An element can be oxidised and reduced in the same reaction if different atoms of that element end up in different products. Follow the oxidation states separately. An increase identifies oxidation and a decrease identifies reduction.
The 2027 SEC G3 Chemistry syllabus (K324) covers redox chemistry. The example below uses a supplied equation to practise interpreting oxidation states. The term disproportionation describes this pattern.
- Write the element’s starting oxidation state.
- Calculate its oxidation state in each product.
- Compare each product with the starting state separately.
Worked example | Chlorine forms chloride and hypochlorite
The practice equation is Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O. Chlorine in elemental Cl₂ has oxidation state 0. In chloride, Cl⁻, its oxidation state is −1.
In ClO⁻, oxygen has oxidation state −2. The sum must equal the ion’s charge, −1. Chlorine must therefore have oxidation state +1.
The chlorine atoms that become chloride go from 0 to −1 and are reduced. Those that become hypochlorite go from 0 to +1 and are oxidised. A complete answer names each product and its change.
Why an average can hide what happened
Averaging the two product oxidation states gives zero. That average hides the two opposite changes. Keep the product ions in separate lines of working.
You can check the supplied equation by counting two chlorine atoms, two oxygen atoms and two hydrogen atoms on each side. The total charge is −2 on each side. This checks the equation, while the separate oxidation states identify the redox changes.
Try the same comparison with hydrogen peroxide
In 2H₂O₂ → 2H₂O + O₂, oxygen has oxidation state −1 in hydrogen peroxide. It becomes −2 in water and 0 in elemental oxygen. Identify which product contains reduced oxygen and which contains oxidised oxygen.
Oxygen is reduced in water and oxidised in O₂. Ask a study partner to point to the substance where oxygen has oxidation state −1. The formula H₂O₂ is the clue that this is a peroxide.
Frequently asked questions
Does every chlorine atom undergo both changes?
Different chlorine atoms end up in the two products. Compare the state of chlorine in each product with the starting state.
Why is the oxidation state of Cl₂ zero?
Chlorine is in elemental form in Cl₂.
Official information
SEAB’s 2027 SEC G3 Chemistry syllabus (K324), Section 7 Redox Chemistry. Checked 10 November 2026.