Physics and chemistry Year 8: Chemical reactions, conservation of mass and combustion
Physics and chemistry, Year 8 (Portugal): In a reaction atoms are rearranged, never lost; how to balance an equation, tell fuel from oxidiser in a combustion, and why burning fuels matters for air and climate.
Atoms swap partners
In a chemical reaction the reactants turn into products: new substances with different properties. But atoms are neither created nor destroyed; they are only rearranged, old bonds breaking and new ones forming. So the total number of atoms of each element is the same before and after. A chemical equation shows this: reactants on the left, products on the right, and coefficients that balance the atoms.
Combustion: what burns and what feeds it
A combustion is a reaction in which a fuel reacts with an oxidiser (the comburent), almost always the oxygen in air, releasing energy as heat and light. The fuel is what burns (wood, natural gas, magnesium); the comburent is what feeds the burning (oxygen). Lab combustions: carbon gives carbon dioxide, C + O₂ → CO₂; sulfur gives sulfur dioxide, S + O₂ → SO₂; magnesium gives magnesium oxide, 2 Mg + O₂ → 2 MgO.
Balancing methane's combustion
Methane (natural gas) burns in air: CH₄ + O₂ → CO₂ + H₂O. Count atoms. Left: 1 C, 4 H, 2 O. Right: 1 C, 2 H, 3 O. Balance with coefficients, never with subscripts: 2 H₂O gives 4 H; now the right has 2 + 2 = 4 O, so 2 O₂ on the left. CH₄ + 2 O₂ → CO₂ + 2 H₂O. Check: 1 C, 4 H and 4 O on each side. Methane is the fuel; oxygen is the comburent.
Mass neither vanishes nor appears
A sheet of paper burns and the ash weighs less: has mass vanished? No: the gases (CO₂ and water vapour) escaped into the air. In a closed container the total mass before and after is equal: this is Lavoisier's Law of Conservation of Mass. The opposite also fools people: 2.4 g of magnesium burning gives 4.0 g of white powder (magnesium oxide), heavier, because it took 1.6 g of oxygen from the air (4.0 − 2.4 = 1.6). Rule: always count the gases.
Pollutants and climate
Burning fossil fuels (petrol, diesel, coal, natural gas) releases CO₂, a greenhouse gas: more of it in the atmosphere strengthens the greenhouse effect and feeds climate change. Incomplete combustion also releases carbon monoxide, CO, which is toxic. Fuels containing sulfur release SO₂, which causes acid rain, and soot particles harm breathing. Ways to reduce them: public transport, energy efficiency, filters and catalytic converters, and renewable energy.
Test what you learned
In any chemical reaction, what is always the same before and after?
- a) The substances present
- b) The number of molecules
- c) The properties of the material
- d) The number of atoms of each element
Correct answer: d) The number of atoms of each element — Right: atoms are only rearranged, never created or destroyed.
Magnesium burns in oxygen. Which balanced equation is correct?
- a) 2 Mg + O₂ → 2 MgO
- b) Mg + O₂ → MgO
- c) Mg + O₂ → MgO₂
- d) 2 Mg + 2 O₂ → 2 MgO
Correct answer: a) 2 Mg + O₂ → 2 MgO — Right: 2 Mg and 2 O on each side.
2.4 g of magnesium burn in air and give 4.0 g of white powder. What explains the extra mass?
- a) Heating made new magnesium atoms
- b) 1.6 g of oxygen from the air joined the magnesium
- c) The Law of Conservation of Mass does not apply to combustions
Correct answer: b) 1.6 g of oxygen from the air joined the magnesium — Right: 4.0 − 2.4 = 1.6 g. The mass is conserved once you count the oxygen taken from the air.
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