AB + CD → AD + CB
A and C are positive charged cations in this reaction, while B and D are negative charged anions. Double-replacement reactions typically occur in aqueous solution between the compounds. To cause a reaction, one of the products is usually a solid precipitate, a gas, or a molecular compound like water.
A precipitate forms in a double-replacement reaction when the cations from one reactant combine to form an insoluble ionic compound with the anions from the other reactant. The following reaction occurs when aqueous solutions of potassium iodide and lead ( II) nitrate are blended.
3Ba(NO3)2 + 2K3PO4 → 6KNO3 + Ba3(PO4)2
Phương trình số #2
3Ca(OH)2 + 2H3PO4 → 6H2O + CaHPO4
Phương trình số #3
Ca(OH)2 + 2HNO3 → Ca(NO3)2 + 2H2O