How matter changes for good, the four types of reaction, and why some metals react faster than others.
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In a physical change no new substance forms and the change is usually reversible (melting ice, dissolving salt). In a chemical change one or more new substances form with different properties and the change is generally permanent (burning magnesium, rusting, cooking). Signs: a gas is evolved, a precipitate forms, colour changes, heat or light is given out. Reactions need conditions: contact of reactants, heat, light, a solvent such as water, electricity or a catalyst.
Classify as physical or chemical: (a) boiling water, (b) rusting of iron, (c) burning of paper, (d) dissolving sugar.
(a) Physical. (b) Chemical. (c) Chemical. (d) Physical.
Silver bromide in a photographic film darkens in light. Which condition is needed?
Light; the reaction is a photochemical change.
Why does a burning candle show both kinds of change?
The wax melting is physical; the burning of the wax vapour into carbon dioxide and water is chemical.
Combination: two or more substances form one (2Mg + O₂ → 2MgO). Decomposition: one substance breaks into simpler ones, by heat (thermal), light or electricity (CaCO₃ → CaO + CO₂). Displacement: a more reactive element takes the place of a less reactive one in a compound (Zn + CuSO₄ → ZnSO₄ + Cu). Double decomposition: two compounds exchange parts; if an insoluble product (precipitate) forms it is precipitation, and an acid with a base is neutralisation (AgNO₃ + NaCl → AgCl↓ + NaNO₃).
Show that mass is conserved when magnesium burns: 2Mg + O₂ → 2MgO (Mg = 24, O = 16).
Left: 2 × 24 + 32 = 80. Right: 2 × 40 = 80. The magnesium is 60% of the oxide formed, so the white ash weighs more than the ribbon.
Heating 100 g of calcium carbonate gives 56 g of calcium oxide. What mass of CO₂ is released?
100 − 56 = 44 g of carbon dioxide.
Name the type: (a) Zn + H₂SO₄ → ZnSO₄ + H₂, (b) HCl + NaOH → NaCl + H₂O.
(a) Displacement. (b) Double decomposition (neutralisation).
Oxidation is gain of oxygen (or loss of hydrogen or electrons); reduction is loss of oxygen (or gain of hydrogen or electrons). They always occur together in a redox reaction. An exothermic reaction gives out heat (burning, neutralisation); an endothermic reaction takes in heat (decomposition of calcium carbonate). The reactivity series lists metals from most to least reactive: K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Hg, Ag, Au. A metal displaces any metal below it from a solution of its salt.
In CuO + H₂ → Cu + H₂O, name the substance oxidised and the one reduced.
Hydrogen is oxidised (gains oxygen); copper oxide is reduced (loses oxygen).
Will iron displace copper from copper sulphate solution? Will copper displace iron from iron sulphate?
Yes, iron is above copper in the series. No, copper is less reactive than iron.
Is the burning of magnesium exothermic or endothermic?
Exothermic: it gives out heat and a bright white light.
1. Give two examples of a physical change.
2. List three signs of a chemical change.
3. What is a precipitate?
4. Name the type of reaction: 2H₂O → 2H₂ + O₂.
5. Write the equation for the displacement of copper by zinc.
6. Define oxidation and reduction in terms of oxygen.
7. Name two exothermic changes.
8. Arrange in decreasing reactivity: Cu, Mg, Fe, Na.
9. In 2Mg + O₂ → 2MgO, 12 g of magnesium burns. Using the mass ratio 48 : 80, find the mass of MgO formed.
10. Why is the decomposition of CaCO₃ called endothermic?