Acidified potassium dichromate(VI) is a suitable oxidising agent.

The idea

The oxidising agent is potassium dichromate(VI) solution acidified with dilute sulfuric acid. The orange dichromate ion, Cr₂O₇²⁻, in which chromium is in the +6 state, is reduced to the green Cr³⁺ ion: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. The acid supplies the hydrogen ions that half equation needs, so an unacidified solution will not work and the word acidified must appear in your answer. In organic equations you are allowed to write the oxidant simply as [O], which keeps the focus on the organic change. Because the colour change reports on whether oxidation happened, the same reagent doubles as the test-tube test for primary and secondary alcohols and for aldehydes.

What earns the marks

• Naming acidified potassium dichromate(VI), stating that it is acidified.
• The colour change from orange to green.
• Identifying the change as chromium going from the +6 to the +3 oxidation state.
• Using [O] correctly in a balanced organic equation.

Common mistakes and how to put them right

• Writing "potassium dichromate" without acidified, which loses the reagent mark.

• Giving the colour change as orange to blue, or yellow to green.

• Saying the dichromate is reduced to chromium metal rather than to Cr³⁺.

• Calling the dichromate a catalyst while also using it as the oxidising agent.

Exam tips

• AQA treats a reagent as something you could take out of a bottle, so name the acid as well as the dichromate.
• Learn the two colours as a pair, orange and green, and always say which is the starting colour.
• Use [O] to balance organic oxidation equations, then check oxygen and hydrogen separately.