Understanding Double Displacement Reactions

A double displacement reaction, also known as a metathesis reaction, involves the exchange of ions between two reacting ionic compounds. The general form is:

AB + CD → AD + CB

Here, A and C are typically cations (positive ions), and B and D are anions (negative ions). They swap partners.

Analyzing Reaction Characteristics

• Option 1: only one product is formed - This is incorrect. Typically, double displacement reactions produce two new compounds.
• Option 2: the reaction mixture is always colourless - This is incorrect. Many double displacement reactions result in coloured precipitates or solutions, depending on the ions involved.
• Option 3: exchange of metal ions between two metal salts - This accurately describes the core mechanism. The positive ions (cations), often from metal salts, exchange their negative partners (anions).
• Option 4: the reaction requires heat to initiate - This is incorrect. Many double displacement reactions, especially precipitation reactions, occur spontaneously at room temperature.

Therefore, the defining characteristic common to all double displacement reactions is the exchange of ions (specifically, cations and anions) between the reactant compounds.