Testing for Ions

The water that you drink, the pool that you swim and the food that you eat all have many different chemicals. However, there are some chemicals that we can ingest but this is to a certain point. When looking at swimming pools, for example, chlorine is added to kill any bacteria, but how can we ensure that it is in there? This is where testing for ions comes in, as we can take a sample of water and test for the chloride ions. We will be going through this test, as well as many others.

• We will first go over the test for metal ions; this will include the flame test and using sodium hydroxide solution.
• After, we will explore the test for carbonate ions.
• We will then go on to the test for sulphate ions.
• Then we will explore the test for halide ions.
• Finally, we will go through the test for ammonium ions.

Test for metal ions

Metal ions are cations. This means that they are positively charged and have lost electrons. To detect metal ions we can use two types of tests; using a flame and using sodium hydroxide. Metal ions are quite interesting because they are different bright colours, this makes it easier to distinguish between them and also makes it more fun to learn. So let us explore them both.

Flame test

The flame test is quite exciting and you might even carry it out in your classroom. During this test, each metal cation produces a different coloured.

Before we explore those different colours let us go through how we experiment:

1. First, get a nichrome wire loop and dip it into concentrated hydrochloric acid, then heat it. This is to clean the wire.
2. Then dip it into the acid again and then into the metal compound that is being tested.
3. Now hold the loop in the blue flame of a Bunsen burner.
4. You will see a specific colour, and you can compare this to when we explore the different colours. If a metal compound has a mixture of metals, some of the flame colours may be dominated by others.

Now to explore the different colours you will see and match them to the right metal ion. Lithium ions, Li⁺, produce a crimson flame colour, sodium ions (Na⁺) produce a yellow flame colour, potassium ions, K⁺, produce a lilac flame colour, calcium ions (Ca²⁺) produce an orange-red flame colour, and copper ions, Cu²⁺, produces a green flame colour. This is summarised in the table below.

Table 1: Different metal ions and the coloured flame they produce in the flame test.

Using sodium hydroxide

We can also use sodium hydroxide solution to detect some unknown compounds. But this has a smaller range and we do not have the same reaction for all the metal ions. The first group is aluminium ions Al³⁺, calcium ions Ca²⁺ and magnesium ions Mg²⁺. When sodium hydroxide is added to the compound with these ions, a white precipitate is formed.

If you carry on adding sodium hydroxide solution and the precipitate dissolves, this means the substance has aluminium ions. However, if the precipitate does not dissolve it can be calcium ions or magnesium ions, which can be tested further with the flame test that we explored above.

For the second group of metal ions, we can sodium hydroxide solution to detect produced coloured precipitates. If a blue precipitate appears, copper (II) ions are in the substance, if a green precipitate is produced, iron (II) ions are present and if a brown precipitate is formed, iron (III) is present. If you go on to study A-Level chemistry you will also cover this topic so it is important that you also understand it at the GCSE level.

Test for carbonate ions

When testing for carbonate ions, we have a two-stage process:

1. We first start with the solution we are testing, to which we add some dilute acid. This will produce carbon dioxide gas.
2. We can then test this gas by bubbling it through a test tube with lime water. If carbonates are present the limewater will turn from a clear solution to a milky cloudy white colour.

Looking at the diagram above you can see that the test tube with our sample has a bung and a tube that connects it to the tube with the limewater. This is to ensure none of the carbon dioxide escapes and can be used to test the presence of carbonate ions.

Test for halide ions

Our next test is to detect the different halide ions, these are chloride ions, Cl^-; bromide ions, Br^-; and iodide ions, I^-.

For this reaction, we start by adding some dilute nitric acid to the substance we are testing. This removes any carbonate ions produce, which can affect our results.

We then add some silver nitrate and if a precipitate forms, this means halide ions are present.

This test also allows us to determine exactly which halide ion is present, so there is no need to carry out any additional tests. To distinguish between the different halides, we will observe the colour of the precipitate produced. If a yellow precipitate is produced, iodide ions are present, if a cream precipitate is produced, bromide ions are present, and if a white precipitate is produced, chloride ions are present. This is summarised in the tablet below.

Table 2: Different halide ions and the coloured precipitate they produce when reacting with nitric acid and silver nitrate.

Test for ammonium ions

The final test that we are going to explore is how to detect ammonium ions (NH4+). You can find ammonium ions in a variety of different substances such as ammonium solution or any ammonium salt like ammonium chloride.

We start the process by adding dilute sodium hydroxide to our substance, we then go on to gently heat this combination.

If ammonium ions are present, the ions will convert to ammonia gas. You can then test this using damp red litmus paper, where if there are ammonium ions, will turn blue.