Theories of acids and bases
3 marks · NormalExplain how the hydroxonium ion (H_3O^+) is formed when an acid dissolves in water, and describe its role in acidbase reactions.
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Also accept: H^+(aq) attaches to H_2O to form H_3O^+; Hydronium or hydroxonium ion acts as the proton donor in further reactions
Common mistakes: Not stating that H^+ bonds to H_2O; Confusing H_3O^+ with H_2O or H^+
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Marking points
M1 · 1 mark · States that when an acid dissolves in water, it donates a proton (H^+) to a water molecule.
M2 · 1 mark · Correctly describes formation of H_3O^+ from H^+ and H_2O (with clear proton transfer).
M3 · 1 mark · Clearly describes the role of H_3O^+ as the actual acid species (e.g., responsible for acidity, involved in proton transfer to bases).
Worked solution
When an acid dissolves in water, it donates a proton (H^+) to a water molecule. The H^+ ion attaches to the oxygen atom in H_2O, forming the hydroxonium ion, H_3O^+.
The H_3O^+ ion is responsible for acidity in solution, as it can donate a proton to other bases in proton transfer reactions.