Catalytic decomposition of hydrogen peroxide
This particular reaction is often called the ‘elephant toothpaste’ demo. It is the decomposition of hydrogen peroxide using iodide ion as catalyst. Detergent is added to the reaction to produce the copious quantity of bubbles. The mechanism is claimed to be:

H₂O₂(aq) + I^-(aq) → H₂O(l) + IO^-(aq)

H₂O₂(aq) + IO^-(aq) → H₂O(l) + I^-(aq) + O₂(g)

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Plastic sulfur
Powdered sulfur is heated until it melts to give a free-flowing pale-yellow liquid of S 8 molecules. Additional heating converts it to a thick, viscous, dark red-brown liquid of long chain sulfur polymers. The viscosity decreases as the temperature is increased further, the chains breaking into shorter lengths. When this is poured into water, the supercooled liquid polymer can be stretched. Over time, the sulfur reconverts to its ring form.

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Sulfite ion as a reducing agent
Sulfite ion will reduce purple permanganate ion to colorless manganese(II) ion in acid solution.

SO₃^2-(aq) + H₂O(l) → SO₄^2-(aq) + 2H⁺(aq) + 2e^-

MnO₄^-(aq) + 8H⁺(aq) + 5e^-→ Mn²⁺(aq) + 4H₂O(l)

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Thiosulfate ion as a reducing agent
Thiosulfate reduces brown iodine solution to colorless iodide ion, itself being oxidized to the tetrathionate ion.

2S₂O₃^2-(aq) → S₄O₆^2-(aq) + 2e^-

I₂(aq) + 2e^-→ 2I^-(aq)

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Unique reaction of the thiosulfate ion
A unique reaction of the thiosulfate ion is that with cold iron(III) ion. A deep purple complex is formed:

Fe³⁺(aq) + 2S₂O₃^2-(aq) → [Fe(S₂O₃)₂]^-(aq)

Upon warming, the iron(III) ion is reduced to iron(II) ion.

[Fe(S₂O₃)₂]^-(aq) + Fe²⁺(aq) → 2Fe²⁺(aq) + S₄O₆^2-(aq)

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