Ion Identification

Distinguishing White Precipitates

Why does adding more of a chemical sometimes make a solid disappear entirely? Both calcium ($\text{Ca}^{2+}$) and zinc ($\text{Zn}^{2+}$) ions form white precipitates when a few drops of sodium hydroxide are added. To tell them apart, you must add excess sodium hydroxide carefully, as adding it too quickly might cause the zinc precipitate to dissolve before you even notice it. Zinc hydroxide is amphoteric, meaning it can act as both an acid and a base. This property allows the white zinc precipitate to dissolve in excess alkali, leaving a colourless solution. In contrast, the white calcium hydroxide precipitate remains completely insoluble.

If the white precipitate does not dissolve, you can confirm the presence of calcium using a flame test, which will produce a characteristic orange-red flame.

Identifying Carbonate and Sulfate Anions

You can snap a piece of chalk in half with your hands, but dropping it into acid causes it to rapidly bubble and break down. This bubbling is called effervescence, and it is the primary test for a carbonate anion ($\text{CO}_3^{2-}$). Simply add a dilute acid, like hydrochloric acid ($\text{HCl}$), to your sample. To confirm the evolved gas is carbon dioxide, connect a delivery tube quickly and bubble the gas through limewater, which will turn from colourless to milky. To test for sulfate ions ($\text{SO}_4^{2-}$), you must first add dilute hydrochloric acid to acidify the sample and destroy any carbonates that could cause a false positive. Next, add a few drops of aqueous barium chloride ($\text{BaCl}_2$). If sulfate ions are present, a dense white precipitate of barium sulfate forms.

If you plan to test for halides directly after, you should acidify with dilute nitric acid and use barium nitrate instead. This prevents you from accidentally introducing chloride ions from the hydrochloric acid.

Identifying Halide Anions

Early photographic film relied entirely on the light-sensitive properties of silver compounds to capture images. In the laboratory, we use aqueous silver nitrate ($\text{AgNO}_3$) to test for halide ions ($\text{Cl}^{-}$, $\text{Br}^{-}$, $\text{I}^{-}$). First, the solution is acidified with dilute nitric acid ($\text{HNO}_3$) to remove interfering carbonate impurities. Never use hydrochloric or sulfuric acid here, as they contain ions that will ruin the test. Upon adding silver nitrate, distinct precipitates form depending on the halide present. Chloride forms a white precipitate ($\text{AgCl}$), bromide forms a cream precipitate ($\text{AgBr}$), and iodide forms a yellow precipitate ($\text{AgI}$).

To further confirm the results, dilute or concentrated ammonia can be added. Silver chloride dissolves in dilute ammonia, silver bromide requires concentrated ammonia to dissolve, and silver iodide remains entirely insoluble.