8.19—Reactions

Know reactions, including ionic equations where appropriate, for identifying: (i) carbonate ions, CO3^2−, and hydrogencarbonate ions, HCO3−, using aqueous acid to form carbon dioxide and limewater to test the gas; (ii) sulfate ions, SO4^2−, using acidified barium chloride solution; and (iii) ammonium ions, NH4+, using sodium hydroxide solution and warming to form ammonia, tested with litmus and HCl fumes.

Use —reactions to connect the rule to the data and decision in the question.

This matters because —reactions determines what can be inferred or chosen; begin with the stated conditions and keep the conclusion tied to the evidence.

Example: apply —reactions to one small, clearly defined case, show the key step or comparison, and explain the result in words.

Boundary: —Reactions is not a universal recommendation. Check the syllabus scope, assumptions, units and the limits of the evidence before generalising.