OXIDATION and REDUCTION always occur TOGETHER in the same reaction — called a REDOX reaction.
In terms of OXYGEN:
OXIDATION = GAIN of oxygen.
REDUCTION = LOSS of oxygen.
Examples:
Mg + O₂ → MgO
Magnesium is OXIDISED — it gains oxygen.
CuO + H₂ → Cu + H₂O
Copper oxide is REDUCED — it loses oxygen.
Hydrogen is OXIDISED — it gains oxygen.
Fe₂O₃ + 3CO → 2Fe + 3CO₂
Iron oxide (Fe₂O₃) is REDUCED — iron loses its oxygen.
Carbon monoxide (CO) is OXIDISED — it gains oxygen to become CO₂.
OIL RIG — Electron Definitions (Overview)
At a more advanced level, oxidation and reduction are defined in terms of ELECTRONS:
OIL RIG:
Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
Think of OIL RIG as a memory device — it only works when you remember what each letter means.
EXAMPLES:
Na → Na⁺ + e⁻ — sodium LOSES an electron → sodium is OXIDISED.
Cl + e⁻ → Cl⁻ — chlorine GAINS an electron → chlorine is REDUCED.
In the reaction: Na + Cl → Na⁺ + Cl⁻
Sodium is OXIDISED (loses e⁻). Chlorine is REDUCED (gains e⁻).
This is a REDOX reaction — both happen simultaneously.
The substance that causes oxidation is the OXIDISING AGENT.
The substance that causes reduction is the REDUCING AGENT.
Note: the oxidising agent is itself REDUCED (it gains electrons). The reducing agent is itself OXIDISED (it loses electrons).
Oxidising and Reducing Agents
REDUCING AGENT — causes reduction of another substance by donating electrons (it is itself OXIDISED):
Carbon in the blast furnace: reduces iron oxide — carbon is the REDUCING AGENT (it gets oxidised to CO₂).
Hydrogen: reduces copper oxide — hydrogen is the REDUCING AGENT (gets oxidised to H₂O).
OXIDISING AGENT — causes oxidation of another substance by accepting electrons (it is itself REDUCED):
Oxygen: oxidises metals (is itself reduced to O²⁻).
Copper oxide: oxidises hydrogen (is itself reduced to Cu).
Common reducing agents: hydrogen, carbon, carbon monoxide, more reactive metals.
Common oxidising agents: oxygen, chlorine, hydrogen peroxide, potassium manganate(VII).
REDOX in everyday contexts:
Rusting — iron is OXIDISED by oxygen and water.
Combustion — fuels are OXIDISED by oxygen.
Metabolic respiration — glucose is OXIDISED.
Photosynthesis — CO₂ is REDUCED to glucose.
⚠️ Common Mistake
In a redox reaction, the REDUCING AGENT is OXIDISED (it gives away electrons/oxygen). The OXIDISING AGENT is REDUCED (it receives electrons/gains oxygen). This is counterintuitive — an oxidising agent doesn't stay oxidised, it gets reduced. Remember: agents do the opposite of what they're called.
📐 Key Equations
Oxidation = gain of oxygen / loss of electrons
Reduction = loss of oxygen / gain of electrons
OIL RIG: Oxidation Is Loss, Reduction Is Gain (electrons)
📌 Key Note
Oxidation: gain O / lose electrons. Reduction: lose O / gain electrons. OIL RIG. Always happen together (redox). Reducing agent: donates electrons → gets oxidised. Oxidising agent: accepts electrons → gets reduced. Carbon and hydrogen are common reducing agents.
🎯 Matching Activity — Oxidised or Reduced?
Identify whether each substance is oxidised or reduced in the reaction. — drag the symbols on the right to match the component names on the left.
Oxidised
Drop here
Reduced
Drop here
Oxidised
Drop here
Reduced
Drop here
Oxidised
Drop here
Fe₂O₃ in Fe₂O₃ + 3CO → 2Fe + 3CO₂ — iron oxide loses oxygen
H₂ in CuO + H₂ → Cu + H₂O — hydrogen gains oxygen
Mg in Mg + O₂ → MgO — magnesium gains oxygen
Na in Na → Na⁺ + e⁻ — sodium loses an electron (OIL RIG)
CuO in CuO + H₂ → Cu + H₂O — copper oxide loses oxygen
🎯 Test Yourself
Question 1 of 2
1. In the reaction CuO + H₂ → Cu + H₂O, what is the reducing agent?
2. What does OIL RIG stand for?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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