CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 22(a)

Observation: no change; the two dry powders simply form a white mixture and no precipitate appears. Justification: the reaction is an exchange of ions, and in the solid (powder) form the ions are held in fixed positions in the crystals, so the ${\text{Ba}}^{2+}$ ions of barium chloride and the ${\text{SO}}_4^{2-}$ ions of sodium sulphate are not free to meet and react. Desired change: dissolve each powder in water separately and mix the two solutions. The ions are then free to move and a white precipitate of barium sulphate forms at once (a double displacement reaction): ${\text{Na}}_2{\text{SO}}_4(\text{aq})+{\text{BaCl}}_2(\text{aq})\to {\text{BaSO}}_4(\text{s})+2\text{NaCl}(\text{aq})$