Precipitate Testing

What are we testing to find? Precipitate tests are used to detect the presence of an ion within a solution. Precipitate tests may test for positive ions, for example magnesium ions (Mg2+). Positive ions are called cations. Precipitate tests may also test for negative ions, for example carbonate ions (CO32+). Negative ions are called anions.

How do we do the test? We take the solution, which is in a test tube, and add another solution to this. At the start of the test both solutions are clear, even though they may be coloured. If the ion we are testing for is present, then a solid will be produced. The test tube will now contain a white or coloured solid which is clearly visible. This solid is called a precipitate. It is a new salt which is insoluble in water.

What results might we get? If the result of adding the two solutions was still a clear liquid then the ion being tested for has not been shown to be present. We may get a white precipitate, or the precipitate may be coloured. This is helpful because sometimes the colour gives us more information to identify the ions present.

Why does this test work? The reason this works is because if a salt is soluble, when placed in water its ions are able to react with other ions. The salt contains anions and cations. An example is copper sulphate which contains Cu2+ cations and SO42- anions. Most salts are soluble, but there are a few rules of solubility, and a few salts are known to be insoluble. If the cation and anion of an insoluble salt are present in the same solution then they will form a solid - and we can see that as a precipitate.

Some rules of solubility of salts 1 All nitrates are soluble. Most sulphates are soluble. However barium sulphate (BaSO4) is known to be insoluble and will form a precipitate, where barium and sulphate ions are present together. Most carbonates are insoluble and will not dissolve in water. However group 1 metals form soluble carbonates. We can therefore make solutions of sodium carbonate and potassium carbonate.

Some rules of solubility of salts 2 Most hydroxides are insoluble and will not dissolve in water. However group 1 metals form soluble hydroxides. We can therefore make solutions of sodium hydroxide and potassium hydroxide. Most halides are soluble. Halides are salts of halogen elements including chlorine, bromine and iodine. A notable exception is that the halides of silver are insoluble. Silver chloride is a white insoluble solid; silver bromide is a cream insoluble solid; and silver iodide is a yellow insoluble solid.

Summary Precipitation reactions are a method of detecting the presence of unknown cations or anions by the addition of a known solution to our sample with unknown ions. The ions can be shown to be present if we are able to make a precipitate.