Advanced Solubility and Precipitation Tests

Sometimes just putting salt into a solution doesn’t provide too much information. In many cases, we need to introduce more chemicals into the solution in order to identify the salt we are working with.

In order to do this, we need to take a test tube, and fill it with a little bit of the salt and some DI water. Then, you will add some of a known chemical in an attempt to have a reaction occur. Typically, you have substitution reactions occur:

$AB+CD\rightarrow AD+CB$

In most cases, however, this type of reaction doesn’t occur as it is shown, as if this reaction is used to identify a precipitate, only one of those four compounds exists as a solid, with the rest existing in solution. Therefore, the actual reaction would be a combination reaction:

$A^++D^-\rightarrow AD$

Typically, these reactions only work when your initial salts are soluble. Therefore, if your initial salt is insoluble, these are not very good tests for identifying your salt.

However, if the salt is indeed soluble, then through a variety of additions, you can identify the presence of various compounds in solution.

As we mentioned in the end of the previous section, Silver Nitrate is one of those chemicals.

Lead in a dissolved lead(II) salt is the cation, Pb²⁺. The accompanying negatively charged species is the anion. Tests for an anion and tests for a metal cation answer different questions, so label the ion being investigated before interpreting any precipitate.

If you didn’t know, a precipitate is the term we use for solids that form as a result of a chemical reaction. You can have various precipitate colors and textures. Typically, the easiest way to make sure you have a precipitate is to look for a change in the solution. Soluble salts have clear solutions, so if you start to see a cloudiness developing, or if you see solids forming on the bottom, then you have precipitates in your solution.

Aside from Nitrate and Acetate, it is a bit harder to identify the anions through simple addition of compounds. Now you’ll have to start looking at pH if you want to identify components past this point.

The simplest pH-related test is the test for Ammonia. If you have ammonium as your cation, then you can tell by adding some sodium hydroxide to solution. If ammonium is present, it will react with the hydroxide ion:

$NH_{4 (aq)}^++OH^-_{(aq)}\rightarrow NH_{3(g)}+H_2O_{(l)}$

An instructor may provide reviewed observations showing that an ammonium salt releases ammonia under appropriate test conditions. Do not identify ammonia by directly sniffing a sample. Use the approved indicator method and ventilation procedure and compare the observed response with controls. This learning task interprets supplied evidence; the archived source’s odor instruction is superseded.

One good choice to adjust the pH of your solution is Nitric Acid. When this acid is added to solutions of insoluble Carbonates, Sulfides, and Phosphates, it causes them to become soluble.

Also by adding Nitric Acid to solution, you are introducing the nitrate anion, which doesn’t precipitate with the cations in your salt. This makes Nitric Acid a great choice to lower the pH of your solution.

The original student instruction to generate hydrogen sulfide is superseded. Hydrogen sulfide is hazardous, and any use requires a separately reviewed institutional protocol and trained supervision. For the chemistry task, use teacher-supplied sulfide-precipitation observations and a candidate-ion table to explain the evidence. The source archive remains available for historical fidelity, not as an executable student procedure.

The reactions for the production of these gases with some salts you might have are:

$CaCO_3 + 2H^+\rightarrow Ca^{2+} + H_2O+CO_2$
$FeS+2H^+ \rightarrow Fe^{2+}+H_2S$

Once you have your salt dissolved (if it was insoluble, the Nitric Acid should dissolve it), then you can go ahead and start performing more tests to check for precipitate formation.

Metal ions can be identified by addition of Sodium Hydroxide to the solution. When metal ions react with the Hydroxide ion, they form insoluble Hydroxides. Sometimes, if you keep going, you get the precipitates to re-dissolve once enough of the hydroxide ion is present. If that happens, you have a cation that is amphoteric. This just means that it forms an insoluble hydroxide complex that can react further with hydroxide to produce a soluble ion. The metals that can do this include Aluminum, Chromium, Lead, Zinc, and Tin. Therefore, if you add sodium hydroxide to your solution and it results in a precipitate that disappears, then you have one of those ions.

You might notice that Zinc occurs in both of those lists. Therefore, these two tests allow you to determine the presence of Zinc in solution.

Instead of Nitric Acid, you can use Hydrochloric (HCl) or Sulfuric (H2SO4) to dissolve insoluble salts and to check for precipitate formation from insoluble salts that develop with the Cl- or SO42- ions. For example, if you have Lead Nitrate as your salt, it will be soluble in water, and if you add some Hydrochloric acid, it will form the insoluble Lead Chloride precipitate. Similarly, Sulfuric can identify the presence of Barium, Calcium, or Strontium.