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
H₂ in CuO + H₂ → Cu + H₂O — hydrogen gains oxygen
Mg in Mg + O₂ → MgO — magnesium gains oxygen
Fe₂O₃ in Fe₂O₃ + 3CO → 2Fe + 3CO₂ — iron oxide loses oxygen
Na in Na → Na⁺ + e⁻ — sodium loses an electron (OIL RIG)
CuO in CuO + H₂ → Cu + H₂O — copper oxide loses oxygen
⭐ Higher Tier Only
Define precisely: oxidation = loss of electrons; reduction = gain of electrons (OIL RIG). Write half equations for oxidation and reduction. Identify oxidising agents (gain electrons, are reduced) and reducing agents (lose electrons, are oxidised). Apply to displacement and electrolysis reactions.
🎯 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?
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