Have you ever wondered what causes the bright colours of your local area's annual firework display? These bold and brilliant hues are all thanks to different metals1. When burnt, they produce a gloriously coloured flame. Add various combinations of fuses and charges to control the explosion, and BOOM! You have a spectacular firework show on your hands!
But this simple principle has an important application: burning an unknown compound is a great way to test for metal ions. In this article, we'll explore how we can use so-called flame tests to identify unknown metal ions. We'll also look at the sodium hydroxide test and tests for other types of ions.
- This article is about tests for metal ions in chemistry.
- We'll start by defining metal ion before looking at their types.
- We'll then find out two ways of testing for metal ions: flame tests and the sodium hydroxide test.
- This will involve exploring their methods and results. We'll also take a deep dive into the chemistry behind flame tests.
- After that, we'll introduce you to tests for negative ions.
- Finally, we'll summarise some of the advantages and disadvantages of tests for metal ions.
Metal ion: definition and types
so, fireworks contain metals, and these metals are responsible for their bright colours. For example, red fireworks often contain lithium. However, the lithium in fireworks isn't in the form of lithium atoms. Instead, it is in the form of lithium ions, a type of metal ion. Let's look at metal ions a little more closely.
Metal ion definition
Firstly, what are metal ions?
Metal ions are positively charged particles formed when a metal atom loses one or more electrons.
For example, lithium atoms have 3 protons and 3 electrons. They have a neutral charge, and we represent them using the letters Li. Calcium ions, on the other hand, have 3 protons but just 2 electrons. They have a positive charge of +1. We represent them using Li+.
Remember that electrons are negative. This means that when an atom loses electrons, it becomes positively charged. We call positively charged ions cations. Note that all metal ions are positive, and so are all cations, but not all cations are metal ions!
Types of metal ion
There are many types of metal ions. Here are some of the more common metal ones that you'll come across in your GCSE studies. We've included information about their charges, electronic structures, and the number of electrons they've lost compared to their neutral atom:
| Name | Formula | Charge | Electronic structure | Number of electrons lost |
| Lithium | Li+ | +1 | 2, 0 | 1 |
| Sodium | Na+ | +1 | 2, 8, 0 | 1 |
| Potassium | K+ | +1 | 2, 8, 8, 0 | 1 |
| Magnesium | Mg2+ | +2 | 2, 8, 0 | 2 |
| Calcium | Ca2+ | +2 | 2, 8, 8, 0 | 2 |
| Aluminium | Al3+ | +3 | 2, 8, 0 | 3 |
| Copper(II) | Cu2+ | +2 | * | 2 |
| Iron(II) | Fe2+ | +2 | * | 2 |
| Iron(III) | Fe3+ | +3 | * | 3 |
Copper and iron are both transition metals. As a result, they have slightly more complicated electronic structures. You'll find out more about the arrangement of electrons in transition metals if you study chemistry at A-level; the article Electron Configuration should help you. But for a refresher on the basics of electron arrangement in ions and atoms, visit Electronic Structures.
Metal ions and ionic compounds
Metal ions aren't usually found on their own. Instead, they bond with negative ions to form an ionic compound. For example, the lithium ions in red fireworks are often bonded to carbonate ions (CO3-). However, we can separate the ions in ionic compounds by dissolving them in water. This forms an aqueous solution.
If you are confused about the difference between atoms and ions, we recommend checking out Ions and Isotopes. Likewise, for more about ionic bonding and compounds, head over to the two articles with the relevant names: Ionic Bonding and Ionic Compounds.
Tests for positive metal ions
Now we know what metal ions are, we can look at how you test for them. There are two main tests for positive metal ions that you need to know about for your GCSE exam:
- Flame tests.
- The sodium hydroxide test.
We'll first look at flame tests. Certain metal ions burn with bright colours when you hold them in a Bunsen burner flame, and this is one way of identifying them. You'll learn about this experiment's method and the results that you should expect to see.
We'll then introduce you to the sodium hydroxide test. Certain metal ions form a hydroxide precipitate when mixed with sodium hydroxide (NaOH). The precipitate can also be used to identify the unknown metal ion. Once again, you'll find out about the method, alongside the results.
Test for metal ions: flame test
If you dip a metal rod in a solution containing a metal ion, the rod becomes covered in the ion. You can then try holding the damp end of the rod in the flame of a Bunsen burner. Some metal ions cause the flame to change colour. As a result, these so-called flame tests are a handy way of testing for metal ions.
Which colour would you like to see - perhaps a brilliant red, or maybe a sunny yellow? If your solution contains a mixture of different metal ions, you could even produce a rainbow of hues! However, it is more likely that the colour of one ion will mask the colours of the others. This means that flame tests aren't the best way of analysing mixtures.
Method
Here's how you carry out flame tests to identify unknown metal ions:
- Dip a wire loop into concentrated hydrochloric acid (HCl) then hold it in a blue Bunsen burner flame. This cleans the loop.
- Dip the loop in the concentrated acid a second time before dipping it into a solution containing an unknown metal ion. Alternatively, you can dip the loop into a powdered ionic solid.
- Hold the loop in the Bunsen burner flame once again. Note the appearance of the flame. Does it stay blue, or does it turn a different colour?
- Continue holding the loop in the flame until the flame burns clear. Then, repeat the process with a different metal ion solution or ionic solid. Make sure to clean the loop in between each test, as you did in step 1.
Fig. 1: A diagram showing the process of a flame test for metal ions.
Ideally, the wire loop should be made from platinum because this metal doesn't affect the colour of the flame. In addition, it is stable under high temperatures. However, platinum is generally too expensive to be used in schools, so nichrome (a mixture of nickel and chromium) is favoured instead.
Results
You should find that some metal ions burn with bright colours. However, some have no visible effect on the flame at all. With any luck, you'll have the following results:
| Metal ion | Flame colour |
| Lithium (Li+) | Crimson |
| Sodium (Na+) | Yellow |
| Potassium (K+) | Lilac |
| Calcium (Ca2+) | Orange-red |
| Copper (Cu+) | Green |
Here's a visual guide to the flame colours produced when burning the metals in the table above:
Fig. 2: The flame colours produced by various metal ions in flame tests..
The chemistry behind flame tests
If you've read the article Atomic Structure and the Periodic Table, you'll know that all atoms and (almost all) ions contain electrons, found in shells orbiting the nucleus. In Electronic Structures, you expand on this knowledge to learn that different electron shells have different energy levels. Those closer to the nucleus are lower in energy, whereas those further from the nucleus are higher in energy. These different energy levels are responsible for the brilliant colours you see in flame tests.
When you burn certain metals or metal ions, some of their electrons jump from one shell to a second. The second shell has a higher energy level.
When an electron jumps to a higher energy shell, we say that it is excited. But excited electrons make the metal unstable, and so the electron quickly returns to its starting shell. As it falls back down, it releases energy in the form of visible light. The colour of the light is related to the difference between the energy levels of the two shells. Because metals have different electronic structures, their electrons jump to different shells, and so this means that different metals burn with different coloured flames.
Test for metal ions: the sodium hydroxide test and hydroxide precipitates
We've finished looking at flame tests - that's the first way of identifying metal ions covered. However, there is a common alternative test for metal ions. It is known as the sodium hydroxide test and works by forming a metal hydroxide precipitate. You see, some metal hydroxide precipitates are completely insoluble. Others are soluble, but only in an excess of sodium hydroxide. In addition, some are white, whilst others are coloured. The hydroxide precipitate helps us to identify the metal ion.
Method
To carry out the sodium hydroxide test:
- Pour approximately 10 cm3 of a metal cation solution into a test tube.
- Add a few drops of sodium hydroxide solution (NaOH) to the test tube using a pipette.
- Observe any changes, noting down the results.
- Gradually add more sodium hydroxide until it is in excess, once again noting any observations.
- Repeat the experiment with different metal cation solutions.
Results
Like with the flame test, you should find that some metal ions give an obvious result in this experiment. On the other hand, others produce no visible change whatsoever. Here are the results that you should expect to see when you carry out the sodium hydroxide test for metal ions:
Ion present | Observation |
Small volume of NaOH | Excess NaOH |
| Ca2+ | White precipitate | Insoluble |
| Mg2+ | White precipitate | Insoluble |
| Al3+ | White precipitate | Precipitate dissolves |
| Cu2+ | Blue precipitate | Insoluble |
| Fe2+ | Green precipitate that slowly turns brown ('Dirty' brown) | Insoluble |
| Fe3+ | Red-brown precipitate | Insoluble |
We cover the sodium hydroxide test in much more detail in another article. There, you'll discover the chemistry behind the test and learn how to write balanced chemical equations for the reactions that take place. We also give you a handy flow chart that summarises the results. Head over to Sodium Hydroxide Test for more.
Test for negative ions
Flame tests and the sodium hydroxide test both help identify positive metal cations. However, they aren't good at testing for negative ions, known as anions. Instead, we use other tests:
We test for carbonate ions (CO32-) using hydrochloric acid (HCl).
We identify sulfate ions (SO42-) using hydrochloric acid (HCl) and barium chloride solution (BaCl2).
Finally, we detect halide ions using nitric acid (HNO3) and silver nitrate solution (AgNO3).
Visit Test for Anions for all the information you need to know about these three tests, including the methods, results, and chemical explanation.
Test for metal ions: pros and cons
Finally, let's consider the pros and cons of tests for metal ions. You might have already thought of some advantages and disadvantages of the tests whilst reading this article. If not, try to think of some now - critiquing chemical processes and techniques is a useful skill. For example, do flame tests work for all metals? Are they easy to carry out, or do they require specialist equipment? Furthermore, do the results tell you anything about the amount of metal ion present?
Well, both flame tests and the sodium hydroxide test are examples of simple chemical tests. They don't require any specialist equipment and are cheap and straightforward to carry out in any lab. However, they have their downsides:
- Chemical tests for metal ions are not as accurate, sensitive, or precise as instrumental methods of chemical analysis, such as flame emission spectroscopy. They are much slower, too.
- Chemical tests also don't tell you about the concentration or amount of metal ions present. As a result, these tests are better used qualitatively, not quantitatively.
- In addition, flame tests and the sodium hydroxide test can't be used with trace samples.
- They can also both give similar results for multiple different cations. Plus, not all metals give visible changes in these two tests. This means you need to carry out further tests to determine their identity.
- For example, lithium isn't the only metal that burns with a red flame. Metals such as strontium do too, and it can be tricky to tell the different hues apart.
- Likewise, in the sodium hydroxide test, Ca2+, Mg2+, and Al3+ ions all produce a white precipitate.
- Finally, they can't be used for mixtures of metal ions.
Above, we mention instrumental methods of chemical analysis. These techniques are much more sensitive and accurate than basic chemical tests, but often require expensive machinery and specialist training. Learn all about them in Instrumental Analysis.
Test for Metal Ions - Key takeaways
- Metal ions are positively charged particles formed when a metal atom loses one or more electrons.
- Examples of metal ions include Li+, K+, Mg2+, and Al3+. Because they are positively charged, all metal ions are cations.
- We can test for positive metal ions using flame tests and the sodium hydroxide test:
- Adding certain metal ions to a flame turns the flame a different colour.
- Adding sodium hydroxide to certain metal ions forms a solid hydroxide precipitate. The precipitates differ in colour and solubility.
- Like all chemical tests, tests for metal ions are cheap and simple to carry out. However, they are less accurate and less sensitive than instrumental methods of chemical analysis.
References
- Helmenstine, Anne Marie, Ph.D, 'The Chemistry of Firework Colors.' ThoughtCo (16/02/2021)
Explanations 
Exams 
Magazine 
