How redox reactions depend on the medium of the solution
The same oxidising agent turns into different products in different media. The violet-pink permanganate ion MnO₄⁻ is reduced in acidic medium by taking 5 electrons to nearly colourless Mn²⁺, in neutral or weakly alkaline medium by taking 3 electrons to a brown MnO₂ precipitate, and in strongly alkaline medium by taking 1 electron to green MnO₄²⁻ (manganate). So permanganate is the strongest oxidising agent in acidic medium. The acidic medium is usually made with H₂SO₄, because permanganate oxidises hydrochloric acid to Cl₂. Dichromate K₂Cr₂O₇ (orange) is reduced in acidic medium by taking 6 electrons to green Cr³⁺; in alkaline medium chromate CrO₄²⁻ (yellow) is stable: the equilibrium 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O shifts right when acid is added and left when alkali is added. In the laboratory such experiments are carried out only by the teacher.
Teacher-only demonstration: in three test tubes the same permanganate and sulfite solutions receive dilute H₂SO₄, water and alkali respectively. Students in safety goggles observe the colour changes from a distance (colourless, brown precipitate, green) and record the result in a table.