Chemistry describes change, and equations are its shorthand. Learn to read them, balance them and predict how fast and how energetically a reaction happens.
Rust forming on a gate, a candle burning, bread rising and a battery powering a phone are all chemical reactions. Chemists record them in a compact written language: the chemical equation. This article teaches you to read that language, write it correctly, balance it, and understand how fast and how energetically reactions happen.
In a physical change, such as melting ice or dissolving sugar, no new substance forms and the change can usually be reversed easily. In a chemical change (a chemical reaction), one or more new substances form with different properties, and the change is usually hard to reverse. During a reaction the reactants (starting materials) are changed into products.
Signs that a chemical reaction may have happened include: a gas is given off (bubbles); a colour change; a solid forms from a solution (a precipitate); a temperature change; light or sound is produced. These signs are clues, not proof: boiling also makes bubbles, and dissolving can change temperature, so check whether a new substance is formed.
The law of conservation of mass says that matter is not created or destroyed in a chemical reaction. The atoms are rearranged: bonds in the reactants are broken and new bonds form in the products. So the total mass of the products equals the total mass of the reactants. This was established through careful weighing experiments by the French chemist Antoine Lavoisier in the 1700s.
If a reaction seems to lose mass, a gas has probably escaped (for example, carbon dioxide from marble chips and acid in an open flask). If it seems to gain mass, a gas from the air has probably joined in (for example, magnesium burning in oxygen to form heavier magnesium oxide).
A word equation names the substances: magnesium + oxygen → magnesium oxide. A symbol equation uses formulae and must be balanced, with the same number of each type of atom on both sides:
The numbers in front of formulae are coefficients; they show how many units of each substance take part. Never change the small numbers inside a formula (subscripts) to balance an equation, because that would change the substance. For example, H₂O is water but H₂O₂ is hydrogen peroxide, a different compound.
State symbols show the physical state: (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water. Example:
Question: Balance Fe + O₂ → Fe₂O₃.
Check: Fe 4 = 4; O 6 = 6.
Question: Balance C₂H₅OH + O₂ → CO₂ + H₂O.
Check: C 2 = 2; H 6 = 6; O 1 + 6 = 7 and 4 + 3 = 7.
Question: Balance Al + HCl → AlCl₃ + H₂.
Check: Al 2 = 2; H 6 = 6; Cl 6 = 6.
When ionic compounds react in solution, some ions do not change; they are called spectator ions. An ionic equation shows only the particles that actually react, so it is a cleaner description of what happens. To write one: split every soluble ionic compound (aq) into ions, cancel the spectator ions, and write what is left.
Question: Write the ionic equation for AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
This ionic equation is the same for any soluble silver salt mixed with any soluble chloride.
The ionic equation for any acid–alkali neutralisation is H⁺(aq) + OH⁻(aq) → H₂O(l), as you saw in the Acids, Bases and Salts article.
| Type | What happens | Example |
|---|---|---|
| Combination (synthesis) | Two or more substances join to form one product | 2Mg + O₂ → 2MgO |
| Decomposition | One compound breaks into two or more simpler substances | CaCO₃ → CaO + CO₂ (heat) |
| Displacement | A more reactive element replaces a less reactive one in a compound | Zn + CuSO₄ → ZnSO₄ + Cu |
| Combustion | A substance burns in oxygen, giving out energy | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Neutralisation | An acid reacts with a base to form a salt and water | NaOH + HCl → NaCl + H₂O |
| Precipitation | Two solutions react to form an insoluble solid | Ag⁺ + Cl⁻ → AgCl |
| Redox | Electrons are transferred: one species is oxidised and one reduced | Zn + Cu²⁺ → Zn²⁺ + Cu |
Many reactions fit more than one description. For example, zinc displacing copper is both a displacement and a redox reaction.
Every reaction involves energy changes, because breaking bonds needs energy and making bonds releases it.
The energy difference is the enthalpy change of the reaction. The peak on the curve shows the activation energy, the minimum energy that colliding particles need in order to react. It is like the push needed to start a ball rolling over a hill. Even exothermic reactions need that initial energy, which is why a match must be struck to start burning.
Question: In a reaction, bond breaking absorbs 1000 kJ and bond making releases 1350 kJ. Is it exothermic or endothermic, and what is the energy change?
The rate of reaction is how quickly reactants are used up or products are formed. According to collision theory, particles must collide with enough energy (at least the activation energy) and the right orientation for a reaction to happen. Anything that increases the number of successful collisions per second speeds the reaction up.
| Factor | Effect on rate | Explanation |
|---|---|---|
| Higher temperature | Faster | Particles move faster and collide more often, and more of the collisions have enough energy |
| Higher concentration (or gas pressure) | Faster | More particles in the same volume, so collisions are more frequent |
| Larger surface area (smaller pieces) | Faster | More particles at the surface are exposed to collisions |
| Catalyst | Faster | Provides an alternative route with lower activation energy; not used up |
A catalyst speeds up a reaction without being used up. For example, manganese(IV) oxide catalyses the breakdown of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂. Enzymes are biological catalysts. Many industrial processes rely on catalysts, which saves energy and money.
Question: Marble chips react with acid. Powdered marble gives a steeper curve than lumps but the same final volume of gas. Why?
Answer: Powder has a larger surface area, so more collisions happen per second and the reaction is faster. The mass of marble is the same, so the same amount of carbon dioxide forms in the end.
Some reactions can go in both directions. They are shown with a double arrow, ⇌. For example, blue hydrated copper(II) sulfate loses water when heated to give white anhydrous copper(II) sulfate, and the white solid turns blue again when water is added: CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O. The reverse reaction is used as a simple test for water. In a closed system, a reversible reaction can reach dynamic equilibrium, where the forward and reverse reactions continue at the same rate so the amounts stay constant. The Haber process for ammonia, N₂ + 3H₂ ⇌ 2NH₃, is an important industrial example.
Hydrogen mixed with air or oxygen is explosive, and heating chemicals can cause splashes. Wear eye protection in all experiments with reactions, tie back long hair and follow your teacher’s instructions.
Try each question first, then tap it to reveal the answer.
Answer: 2H₂ + O₂ → 2H₂O.
Answer: CH₄ + 2O₂ → CO₂ + 2H₂O.
Answer: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Answer: Endothermic: it needs a continuous supply of heat.
Answer: Particles move faster, so collisions are more frequent and a greater fraction have enough energy to react.
Answer: It lowers it by providing an alternative reaction pathway.
Answer: The magnesium combined with oxygen from the air, so the product MgO has extra mass.
Suggested use: have students weigh a sealed flask before and after a reaction (such as dilute acid with sodium hydrogencarbonate in a closed system) to show conservation of mass, then repeat open to explain the apparent loss. A temperature-probe experiment with dissolving salts shows exothermic and endothermic changes. Parents can reinforce balancing by using modelling clay or coloured beads for atoms. Please follow your own syllabus for required reaction types and notation.
These reputable sources were consulted to verify the facts in this article:
Copyright note: This article is original EDUSAMBAM educational writing. The diagrams are original inline SVG schematics created for this page and are not copied from any source. Links open external websites; the sources were used to check facts and are suggested for further study.
Last updated: 21 October 2026. Written for students up to age 18 and checked against standard school-level chemistry references. This is general education, not a substitute for your teacher, textbook or syllabus.
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