Chapter 6: Chemical Reactions — Part 1

Topic 6.1 · Physical and chemical changes

Physical Changes

Physical changes occur when changes of state happen, such as melting, freezing, boiling, or evaporating.
In these changes, no new substances are made because the chemical composition of the elements or compounds remains the same.
Physical changes are generally reversible and involve no significant energy change.
An example of a physical change is the fractional distillation of crude oil, which separates mixtures of substances that are not chemically combined.

Chemical changes require a chemical reaction where the products have a different chemical composition than the reactants.
These reactions are typically irreversible and involve a large energy change, where thermal energy is either taken in (endothermic) or released (exothermic).
A colour change can often be observed during a chemical reaction.
During chemical reactions, atoms are not created or destroyed, meaning mass is conserved; the total mass of the reactants equals the total mass of the products.
Examples include cooking, digestion, burning (combustion), and neutralisation reactions to produce salts.

Exam Traps

Do not say atoms are created or destroyed in a chemical reaction — only rearranged.

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