AQA GCSE Combined Science Chemistry - chemical changes
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25 Terms
What is an acid?
A substance that forms H⁺ ions in aqueous solution.
What is an alkali?
A soluble base that forms OH⁻ ions in aqueous solution.
What is the pH scale?
A scale from 0 (strong acid) to 14 (strong alkali) showing acidity/alkalinity.
What does a pH of 7 mean?
The solution is neutral.
What is a base?
A substance that neutralises an acid (includes alkalis, metal oxides, metal hydroxides).
What happens in a neutralisation reaction?
Acid + Base → Salt + Water
What is a salt?
A compound formed when the hydrogen in an acid is replaced by a metal or ammonium ion.
What salt is formed from hydrochloric acid?
A chloride (e.g. sodium chloride).
What salt is formed from sulfuric acid?
A sulfate (e.g. copper sulfate).
What salt is formed from nitric acid?
A nitrate (e.g. potassium nitrate).
How do you make soluble salts from an insoluble base?
Add excess base to warm acid → filter → evaporate the solution.
Why is excess base added when preparing a salt?
To ensure all the acid is neutralised and no acid remains.
How do you obtain dry crystals of a salt?
Evaporate water from the filtered solution (crystallisation).
What is the test for hydrogen gas?
A lit splint causes a ‘squeaky pop’.
What is the test for carbon dioxide?
Turns limewater cloudy.
What happens when metals react with acids?
Metal + Acid → Salt + Hydrogen
What is the reactivity series of metals (top 5)?
Potassium, sodium, calcium, magnesium, aluminium
What gas is released when a metal reacts with water or acid?
Hydrogen gas (H₂)
What is displacement?
A more reactive metal displaces a less reactive one from its compound.
What is a redox reaction?
A reaction where one substance is oxidised and another is reduced.
What does oxidation mean?
Loss of electrons.
What does reduction mean?
Gain of electrons.
What is electrolysis?
The process of breaking down a compound using electricity.
Why can ionic compounds only be electrolysed when molten or in solution?
Because ions are free to move and carry charge.
What are the electrodes called?
Anode (positive), Cathode (negative)