French chemist Antoine Lavoisier is credited with finding that mass is conserved in chemical reactions. His work on reactions ran from 1772 to 1794, and his textbook, Traité élémentaire de chimie, was published in 1789.
Balanced equations show the law at work
Because atoms are never lost or made, a symbol equation must be balanced: there must be the same number of atoms of each element on both sides. In a balanced equation the total of the relative formula masses of the reactants (in the quantities shown) equals the total for the products.
To balance an equation you change the big numbers in front of formulae, which are multipliers for the whole formula. You never change the small subscript numbers, because those are part of the formula itself. Changing H₂O to H₂O₂ would give you hydrogen peroxide, a different substance, not more water. For a full step-by-step method, work through our guide on how to write chemical formulae.
Why mass sometimes seems to change
If mass is conserved, why does a pile of ash weigh less than the wood that made it? The answer is nearly always a gas. In an open container, a gas can escape or be taken in from the air without you noticing.
- When a metal burns in oxygen, the oxide it makes is heavier than the metal, because oxygen from the air has joined it.
- When a metal carbonate is heated, carbon dioxide is released and escapes, so the solid that is left (the metal oxide) is lighter.
Use the calculator below to test the law for yourself. Choose a reaction and a mass of the starting reactant, and it works out the masses of the other substances so you can compare the totals before and after.
Now try an exam-style question on the same idea.
10.0 g of calcium carbonate is heated in an open crucible and 5.6 g of solid is left. Explain the loss of mass. (2 marks)
The calcium carbonate decomposed to make calcium oxide and carbon dioxide gas. The carbon dioxide escaped from the open crucible, so the solid is lighter. No atoms were destroyed: the mass of calcium oxide (5.6 g) plus the carbon dioxide (4.4 g) still adds up to the original 10.0 g.
The next section looks at how you can tell, from observations alone, that a reaction has taken place.
Signs of a chemical reaction
How can you tell that a chemical reaction has happened? Chemists look for clues. The signs of a chemical reaction are observations that suggest a new substance has formed: a colour change, a gas given off, a temperature change, a precipitate, light, or a new smell. A single clue is not proof, so always ask the real question: has a new substance been made?
Colour change
A new colour often means a new substance. The tests in the AQA GCSE specification rely on this. When sodium hydroxide solution is added to a solution containing copper(II) ions, a blue precipitate appears. Iron(II) ions give a green precipitate and iron(III) ions give a brown one.
Be careful, though. Diluting a coloured solution makes it paler without any reaction, so a colour change counts as evidence only when the colour is different, not just fainter.
Gas given off
Fizzing or bubbles often mean that a gas is being made. When magnesium is added to hydrochloric acid, hydrogen gas bubbles off:
Mg + 2HCl → MgCl₂ + H₂
Bubbles alone are not proof, because boiling water also bubbles and no new substance is made. To be sure which gas has formed, use the standard gas tests from the AQA specification:
- Hydrogen: a burning splint makes a squeaky “pop”.
- Oxygen: a glowing splint relights.
- Carbon dioxide: limewater (calcium hydroxide solution) turns milky.
- Chlorine: damp litmus paper is bleached white.
The milky colour in the limewater test is a reaction in its own right. Carbon dioxide reacts with calcium hydroxide to make solid calcium carbonate:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Temperature change
Many reactions make their surroundings hotter or colder. A reaction that transfers energy to the surroundings is exothermic, and the temperature rises. One that takes in energy is endothermic, and the temperature falls. We look at both in detail later in this article.
Again, treat this as a clue only. Some substances change the temperature of water simply by dissolving in it, and dissolving is not always a chemical reaction. That is why the AQA required practical on temperature changes includes reactions such as acid with metals and neutralisations, where a new substance is clearly made.
A precipitate forms
A precipitate is an insoluble solid that forms when two solutions are mixed. The liquid turns cloudy, or solid particles swirl around and then settle. The classic school demonstration mixes colourless solutions of lead(II) nitrate and potassium iodide to make bright yellow lead(II) iodide:
Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
Check the atoms: both sides have 1 lead, 2 nitrogen, 6 oxygen, 2 potassium and 2 iodine, so the equation is balanced. Lead compounds are toxic, so this is a teacher demonstration only and not a home experiment.
Precipitates are also the basis of the GCSE tests for negative ions. With silver nitrate (and dilute nitric acid), chloride ions give a white precipitate, bromide ions a cream one and iodide ions a yellow one. Sulfate ions with barium chloride (and dilute hydrochloric acid) give a white precipitate.
Light and a new smell
Some reactions give out light. Burning magnesium ribbon produces a brilliant white light and leaves a white powder, magnesium oxide:
2Mg + O₂ → 2MgO
A new smell can also point to a new substance, although smell is a poor test and some chemicals are dangerous to sniff. In school labs you are taught to waft, never to sniff directly.
Chemical change vs physical change
Every change in matter is either physical or chemical. Understanding the difference between a physical change and a chemical change is one of the most common KS3 and GCSE questions. The test is simple: new substance? If a new substance is formed, it is a chemical change (a chemical reaction). If the substance is still the same, only in a different state or shape, it is a physical change.
Melting ice is the standard example of a physical change. Water stays H₂O whether it is solid, liquid or gas; only the arrangement and movement of the particles change. Rusting iron is a chemical change, because a new substance forms. Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O, where the amount of water varies. The reaction can be written as:
4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O
Both air (oxygen) and water are needed for iron to rust. Rusting is an example of corrosion, and it can be prevented with barriers such as greasing, painting or electroplating. For a deeper look, read is iron rusting a chemical change?
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance made? | No | Yes |
| Bonds between atoms | Particles move or change arrangement, but the substance is the same | Bonds break and new bonds form |
| Example | Melting ice, dissolving sugar, filtering sand from water | Rusting iron, burning magnesium, neutralising an acid |
| Usual sign | Change of state, shape or size | Colour change, gas, precipitate, temperature change, light |
| Mass | Conserved | Conserved (check that no gas escaped or entered) |
Is it a chemical reaction? A quick checklist
Next time you are unsure, run through these questions. Your ticks are remembered in your browser.
- Has a new substance formed?
- Does the product have different properties from the reactants?
- Was there an energy change, such as heat or light?
- Was a gas or a precipitate made?
- Can the products NOT be separated by a physical method such as filtering or evaporating?
If you can answer yes to the first question, you have a chemical reaction. The others are supporting evidence. Try two exam-style questions on the same idea.
Is dissolving sugar in tea a chemical reaction? (2 marks)
No. The sugar is still sugar; it has only spread out among the water particles, so no new substance has been made. The sugar can be recovered by evaporating the water.
Give two observations that show a reaction when magnesium burns. (2 marks)
A bright (brilliant) white light is given out, and a white powder (magnesium oxide) is left. Heat is also released.
Types of chemical reactions
There are millions of different reactions, but most of them fit a small number of patterns. Textbooks do not all classify reactions in exactly the same way, and many reactions belong to more than one type. This article follows the ones that GCSE courses focus on. The table is a quick map, and each type is explained below it.
| Type | Pattern | Example |
|---|---|---|
| Combination (synthesis) | A + B → AB | 2Mg + O₂ → 2MgO |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ |
| Displacement | A + BC → AC + B | Zn + CuSO₄ → ZnSO₄ + Cu |
| Precipitation (double displacement) | AB + CD → AD + CB | BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl |
| Neutralisation | acid + base → salt + water | HCl + NaOH → NaCl + H₂O |
| Combustion | fuel + oxygen → oxides + energy | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Oxidation and reduction | gain or loss of oxygen (or electrons) | 2Mg + O₂ → 2MgO |
| Reversible | A + B ⇌ C + D | N₂ + 3H₂ ⇌ 2NH₃ |
Combination (synthesis) reactions
In a combination reaction, two or more substances join to make a single product. The general pattern is A + B → AB. Elements combine with oxygen to make oxides, and some compounds combine too:
2H₂ + O₂ → 2H₂O
2Mg + O₂ → 2MgO
CaO + H₂O → Ca(OH)₂
The last equation is calcium oxide (quicklime) reacting with water to make calcium hydroxide.
Decomposition reactions
Decomposition is the opposite pattern: one compound breaks down into two or more simpler substances, AB → A + B. When heat does the work it is called thermal decomposition, and in the AQA specification these reactions are endothermic.
CaCO₃ → CaO + CO₂
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Hydrogen peroxide decomposes into water and oxygen, and the reaction is much faster with manganese(IV) oxide added:
2H₂O₂ → 2H₂O + O₂
The manganese(IV) oxide is a catalyst. A catalyst changes the rate of a reaction without being used up, by providing a different pathway with a lower activation energy. Our article on catalysts, promoters and accelerators goes into more detail. Electricity can also decompose a compound; the electrolysis of water gives hydrogen and oxygen:
2H₂O → 2H₂ + O₂
Displacement reactions
In a displacement reaction a more reactive element takes the place of a less reactive one in a compound. The AQA specification gives the reactivity order for common metals as potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper, from most to least reactive. Hydrogen and carbon are often included in the series too.
Zn + CuSO₄ → ZnSO₄ + Cu
Fe + CuSO₄ → FeSO₄ + Cu
Zinc and iron are both more reactive than copper, so they push it out of copper(II) sulfate solution as reddish-brown copper metal. Reverse the pair and nothing happens. Copper is more reactive than silver, so a copper wire placed in silver nitrate solution slowly grows crystals of silver:
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
Metals also displace hydrogen from acids, which is where the bubbles come from:
Mg + 2HCl → MgCl₂ + H₂
Zn + 2HCl → ZnCl₂ + H₂
The same idea works for non-metals. A more reactive halogen displaces a less reactive one from a solution of its salt, so chlorine displaces bromine from potassium bromide:
Cl₂ + 2KBr → 2KCl + Br₂
For a full guide to the series, see reactivity of metals and chemical equations.
Precipitation (double displacement) reactions
Here two soluble compounds swap partners, and one of the new compounds is insoluble, so it appears as a precipitate. The pattern is AB + CD → AD + CB.
BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
AgNO₃ + NaCl → AgCl + NaNO₃
The first is the basis of the sulfate test (white barium sulfate), the second of the chloride test (white silver chloride). The lead(II) iodide reaction earlier in this article is another example.
Neutralisation
Acids react with alkalis and other bases to make a salt and water. Acids also react with metal carbonates to make a salt, water and carbon dioxide. The salt depends on the acid: hydrochloric acid makes chlorides, nitric acid makes nitrates and sulfuric acid makes sulfates.
HCl + NaOH → NaCl + H₂O
In terms of ions, every neutralisation comes down to the same change, which higher-tier students should know:
H⁺ + OH⁻ → H₂O
The reaction of baking soda (sodium hydrogencarbonate) with vinegar (dilute ethanoic acid) is an acid–carbonate reaction (a type of neutralisation) that gives off carbon dioxide. Unlike most neutralisations it is endothermic, so the mixture gets slightly colder:
NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂
See acids and bases explained for pH, indicators and more salts.
Combustion
Combustion is burning: a fuel reacts with oxygen and releases energy. In the complete combustion of a hydrocarbon, the products are carbon dioxide and water, and both the carbon and the hydrogen are oxidised.
CH₄ + 2O₂ → CO₂ + 2H₂O
A candle burning completely follows the same pattern, with wax as the fuel. Our complete lesson on combustion covers incomplete combustion as well.
Oxidation and reduction (redox)
At GCSE, oxidation means gaining oxygen and reduction means losing oxygen. Burning magnesium is oxidation, and so is rusting, which is simply a slow oxidation of iron. Metals that are less reactive than carbon can be extracted from their oxides by reduction with carbon.
Higher-tier students also learn the electron definition, remembered as OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
Reversible reactions
In a reversible reaction the products can react to re-form the reactants. It is shown with a double arrow, ⇌, and the direction can be changed by changing the conditions. If the reaction is exothermic in one direction, it is endothermic in the other by the same amount of energy.
NH₄Cl ⇌ NH₃ + HCl
Another example is blue hydrated copper(II) sulfate, which turns white when heated and loses its water. Add water back and it turns blue again. The Haber process, which makes ammonia, is a reversible reaction too:
N₂ + 3H₂ ⇌ 2NH₃
In a closed system a reversible reaction reaches equilibrium, which is when the forward and reverse reactions happen at the same rate.
Name that reaction type
Test yourself. Read the equation on the front of each card, say the type out loud, then tap to check.
Exothermic and endothermic reactions
Every chemical reaction involves energy. As we saw earlier, energy must be supplied to break the bonds in the reactants, and energy is released when the bonds in the products form. Which of the two is larger decides whether the reaction warms or cools its surroundings. Energy is conserved overall: it is transferred, not lost.
- In an exothermic reaction, the energy released in forming new bonds is greater than the energy needed to break existing bonds. Energy is transferred to the surroundings, and their temperature rises.
- In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released in forming new bonds. Energy is taken in from the surroundings, and their temperature falls.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy direction | Released to the surroundings | Taken in from the surroundings |
| Temperature of surroundings | Rises | Falls |
| Examples | Combustion, many oxidation reactions, neutralisation | Thermal decomposition, citric acid with sodium hydrogencarbonate |
| Everyday uses | Self-heating cans, hand warmers | Some sports injury packs |
Even an exothermic reaction usually needs a push to get started. Particles must collide with at least a minimum amount of energy, the activation energy, before they can react. That is why a match must be struck before it will burn, even though burning then releases plenty of energy.
Temperature changes in reacting solutions are an AQA required practical. Students measure the temperature rise for acids with metals, acids with carbonates, neutralisations and the displacement of metals.
Examples of chemical reactions in everyday life
Once you know what to look for, you start to see chemical reactions everywhere. Here are the examples that show up most often in lessons and exam papers.
Burning and rusting
Burning a candle is combustion: the wax reacts with oxygen to make carbon dioxide and water, releasing heat and light. Rusting is the slow oxidation of iron in the presence of air and water. They look nothing alike, but both are oxidation reactions, and both make new substances. The fast and slow versions sit side by side in the carousel earlier in this article.
Cooking and baking
Frying an egg is usually classed as a chemical change. Heat unfolds (denatures) the proteins in the egg white, and the unfolded proteins then bond to one another to form a solid network. Once that has happened, it cannot be reversed: you cannot uncook an egg.
Baking soda is sodium hydrogencarbonate. When it meets an acid such as vinegar it fizzes, as carbon dioxide is released:
NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂
Bakers use the same idea to make cakes rise. For the difference between the two raising agents, see baking soda vs baking powder.
Reactions in living things
Living things are chemical factories. In photosynthesis, plants turn carbon dioxide and water into glucose and oxygen, using energy from light. Because it takes in light energy, it is an endothermic reaction.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Count the atoms to check: 6 carbon, 12 hydrogen and 18 oxygen on each side. The answer to “is photosynthesis a chemical reaction?” is therefore a firm yes. Read more in our article on photosynthesis.
In aerobic respiration, cells use glucose and oxygen to release energy, and the reaction is exothermic:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Our guide to cellular respiration explains how this energy is captured.
Frequently asked questions about chemical reactions
What is a chemical reaction in simple terms?
A chemical reaction is a process in which one or more starting substances, the reactants, change into different substances, the products. Bonds between atoms break and new bonds form, so the atoms are rearranged. The products have different properties from the reactants, for example hydrogen and oxygen gases making liquid water.
What are the signs of a chemical reaction?
Common signs are a colour change, bubbles of gas, a temperature change, a precipitate (an insoluble solid forming in a solution), light given out and a new smell. These are clues rather than proof. The real test is whether a new substance has been made, which is why boiling water does not count.
What are the main types of chemical reactions?
The main types at GCSE are combination (synthesis), decomposition, displacement, precipitation, neutralisation, combustion, oxidation and reduction, and reversible reactions. Classifications vary between textbooks, and many reactions fit more than one type. Burning magnesium, for example, is both a combination reaction and an oxidation.
What is the difference between a physical change and a chemical change?
A physical change, such as melting ice or dissolving sugar, does not make a new substance. A chemical change, such as rusting or burning, does. Chemical changes break and make bonds, and often show signs such as a gas, a colour change or a temperature change. Reversibility is not the test.
Are atoms destroyed in a chemical reaction?
No. The law of conservation of mass says that no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants. Atoms are only rearranged into new combinations. That is why a symbol equation must have the same number of atoms of each element on both sides.
Why does mass sometimes seem to change in a reaction?
The change is almost always due to a gas in an open container. When a metal burns in oxygen, the oxide is heavier because oxygen joined it from the air. When a metal carbonate is heated, carbon dioxide escapes and the solid left behind is lighter. In a sealed container, the total mass stays the same.
Is rusting a chemical reaction?
Yes. Iron reacts with oxygen and water to form hydrated iron(III) oxide, Fe₂O₃·xH₂O, which is rust. This is a new substance with different properties from iron, so it is a chemical change. Both air and water are needed. Greasing, painting and electroplating prevent rusting by keeping them away from the metal.
Is photosynthesis a chemical reaction?
Yes. In photosynthesis, carbon dioxide and water react to make glucose and oxygen, using energy from light: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. New substances form and the equation is balanced, with 6 carbon, 12 hydrogen and 18 oxygen atoms on each side. It is an endothermic reaction.
What is the difference between exothermic and endothermic reactions?
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings rises. Combustion and neutralisation are examples. An endothermic reaction takes in energy from the surroundings, so they get colder. Thermal decomposition is an example. In both cases, energy is conserved overall.
