An iron nail dipped in copper sulphate solution turns brown. This is due to which one of the following types of reactions ?
- (a)Addition reaction
- (b)Decomposition reaction
- (c)Substitution reaction
- (d)Displacement reaction
Correct — D, Displacement reaction. Iron is more reactive than copper, so when an iron nail is dipped in copper sulphate solution the iron displaces copper from the salt: Fe + CuSO4 gives FeSO4 + Cu. The freed copper deposits on the nail as a reddish-brown coating (and the blue solution slowly turns pale green). A more reactive metal ousting a less reactive one from its salt is a displacement reaction.
- (a)Addition reaction — Addition reactions occur when atoms add across the double bond of an unsaturated compound; no double bond is involved here, so it does not apply.
- (b)Decomposition reaction — Decomposition is a single compound breaking into simpler products. Here a metal replaces another in a salt — that is not decomposition.
- (c)Substitution reaction — This is the tempting trap. 'Substitution' usually describes an atom or group being replaced in an organic molecule; a more reactive metal ousting a less reactive one from its salt is specifically called a displacement reaction.
In a displacement (single-replacement) reaction, a more reactive element takes the place of a less reactive one in a compound. Metal reactivity is ranked by the reactivity series; a metal higher in the series can displace any metal below it from that metal's salt solution.
The colour change is the giveaway: brown copper coats the iron nail while the blue copper-sulphate solution fades. Iron sits above copper in the reactivity series, so it displaces copper. Do not mislabel this as 'substitution', which belongs to organic chemistry.
- The reaction is Fe + CuSO4 gives FeSO4 + Cu.
- Iron is more reactive than copper (higher in the reactivity series), so it displaces copper.
- Reddish-brown copper deposits on the nail; the blue solution turns pale green.
- It is a displacement reaction and also a redox reaction (iron is oxidised, copper reduced).
- Calling it a 'substitution' reaction — that term belongs to organic chemistry.
- Missing that a displacement reaction is also a redox reaction.
Asked as a 'which type of reaction' item using the classic iron-nail-in-copper-sulphate demonstration.
No directly related past PYQ was found.
- practice — not a real PYQ
When an iron nail is placed in copper sulphate solution, the colour of the solution changes from
- (a)green to blue
- (b)blue to pale green
- (c)colourless to blue
- (d)blue to red
Answer(b) blue to pale green — copper is displaced and iron(II) sulphate forms.
- practice — not a real PYQ
A displacement reaction takes place when
- (a)a less reactive metal is placed in the salt of a more reactive metal
- (b)a more reactive metal is placed in the salt of a less reactive metal
- (c)two compounds exchange ions
- (d)a compound breaks into simpler substances
Answer(b) a more reactive metal is placed in the salt of a less reactive metal — as iron displaces copper.