NCERT Class 10 Science | Chapter: Metals and Non-Metals | Texcellency Book Series

✅ Answer in Brief (For Quick Revision)

The reactivity order is: Mg > Zn > Cu (magnesium most reactive, copper least reactive).

Displacement reactions take place in three cases: 🔵 ZnO + Mg → MgO + Zn ✅ (Mg more reactive than Zn — displaces Zn from ZnO) 🔵 CuO + Mg → MgO + Cu ✅ (Mg more reactive than Cu — displaces Cu from CuO) 🔵 CuO + Zn → ZnO + Cu ✅ (Zn more reactive than Cu — displaces Cu from CuO)

No displacement in the remaining six combinations — either the same metal meets its own oxide (no reaction) or the added metal is LESS reactive than the metal in the oxide (cannot displace).

⭕ Important correction from the existing post: ZnO + Mg IS a displacement reaction — magnesium is more reactive than zinc. The existing post incorrectly states no reaction for ZnO + Mg. Always check reactivity order: if the added metal is ABOVE the oxide’s metal in the reactivity series → displacement occurs.

🏭 The Seniority Analogy — The Golden Rule

Imagine a company with employees ranked by seniority (the reactivity series). Each employee is sitting in a chair (occupying an oxide — sitting combined with oxygen). A new employee (the added metal) wants to sit in that chair.

🔵 If the new employee is MORE SENIOR (more reactive): They say “I outrank you — get up, I’m taking this chair.” The less senior employee (less reactive metal) is displaced — displaced from the oxide. Displacement reaction occurs. 🔵 If the new employee is LESS SENIOR (less reactive): They cannot claim the chair. The current occupant stays. No reaction. 🔵 If it is the SAME employee trying to sit in their own chair: They are already there — nothing changes. No reaction.

Seniority = reactivity. More reactive = higher seniority = wins the chair (the oxide). This is the entire principle of displacement reactions in one analogy.

🔴 The Foundation — The Reactivity Series and the Three Metals

Before analysing all 9 combinations, you must have the reactivity order firmly in mind.

The reactivity series (relevant portion): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Pt > Au

Among our three metals: Mg > Zn > Cu

🔵 Magnesium (Mg): Most reactive of the three — reacts vigorously with oxygen, water (hot), and acids. Burns with a brilliant white flame in air. 🔵 Zinc (Zn): Moderately reactive — above iron in the reactivity series. Reacts with dilute acids, displaces copper from copper salt solutions. 🔵 Copper (Cu): Least reactive of the three — below hydrogen in the reactivity series. Does not react with dilute acids or water. Can be displaced by both Mg and Zn.

The displacement rule for metal + metal oxide (heated): A more reactive metal CAN displace a less reactive metal from its oxide. A less reactive metal CANNOT displace a more reactive metal from its oxide. A metal CANNOT displace itself from its own oxide (no driving force).

🔶 Systematic Analysis — All 9 Combinations

This question involves 3 oxides (MgO, ZnO, CuO) × 3 metals (Mg, Zn, Cu) = 9 possible combinations. Let us go through every single one.

COMBINATIONS WITH MgO (Magnesium Oxide):

Combination 1 — MgO + Mg: 🔵 Same metal + its own oxide → No reaction 🔵 There is no driving force — Mg cannot displace itself from MgO 🔵 Result: No reaction ❌

Combination 2 — MgO + Zn: 🔵 Zn is LESS reactive than Mg (Mg > Zn in reactivity series) 🔵 Zn cannot displace Mg from MgO — a less reactive metal cannot displace a more reactive one 🔵 Result: No reaction ❌

Combination 3 — MgO + Cu: 🔵 Cu is LESS reactive than Mg (Mg > Cu in reactivity series — by a large margin) 🔵 Cu cannot displace Mg from MgO 🔵 Result: No reaction ❌

Verdict for MgO: MgO shows NO displacement reactions with any of the three metals. Magnesium is the most reactive — its oxide is the most stable — no metal among the three can displace it.

COMBINATIONS WITH ZnO (Zinc Oxide):

Combination 4 — ZnO + Mg: ⭕ THE CRITICAL COMBINATION — existing post has this wrong 🔵 Mg is MORE reactive than Zn (Mg > Zn in reactivity series) 🔵 Mg CAN and DOES displace Zn from ZnO when heated 🔵 DISPLACEMENT REACTION OCCURS ✅ 🔵 Equation: ZnO + Mg → MgO + Zn (on heating) 🔵 Observation: ZnO (white powder) + Mg (grey/silver metal) → on heating → MgO (white) + Zn (greyish metallic deposit). The zinc is released as free metal. 🔵 This IS a displacement reaction — the existing post incorrectly says no reaction here.

Combination 5 — ZnO + Zn: 🔵 Same metal + its own oxide → No reaction 🔵 Zn cannot displace itself from ZnO 🔵 Result: No reaction ❌

Combination 6 — ZnO + Cu: 🔵 Cu is LESS reactive than Zn (Zn > Cu in reactivity series) 🔵 Cu cannot displace Zn from ZnO 🔵 Result: No reaction ❌

Verdict for ZnO: ZnO shows ONE displacement reaction — only with Mg (the more reactive metal).

COMBINATIONS WITH CuO (Copper Oxide):

Combination 7 — CuO + Mg: 🔵 Mg is MORE reactive than Cu (Mg >> Cu — by a very large margin in reactivity series) 🔵 Mg displaces Cu from CuO vigorously when heated 🔵 DISPLACEMENT REACTION OCCURS ✅ 🔵 Equation: CuO + Mg → MgO + Cu (on heating) 🔵 Observation: Black CuO + grey Mg → on heating → white MgO + reddish-brown Cu metal. The reddish-brown copper deposit is a clear visual sign that displacement has occurred.

Combination 8 — CuO + Zn: 🔵 Zn is MORE reactive than Cu (Zn > Cu in reactivity series) 🔵 Zn displaces Cu from CuO when heated 🔵 DISPLACEMENT REACTION OCCURS ✅ 🔵 Equation: CuO + Zn → ZnO + Cu (on heating) 🔵 Observation: Black CuO + greyish Zn → on heating → white ZnO + reddish-brown Cu metal. Again, the appearance of reddish-brown copper confirms displacement.

Combination 9 — CuO + Cu: 🔵 Same metal + its own oxide → No reaction 🔵 Cu cannot displace itself from CuO 🔵 Result: No reaction ❌

Verdict for CuO: CuO shows TWO displacement reactions — with both Mg and Zn (both more reactive than Cu). Copper is the least reactive of the three — its oxide is most easily displaced.

🔷 Summary of All 9 Combinations — Complete Table

MgO + Mg: No reaction (same metal) MgO + Zn: No reaction (Zn less reactive than Mg) MgO + Cu: No reaction (Cu less reactive than Mg) ZnO + Mg: ✅ DISPLACEMENT — ZnO + Mg → MgO + Zn (Mg more reactive than Zn) ZnO + Zn: No reaction (same metal) ZnO + Cu: No reaction (Cu less reactive than Zn) CuO + Mg: ✅ DISPLACEMENT — CuO + Mg → MgO + Cu (Mg more reactive than Cu) CuO + Zn: ✅ DISPLACEMENT — CuO + Zn → ZnO + Cu (Zn more reactive than Cu) CuO + Cu: No reaction (same metal)

Total displacement reactions: 3 (Combinations 4, 7, 8) Total no reactions: 6 (Combinations 1, 2, 3, 5, 6, 9)

🔴 The Equations — Written Clearly for the Exam

The three displacement reactions with balanced equations:

Reaction 1 — ZnO + Mg → MgO + Zn (on heating) (Zinc oxide + Magnesium → Magnesium oxide + Zinc) 🔵 Mg displaces Zn because Mg > Zn in reactivity 🔵 Magnesium takes oxygen away from zinc — reduces ZnO to Zn metal 🔵 Mg itself gets oxidised (gains oxygen → forms MgO)

Reaction 2 — CuO + Mg → MgO + Cu (on heating) (Copper oxide + Magnesium → Magnesium oxide + Copper) 🔵 Mg displaces Cu because Mg >> Cu in reactivity 🔵 Very vigorous reaction — magnesium reacts intensely with most metal oxides 🔵 Reddish-brown copper metal is released — visible observation

Reaction 3 — CuO + Zn → ZnO + Cu (on heating) (Copper oxide + Zinc → Zinc oxide + Copper) 🔵 Zn displaces Cu because Zn > Cu in reactivity 🔵 This is also an example of thermite-type reaction (solid-solid displacement on heating) 🔵 Reddish-brown copper metal deposited — observable

All three reactions follow the pattern: More reactive metal + Less reactive metal’s oxide → More reactive metal’s oxide + Less reactive metal (free)

🔶 The Double Role — Oxidation and Reduction Happening Simultaneously

Each displacement reaction is also a REDOX reaction — oxidation and reduction both happening in the same reaction:

Taking CuO + Zn → ZnO + Cu as the example:

🔵 Zinc (Zn) is OXIDISED — it gains oxygen (goes from free Zn metal → ZnO). Loss of electrons. Oxidation. 🔵 Copper in CuO is REDUCED — it loses oxygen (goes from CuO → free Cu metal). Gain of electrons. Reduction. 🔵 Zn is the reducing agent — it causes Cu to be reduced (by giving oxygen to CuO’s copper) 🔵 CuO is the oxidising agent — it causes Zn to be oxidised (by supplying oxygen to Zn)

This is a complete redox reaction — the more reactive metal (Zn) acts as the reducing agent and the less reactive metal’s oxide (CuO) acts as the oxidising agent. This pattern holds for ALL three displacement reactions in this question.

🔷 Real-Life Connection — The Thermite Reaction

The most dramatic and famous real-life example of this exact type of reaction — a more reactive metal displacing a less reactive metal from its oxide by heating — is the THERMITE REACTION:

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe + enormous heat (~2500°C) (Iron oxide + Aluminium → Aluminium oxide + Iron + Heat)

🔵 Aluminium is more reactive than iron (Al > Fe in reactivity series) 🔵 Al displaces Fe from Fe₂O₃ — same principle as Mg displacing Cu from CuO 🔵 The reaction releases so much heat (~2500°C) that the iron produced is in MOLTEN form 🔵 This molten iron flows into the gap between two railway track sections and solidifies — welding them together perfectly

The thermite reaction is used across the world to weld railway tracks in situ — the tracks cannot be brought to a factory for conventional welding, so the thermite reaction provides a portable, self-sustaining welding process right on the railway line. Next time you see seamless railway tracks, know that thermite chemistry — the exact same displacement reaction you are studying — made those joins possible.

🔵 Thermite is also used in military incendiary devices, in controlled demolition, and in welding underwater pipelines.

🔴 How to Solve Any Displacement Reaction Question — The Method

This method works for ANY question of this type — not just for Mg, Zn, Cu. Use it every time:

Step 1: Identify the reactivity series position of the metal being added (the free metal) and the metal in the oxide. Step 2: Ask: Is the added metal MORE reactive than the metal in the oxide? 🔵 YES → Displacement reaction occurs. Write the equation: Added metal + Metal oxide → Added metal’s oxide + Displaced metal 🔵 NO (less reactive or equal/same metal) → No reaction

Step 3: Write the balanced equation for each displacement reaction identified. Step 4: State the observation (what you would see in the lab — colour changes, deposits).

Applying to this question: 🔵 Mg (high) vs Zn (lower): Mg > Zn → ZnO + Mg → MgO + Zn ✅ 🔵 Mg (high) vs Cu (much lower): Mg > Cu → CuO + Mg → MgO + Cu ✅ 🔵 Zn (medium) vs Cu (lower): Zn > Cu → CuO + Zn → ZnO + Cu ✅ 🔵 Zn (medium) vs Mg (higher): Zn < Mg → MgO + Zn → No reaction ❌ 🔵 Cu (low) vs Mg (much higher): Cu < Mg → MgO + Cu → No reaction ❌ 🔵 Cu (low) vs Zn (higher): Cu < Zn → ZnO + Cu → No reaction ❌ 🔵 Same metal: No reaction × 3 ❌

🎵 Rhyme to Remember

“Reactivity series — Mg at the top, Then Zn below it — then Cu does stop!* The higher metal always wins the game,* Displaces the lower — that is its fame!* ZnO meets Mg — Mg says ‘move out!’* MgO + Zn is what it’s about!* CuO meets Mg AND Zn — both win the fight,* Two displacement reactions — copper shining bright!* Three displacements total — remember this well:* Mg beats Zn, Mg beats Cu, Zn beats Cu — ring the bell!”*

🧩 Mnemonics

🔵 “Mg > Zn > Cu — More Zealous Creatures” (Most reactive to Least) — M for Mg, Z for Zn, C for Cu. 🔵 “HIGHER DISPLACES LOWER — always, in heating experiments” — one rule for all displacement reactions. 🔵 “3 DISplacements: Mg+ZnO ✅, Mg+CuO ✅, Zn+CuO ✅ — the three Ds” — the only three that work. 🔵 “Same + Same = Nothing” — metal + its own oxide = no reaction, always. 🔵 “THERMITE = Al beats Fe = 2500°C = Railway track welding” — the real-world extension of this concept. 🔵 “CuO is the easiest to displace — copper is least reactive of the three — BOTH Mg AND Zn beat it.”

✅ Exam-Ready Answer (Write This in Board Exam)

Metallic oxides of zinc, magnesium and copper were heated with zinc, magnesium and copper separately. In which cases will you find displacement reactions taking place?

Reactivity order: Mg > Zn > Cu (from the reactivity series)

Rule: A more reactive metal displaces a less reactive metal from its oxide when heated.

Displacement reactions occur in the following three cases:

Case 1: ZnO + Mg → MgO + Zn (on heating) Magnesium is more reactive than zinc — so Mg displaces Zn from ZnO. Observation: ZnO (white) + Mg → MgO (white) + Zn (grey metallic deposit)

Case 2: CuO + Mg → MgO + Cu (on heating) Magnesium is more reactive than copper — so Mg displaces Cu from CuO. Observation: CuO (black) + Mg → MgO (white) + Cu (reddish-brown metallic deposit)

Case 3: CuO + Zn → ZnO + Cu (on heating) Zinc is more reactive than copper — so Zn displaces Cu from CuO. Observation: CuO (black) + Zn → ZnO (white) + Cu (reddish-brown metallic deposit)

No displacement reactions in the remaining combinations: 🔵 MgO + Zn → No reaction (Zn less reactive than Mg) 🔵 MgO + Cu → No reaction (Cu less reactive than Mg) 🔵 ZnO + Cu → No reaction (Cu less reactive than Zn) 🔵 MgO + Mg, ZnO + Zn, CuO + Cu → No reaction (same metal + its own oxide)

Total displacement reactions = 3.

📌 Key Points Checklist

✅ Reactivity order: Mg > Zn > Cu — most to least reactive ✅ Rule: more reactive metal displaces less reactive metal from its oxide (when heated) ✅ 3 displacement reactions: ZnO + Mg → MgO + Zn | CuO + Mg → MgO + Cu | CuO + Zn → ZnO + Cu ✅ 6 no-reaction combinations: MgO + Zn, MgO + Cu, ZnO + Cu (less reactive added) + same metal + own oxide (×3) ✅ ⚠️ Existing post error: ZnO + Mg IS a displacement reaction — Mg is MORE reactive than Zn ✅ All displacement reactions here are also REDOX: added metal = oxidised (gains O) + displaced metal’s oxide = reduced (loses O) ✅ CuO is most easily displaced — Cu is least reactive — both Mg and Zn can displace it ✅ MgO is never displaced — Mg is most reactive — neither Zn nor Cu can beat it ✅ Observation for each: reddish-brown Cu metal appears when CuO is displaced | grey Zn metal appears when ZnO is displaced ✅ Real-life connection: thermite reaction (Fe₂O₃ + 2Al → Al₂O₃ + 2Fe, ~2500°C) used to weld railway tracks — same principle

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