Displacement reaction:
A more reactive element pushes a less reactive element out of its compound.
- Example: Fe + CuSO4 -> FeSO4 + Cu
Iron is more reactive than copper, so it displaces copper. The blue solution turns light green, and a brownish coating forms on the iron nail.
- One element takes the place of another in a compound.
- It depends on the reactivity series.
Double displacement reaction:
Two compounds exchange their ions to form two new compounds.
- Example: Na2SO4 + BaCl2 -> BaSO4 + 2NaCl
A white precipitate of barium sulphate forms.
- Both compounds exchange partners.
- These usually take place in solution.
Is precipitation the same thing?
No, but the two overlap. NCERT calls any reaction that produces a precipitate (an insoluble solid) a precipitation reaction. Many double displacement reactions are also precipitation reactions, like the barium sulphate example above, because one of the new compounds is insoluble. But not every double displacement reaction forms a precipitate. For example, neutralisation such as NaOH + HCl -> NaCl + H2O is a double displacement reaction with no precipitate.
Quick way to tell them apart in an equation:
- An element plus a compound on the reactant side, and a different element set free on the product side: displacement. (An element plus a compound alone is not enough: 2CO + O2 -> 2CO2 is a combination reaction.)
- Two compounds on the reactant side that swap ions: double displacement.
Exam tip: learn one observation (a colour change or a precipitate) for each example along with the type of reaction. A question can ask for either, and the observation also helps you name the type correctly.
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