CBSE Class 10 Answered

Single displacement: This is when one element trades places with another element in a compound. These reactions come in the general form of:

A + BC? AC + B

A and B must be either:

• different metals (hydrogen's behavior as a cation renders it as a metal here), in which case C represents an anion; or
• halogens, in which case C represents a cation.

One example of a single displacement reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:

Mg + 2 H₂O  ?Mg(OH)₂ + H₂

Double displacement: This is when the anions and cations of two different molecules switch places, forming two entirely different compounds. These reactions are in the general form:

AB + CD? AD + CB

One example of a double displacement reaction is the reaction of lead (II) nitrate with potassium iodide to form lead (II) iodide and potassium nitrate:

Pb(NO₃)₂ + 2 KI ? PbI₂ + 2 KNO₃