Definition
A chemical reaction is a process in which one or more substances (the reactants) are transformed into one or more different substances (the products). During a chemical reaction, the atoms of the reactants are rearranged to form new chemical bonds, producing materials with different properties from the starting materials.
Key Characteristics
- Reactants are consumed and products are formed; the total number of atoms is conserved (law of conservation of mass).
- Chemical reactions are represented by chemical equations (e.g., 2H₂ + O₂ → 2H₂O).
- Signs that a chemical reaction has occurred include: change in colour, production of a gas (bubbles), formation of a precipitate, change in temperature, or production of light.
- Reactions can be exothermic (release energy/heat) or endothermic (absorb energy/heat).
- The rate of a reaction can be affected by temperature, concentration, surface area, and catalysts.
- Common types include combustion, oxidation, neutralisation, decomposition, and displacement reactions.
Real-World Example
When iron (Fe) is left in moist air, it reacts with oxygen and water to form iron oxide (rust). This is an oxidation reaction: 4Fe + 3O₂ → 2Fe₂O₃. The rust has different properties from the original iron — it is reddish-brown, flaky, and brittle. This reaction happens slowly at room temperature but speeds up in salty or acidic environments, which is why cars in coastal areas rust faster.
A chemical reaction rearranges atoms from reactants to form new products. No atoms are created or destroyed — they are only reorganised into different arrangements. Energy is either released (exothermic) or absorbed (endothermic) during the process, and observable signs such as colour change, gas production, or temperature change indicate that a reaction has taken place.
Word Equations and Symbol Equations
Reactions are described by equations. A word equation names the substances:
Magnesium + Oxygen → Magnesium oxide
A symbol equation uses chemical formulae:
2Mg + O₂ → 2MgO
The equation must be balanced — the same number of each type of atom appears on both sides, reflecting conservation of mass.
Practice Problems
Is burning a candle a physical change or a chemical reaction? Give two pieces of evidence.
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In the reaction: Zn + H₂SO₄ → ZnSO₄ + H₂, identify the reactants and products. Is this reaction balanced?
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A hand-warmer pack gets hot when activated. Is the chemical reaction inside exothermic or endothermic? Explain.
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A student mixes baking soda and vinegar in a beaker and observes vigorous bubbling. The beaker also feels slightly cooler. What type of reaction is this, and what does the temperature change tell us?