Matter can change in two fundamental ways. Physical changes alter the form of a substance without changing its chemical identity, while chemical changes produce entirely new substances. The Law of Conservation of Mass governs all chemical reactions.
14.1 Physical vs Chemical Change
Definition
Physical change
A physical change is a change in which no new substances with new chemical properties are formed. The chemical composition and formula of the substance remain unchanged. Physical changes are usually easily reversible. Examples: melting, boiling, cutting, dissolving.
Definition
Chemical change (chemical reaction)
A chemical change is a change in which new substances with new chemical properties are formed. The atoms are rearranged to form new bonds. Chemical changes are usually difficult or impossible to reverse. Examples: burning, rusting, cooking, decomposition.
Definition
Law of Conservation of Mass
The Law of Conservation of Mass states that in a chemical reaction, the total mass of the reactants equals the total mass of the products. Matter is neither created nor destroyed in a chemical reaction — the same atoms are present before and after, just rearranged.
Definition
Law of Constant Composition (Definite Proportions)
The Law of Constant Composition states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass, regardless of how the compound was prepared or where it was found.
Signs that indicate a chemical change has occurred
- Change in colour (not simply mixing colours)
- Production of a gas — bubbling or fizzing
- Formation of a precipitate — an insoluble solid that appears in solution
- Release or absorption of energy — production of heat, light or sound
- Formation of a new smell or odour
- The change is usually irreversible under normal conditions
Physical vs Chemical Change
| Property | Physical Change | Chemical Change |
|---|---|---|
| New substances formed? | No | Yes |
| Reversibility | Usually easily reversible | Usually difficult or impossible to reverse |
| Chemical formula | Unchanged | Changes — new formulae |
| Examples | Melting, boiling, dissolving, cutting | Burning, rusting, cooking, decomposition |
The Law of Conservation of Mass means that atoms are rearranged in a chemical reaction but never created or destroyed. If you start with 10 g of reactants and the reaction goes to completion, you will always end up with exactly 10 g of products — provided none of the products escape (e.g. a gas does not leave the container). A balanced chemical equation is the mathematical statement of this law: the same atoms appear on both sides, just in different arrangements.
Worked Example
2 g of hydrogen gas reacts completely with 16 g of oxygen gas to form water. What mass of water is produced?
Given
- Mass of H₂ = 2 g
- Mass of O₂ = 16 g
Find
Mass of water produced
Solution
- 1Apply the Law of Conservation of Mass:
- 2Total mass of reactants = 2 + 16 = 18 g
- 3Total mass of products = total mass of reactants = 18 g
- 4All reactants are converted to water, so mass of water = 18 g
Practice Question
Iron powder (7 g) is mixed with sulphur powder (4 g) and heated strongly to form iron(II) sulphide (FeS). (a) What type of change occurred? Give two reasons. (b) What mass of FeS is formed?
(4 marks)
Exam Tip
For exam questions on chemical change: always check for the 6 signs listed above. The most reliable signs are the formation of a precipitate (new insoluble solid) and a significant energy change (flame, heat, light). A colour change alone does not always confirm a chemical change — mixing two coloured solutions can give a colour change that is purely physical.
14.2 Types of Chemical Reactions
Definition
Synthesis reaction
A synthesis (combination) reaction is a reaction in which two or more reactant substances combine to form a single product. General form: A + B → AB.
Definition
Decomposition reaction
A decomposition reaction is a reaction in which a single compound breaks down to form two or more simpler substances. General form: AB → A + B.
Definition
Exchange (double displacement) reaction
In an exchange reaction, two compounds react and exchange ions or groups to form two new compounds. General form: AB + CD → AD + CB.
Common reaction types and examples
- Synthesis: 2Mg(s) + O₂(g) → 2MgO(s) — magnesium burns in oxygen to form magnesium oxide
- Decomposition: 2H₂O₂(l) → 2H₂O(l) + O₂(g) — hydrogen peroxide decomposes to water and oxygen
- Combustion: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) — methane (natural gas) burns in oxygen
Worked Example
Write a word equation and a balanced formula equation for the burning of magnesium in oxygen.
Given
- Reactants: magnesium and oxygen gas
- Product: magnesium oxide
Find
Word equation and balanced formula equation
Solution
- 1Word equation: magnesium + oxygen → magnesium oxide
- 2Formula equation (unbalanced): Mg(s) + O₂(g) → MgO(s)
- 3Balance: O₂ gives 2 O atoms on left, but MgO has only 1 O on right → place 2 in front of MgO: Mg + O₂ → 2MgO
- 4Now Mg: 1 on left, 2 on right → place 2 in front of Mg: 2Mg + O₂ → 2MgO
- 5Check: Mg: 2 = 2 ✓; O: 2 = 2 ✓
Practice Question
Identify the type of reaction and write the missing product for: CaCO₃(s) → CaO(s) + ?
(3 marks)