Tests for negative ions (anions): the carbonate test with acid, the halide test with silver nitrate, and the sulfate test with barium chloride, including the observations and ionic equations.

What this dot point is asking

OCR wants you to describe the chemical tests for three common negative ions (anions): carbonate (using dilute acid), halide (chloride, bromide and iodide, using silver nitrate), and sulfate (using barium chloride), including the observations and the ionic equations. These complete the qualitative analysis toolkit alongside the cation and gas tests.

Test for carbonate ions

Test for halide ions

The ionic equation for chloride is $\text{Ag}^+(\text{aq}) + \text{Cl}^-(\text{aq}) \rightarrow \text{AgCl}(\text{s})$. The corresponding equations for bromide and iodide are $\text{Ag}^+(\text{aq}) + \text{Br}^-(\text{aq}) \rightarrow \text{AgBr}(\text{s})$ and $\text{Ag}^+(\text{aq}) + \text{I}^-(\text{aq}) \rightarrow \text{AgI}(\text{s})$. The nitric acid is added first to remove carbonate ions, which would otherwise form a silver carbonate precipitate and give a false positive. These are precipitation reactions: two solutions are mixed and an insoluble solid (the silver halide) forms because the silver ion and the halide ion combine into a compound that does not dissolve.

Test for sulfate ions

Why you acidify first

Adding the acid first in the halide and sulfate tests removes carbonate ions, which would otherwise react to form their own precipitate and give a false positive. This makes the test specific to the halide or sulfate. The right acid matters: for the halide test you add nitric acid (not hydrochloric, which would add chloride ions and give a false positive), while for the sulfate test you add hydrochloric acid (not sulfuric, which would add sulfate ions).

A full analysis: combining the tests

In an exam you may be asked to fully identify an unknown salt by combining the cation and anion tests. The usual order is to identify the positive ion first (a flame test, or sodium hydroxide for a coloured precipitate), then identify the negative ion using the carbonate, halide and sulfate tests above. For example, a green flame plus a white precipitate with barium chloride after acidifying would indicate copper sulfate. Always describe both the reagent and the observed result, because the result is what earns the mark.