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Getting started: chemical reactions and conservation of mass
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In a chemical reaction, atoms are rearranged to make new substances. GCSE questions often start with a word equation or a mass change, so let us recap them.
Chemical or physical change?
In a chemical change new substances are made. Signs: fizzing (a gas is made), a colour change, a temperature change, or a precipitate (a solid forming when two solutions are mixed). Most are hard to reverse.
In a physical change no new substance is made: melting, boiling, dissolving. It is easy to reverse.
In a physical change no new substance is made: melting, boiling, dissolving. It is easy to reverse.
Word equations
reactants → products
metal + acid → salt + hydrogen
metal + oxygen → metal oxide (oxidation)
fuel + oxygen → carbon dioxide + water (combustion of a hydrocarbon fuel)
acid + alkali → salt + water (neutralisation)
The salt is named from the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates.
pH: below 7 is acidic, 7 is neutral, above 7 is alkaline. Universal indicator goes red in strong acid, green when neutral and purple in strong alkali.
metal + acid → salt + hydrogen
metal + oxygen → metal oxide (oxidation)
fuel + oxygen → carbon dioxide + water (combustion of a hydrocarbon fuel)
acid + alkali → salt + water (neutralisation)
The salt is named from the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates.
pH: below 7 is acidic, 7 is neutral, above 7 is alkaline. Universal indicator goes red in strong acid, green when neutral and purple in strong alkali.
Conservation of mass
Atoms are not made or destroyed in a reaction, only rearranged, so total mass of reactants = total mass of products.
In an open flask the mass seems to fall if a gas escapes. When a metal burns the mass rises, because oxygen from the air joins on.
In an open flask the mass seems to fall if a gas escapes. When a metal burns the mass rises, because oxygen from the air joins on.
2.4 g of magnesium burns in air and makes 4.0 g of magnesium oxide. What mass of oxygen reacted?
- Mass of magnesium + mass of oxygen = mass of magnesium oxide
- Mass of oxygen = 4.0 − 2.4
Answer: 1.6 g
Chemical changes make new substances; physical changes do not. Learn the word-equation patterns. Mass is conserved: apparent changes come from gases leaving or joining.
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