Chemical Changes and Reactions

The One Thing

A physical change alters the form or state of a substance without producing a new substance, whereas a chemical reaction rearranges atoms to form one or more new substances with different properties. Chemical reactions are represented by balanced equations because atoms are conserved during the rearrangement.

Reactions, Processes and Experiments

Key Terms

• Physical change: A usually reversible change in which no new substance is formed; only the physical properties or state change.
• Chemical change: A change in which one or more new substances are formed with new chemical properties; it is often difficult to reverse.
• Chemical reaction: A process in which reactants are converted into products by breaking old chemical bonds and forming new ones.
• Reactants: The substances present at the beginning of a chemical reaction.
• Products: The new substances formed during a chemical reaction.
• Chemical equation: A symbolic representation of a chemical reaction using chemical formulae and symbols.
• Word equation: A chemical reaction written using the names of the reactants and products.
• Balanced chemical equation: An equation containing equal numbers of atoms of each element on both sides.
• Combination reaction: A reaction in which two or more substances combine to form a single product, such as calcium oxide plus water forming calcium hydroxide.
• Decomposition reaction: A reaction in which one compound breaks down into two or more simpler substances, often due to heat, light, or electricity.
• Displacement reaction: A reaction in which a more reactive element replaces a less reactive element from its compound.
• Double displacement reaction: A reaction in which two compounds exchange ions or groups to form two new compounds.
• Exothermic reaction: A reaction that releases heat energy to the surroundings.
• Endothermic reaction: A reaction that absorbs heat energy from the surroundings.
• Catalyst: A substance that changes the rate of a reaction without being permanently consumed.
• Precipitate: An insoluble solid formed when two solutions react.
• Reversible change: A change that can be changed back to the original form by suitable methods.
• Irreversible change: A change that cannot easily be changed back to the original substances.

Easily Confused

• Physical change vs chemical change: A physical change does not form a new substance, whereas a chemical change forms one or more new substances with different chemical properties.
• Reactants vs products: Reactants are present at the beginning of a reaction; products are formed during the reaction.
• Word equation vs chemical equation: A word equation uses substance names, whereas a chemical equation uses formulae and symbols.
• Subscripts vs coefficients: Subscripts show the number of atoms in a molecule or formula unit and must not be changed when balancing; coefficients are placed before formulae and show relative numbers of molecules or formula units.
• Combination vs decomposition: A combination reaction forms a single product from two or more substances, whereas a decomposition reaction breaks one compound into two or more simpler substances.
• Displacement vs double displacement: In displacement, one element replaces another element in a compound; in double displacement, two compounds exchange ions or groups.
• Exothermic vs endothermic: An exothermic reaction releases heat, whereas an endothermic reaction absorbs heat.
• Precipitate vs gas formation: A precipitate is an insoluble solid formed from reacting solutions, whereas gas formation produces a gaseous product.

What Gets Asked

• Classifying examples as physical or chemical changes: Questions may use melting ice, boiling water, dissolving sugar in water, cutting paper, stretching a rubber band, rusting of iron, burning of wood, curdling of milk, digestion of food, or cooking an egg. The mark-losing error is identifying a change only from whether it appears reversible, rather than deciding whether a new substance has formed.
• Identifying evidence of a chemical reaction: Students may be asked to recognise colour change, gas formation, precipitate formation, temperature change, light emission, or change in smell. A single observation must be interpreted with the substances and conditions involved.
• Writing or completing equations: Questions may require the word equation magnesium + oxygen → magnesium oxide or the balanced equation 2Mg + O2 → 2MgO. A common error is changing subscripts instead of adding coefficients.
• Balancing equations and applying conservation of mass: Students must ensure that each element has equal numbers of atoms on both sides because matter is neither created nor destroyed. The total mass of reactants must equal the total mass of products.
• Classifying reaction types: Questions may use CaO + H2O → Ca(OH)2, CaCO3 → CaO + CO2, Zn + CuSO4 → ZnSO4 + Cu, or AgNO3 + NaCl → AgCl + NaNO3. The specific distinction is whether substances combine, a compound breaks down, an element replaces another, or two compounds exchange ions.
• Identifying energy changes and reaction conditions: Questions may contrast combustion of methane, CH4 + 2O2 → CO2 + 2H2O + heat, with thermal decomposition of calcium carbonate, which requires heat. Marks are lost by confusing heat release with heat absorption or omitting required conditions such as heat, light, electricity, pressure, or a catalyst.