Practical activity

Chemists have created a series of chemical tests that will identify which metal ion is present. Similarly, there is another series of chemical tests for negative ions, so that the overall identity of an unknown salt can be discovered. Many of these tests involve a precipitation reaction. In the online experiment you will identify metal ions in solution.

Task 1: You will perform a series of chemical tests by mixing a metal ion in solution with one reagent.

Task 2: Using observations of chemical tests, you will identify the unknown metal ion present in a solution.

Task 1: Chemical tests for metal ions in solution

First, you will examine the outcome of individually mixing a metal ion in solution with one reagent. Tests will be performed with two different reagents (sodium hydroxide and ammonia solutions). You should look very carefully at what happens to each metal ion solution when adding a reagent (starting with a few drops and then adding reagent in excess). Remember that all your observations should be recorded in your laboratory notebook.

A possible change occurring during the tests is the formation of a coloured precipitate, but don’t be disappointed if you don’t see a change when performing some tests. Remember that negative results are often as important as positive ones and they should be also recorded in your laboratory notebook.

Your observations will show that each metal ion reacts in a different way with each reagent and so these tests provide a means of identifying which metal is present in an unknown solution, which is your second task.

Detailed instructions are provided within the experiment. In preparation, read and reflect on the following summary of the steps you will need to take:

1. Using the dropping pipette, transfer a small volume of a metal ion solution into an empty test tube. Ensure you note down which metal ion is being transferred into each test tube (numbered 1–6).
2. Select one of the reagents (sodium hydroxide solution or ammonia solution) and using the dropper pipette initially add a few drops of your selected reagent into the test tube containing the metal ion solution. Ensure you note down which reagent is being added to each test tube.
3. After watching video showing chemical reaction, record the results of the test by giving a brief description of what has been observed. Don’t be disappointed if you don’t see a change when performing some tests. Remember that negative results are often as important as positive ones, and they should be also recorded.
4. Add excess of your selected reagent.
5. Ensure that the metal ion solution and the reagent solution are mixed well by using a glass stirring rod before recording your final observations.
6. Repeat this test using the second reagent and record your observations.
7. Repeat steps 1–6 for each of the other six metal ion solutions available and record all results

Tables 1 and 2 show templates you could use to record your observations.

Table 1 Qualitative data from reactions of positively charged metal ions in solution with a solution of dilute sodium hydroxide.

Table 2 Qualitative data from reactions of positively charged metal ions in solution with a solution of dilute ammonia.

Click on the icon to access the OpenSTEM Africa Virtual Laboratory (Chemicaltests) homepage. Watch the introductory video before entering the experiment to carry out Task 1.

Task 2: Identification of an unknown metal ion in a water sample

Once you completed chemical tests for the six metal ions in solution, select one of the unknown solutions a–d.

1. Using the dropping pipette, transfer a small volume of the unknown solution into an empty test tubes.
2. Select one of the reagents (sodium hydroxide solution or ammonia solution) and using the dropper pipette initially add a few drops of your selected reagent into the test tube containing the unknown solution.
3. After watching video showing chemical reaction, record the results of the test by giving a brief description of what has been observed.
4. Add excess of your selected reagent.
5. Ensure that the metal ion solution and the reagent solution are mixed well by using a glass stirring rod before recording your final observations.
6. Repeat this test using the second reagent and record observations.

The colour of an unknown solution may provide a clue in the identification of a metal ion however you should always carry out the chemical tests to conclusively confirm which metal ion is present. Table 3 shows a template you could use to record your observations.

Table 3 Qualitative data from the reaction of an unknown sample containing a metal ion with different reagents

Which metal ion do you think is present in your allocated water sample? Explain your choice

Note that unknows samples are randomly allocated and yours will be different to those allocated to your fellow students. Check with your teacher if an identification of a metal ion is a bit challenging.