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Colours and appearances of the common redox species

Why observations carry the marks

Explanatory Note 4 lists "observations for reactions" as one of the six assessable things, and both Merit and Excellence require you to link your equations to what was seen.
An assessment will typically give you a table of substances with their appearances. It is not allowed to tell you which react together — so if you know the colours, you can work out what happened without the table.
Every colour change in this standard is a species being created or destroyed. Name the species, not just the colour.

The oxidants

Species	Appearance	Becomes	Appearance
MnO₄⁻ in acid	purple	Mn²⁺	colourless (very pale pink)
Cr₂O₇²⁻ in acid	orange	Cr³⁺	green
Fe³⁺	yellow-brown	Fe²⁺	pale green
Cu²⁺	blue	Cu	pink-brown solid
I₂	brown in solution	I⁻	colourless
Br₂	orange-brown	Br⁻	colourless
Cl₂	pale yellow-green	Cl⁻	colourless
O₂	colourless gas	oxide / H₂O	—
H₂O₂	colourless	H₂O	colourless
OCl⁻ (bleach)	colourless	Cl⁻	colourless
IO₃⁻	colourless	I₂	brown
conc. HNO₃	colourless	NO₂	brown gas
H⁺	colourless	H₂	colourless gas, bubbles

The reductants

Species	Appearance	Becomes	Appearance
Metals	grey/silver solid (Cu is pink-brown)	metal ions	often colourless
Fe²⁺	pale green	Fe³⁺	yellow-brown
I⁻	colourless	I₂	brown
Br⁻	colourless	Br₂	orange-brown
H₂S	colourless gas, rotten-egg smell	S	yellow solid
SO₂	colourless gas, choking smell	SO₄²⁻	colourless
SO₃²⁻ / HSO₃⁻	colourless	SO₄²⁻	colourless
H₂O₂	colourless	O₂	colourless gas, bubbles
C	black solid	CO₂	colourless gas
H₂	colourless gas	H₂O	colourless liquid

Observations that are not colours

Do not stop at colour. These also count, and often they are the only observation available:

Bubbles of gas — O₂ from peroxide, H₂ from a metal and acid, CO₂ from carbon.
A solid forming or disappearing — a metal coating on another metal, a yellow sulfur precipitate, a metal strip dissolving.
A smell — rotten eggs for H₂S, choking for SO₂. (Never inhale deliberately; waft.)
Heat — many redox reactions are strongly exothermic and the test tube warms.
No change at all — which, as the halogen pages show, is itself evidence.

Writing an observation properly

Say what changed, from what, to what.
- Weak: "It changed colour."
- Better: "The solution turned from purple to colourless."
- Best: "The purple MnO₄⁻ was decolourised as it was reduced to colourless Mn²⁺."
The best version names the species as well as the colours. That is what turns an observation into a link, which is the Merit criterion.

Worked ExampleDeducing a reaction from observations alone

A colourless solution is added to an orange solution that has been acidified. The mixture turns green and a brown colour also develops. Using only the observations, identify the two species that reacted, state what happened to each, and write the overall equation.

Step 1 — Identify the orange solution

An orange solution in an acidified redox context is dichromate, Cr₂O₇²⁻.

The mixture turning green identifies its product as Cr³⁺. Chromium has fallen from +6 to +3, so dichromate was reduced and is the oxidant.

Step 2 — Identify the colourless solution

The colourless solution must contain the reductant. A brown colour developing points to I₂, which means the colourless solution contained iodide ions, I⁻.

Iodine has risen from −1 to 0, so iodide was oxidised and is the reductant.

Step 3 — Write the half equations

$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$

$2\text{I}^- \rightarrow \text{I}_2 + 2e^-$

Step 4 — Combine

Multiply the iodide half by 3 so both involve 6 electrons, then add and cancel:

$\text{Cr}_2\text{O}_7^{2-}(aq) + 14\text{H}^+(aq) + 6\text{I}^-(aq) \rightarrow 2\text{Cr}^{3+}(aq) + 7\text{H}_2\text{O}(l) + 3\text{I}_2(aq)$

Step 5 — Check the observations against the equation

The equation predicts Cr³⁺ (green) and I₂ (brown) as the coloured products, and the disappearance of the orange Cr₂O₇²⁻ — which is exactly the sequence described.

Answer: acidified dichromate (orange) was reduced to Cr³⁺ (green) and acted as the oxidant; iodide (colourless) was oxidised to I₂ (brown) and acted as the reductant.