A Level Chemistry Experiment: How To Identify Cations & Anions

As you’ll already know from your GCSE chemistry studies, cations and anions are ions with opposite but complementary charges. 

When an atom loses one or more electrons it becomes a positively charged cation. On the other hand, if an atom gains one or more electrons it becomes a negatively charged anion. 

Being able to identify cations and anions is important in analytical chemistry because it can help rule out certain chemicals or impurities in a sample. It’s also useful in various fields such as manufacturing, forensics, and environmental science. 

If you’re studying A level chemistry, you may be required to perform an experiment to identify cations and anions. In this practical experiment, we’ll be using flame tests and precipitation reactions to do just that.

Understanding cations and anions

Cations and anions are charged particles (ions) that can be formed either through ionic dissociations in an aqueous solution or energy inputs. 

Ionic compounds, acids, bases, and salts all have constituents that naturally dissociate into anions and cations when they’re dissolved in water. Even pure water exhibits spontaneous ionisation into hydronium and hydroxide ions.

In ionic compounds like table salt (sodium chloride), the electron pairs in the outermost shells are captured by just one atom, molecule or functional group. As a result, the electron clouds in ionic compounds are skewed on one side. These are called electrophiles. Examples of electrophiles are the following:

• Hydronium ion (H3O+)
• Boron trifluoride (BF3)
• Aluminium chloride (AlCl3)
• Fluorine (F2)
• Chlorine (Cl2)
• Bromine (Br2)
• Iodine (I2)

Ions can also form when atoms are subjected to an electrical current (such as in the case of electrolysis) or high-energy radiation. For example, ultraviolet rays can knock out electrons from the gases in the atmosphere, causing them to become ionised. 

What is a cation?

A cation is a positively charged ion, i.e. an atom that has lost one or more of its electrons. The positive charge will depend on how many electrons have been lost. When an atom loses electrons from its outermost shells, it’s called electropositivity. 

What is an anion?

An anion is a negatively charged ion, i.e. an atom that has gained one or more electrons. The negative charge will depend on the number of electrons the ion has gained. An atom’s tendency to gain electrons is called electronegativity.

Required materials and safety precautions

For this experiment, we’ll be using the same chemicals for both the flame test and the precipitation test. The chemicals are of known composition. The main purpose is to demonstrate how specific types of ions produce different colours. For the flame test, we can use the following illustration as a reference.

Source

We will be testing for the following ions:

• Anions – OH–, SO42–, CO32–, Cl–, Br–, I–, NO3–
• Cations – H+, Ca2+, Cu2+, Fe3+, Fe2+, NH4+

Equipment:

• Test tubes
• Graduated cylinder
• Volumetric flask or Erlenmeyer flask
• Beakers
• Bunsen burner
• Wooden applicator sticks
• Lighter or match
• Goggles

Chemicals:

• Full-range indicator paper
• Ammonia solution, 2 mol dm–3 
• Sodium hydroxide solution, 0.4 mol dm–3 
• Hydrochloric acid solution, 0.4 mol dm–3
• Barium chloride solution, 0.1 mol dm–3
• Limewater solution, 0.02 mol dm–3
• Nitric acid, 0.4 mol dm–3
• Silver nitrate solution, 0.1 mol dm–3

You will need access to the following chemicals for testing ions:

• Copper chloride
• Potassium carbonate
• Potassium iodide
• Copper(II) sulphate
• Iron(III) chloride
• Iron(II) sulphate
• Lead nitrate
• Magnesium sulphate

It’s important to remember that many of the chemicals you’ll be working with are corrosive and should be handled with care. They also produce noxious fumes that can irritate your respiratory system. 

To avoid potential injuries and poisoning, always wear goggles, rubber gloves, and other protective gear. Use the fume hood when diluting the chemicals.

Starting the cation/anion experiment

Before you perform the experiment, make sure you have the necessary equipment and materials. Ensure your Bunsen burner is connected to the gas supply correctly and there are no leaks.

Step 1: Prepare the solutions

Soak the applicator sticks in distilled water before soaking them in the test tubes. You may use up to three applicator sticks per test tube.

Dilute the solutions or use pre-diluted standard solutions. Calculate the molarity based on the given volume of samples. Use the graduated cylinder and flask to mix the solutions. Pour each solution into a separate test tube.

Step 2: Perform flame tests for cations

Prepare a lighter with an extended nozzle or match. Turn on the gas and ignite the Bunsen burner. Adjust the gas valve and the collar of the burner until the flame becomes blue. One by one, expose the applicator sticks to the flame and observe the colour changes. Record the results in a table.

Step 3: Conduct precipitation reactions for anions

1. Chloride anions – Add a few drops of dilute nitric acid in a test tube with copper chloride solution. Then add a few drops of silver nitrate solution. You’ll get a white precipitate, which is silver chloride. Silver chloride is soluble in ammonia solution.
2. Iodide anions – In a separate test tube with potassium iodide, add a few drops of diluted nitric acid, followed by a few drops of silver nitrate solution. You’ll get a yellow precipitate of silver iodide.
3. Sulphate anions – Add a few drops of barium chloride to a solution of magnesium sulphate. You’ll get a milky precipitate of barium sulphate.

You can experiment with other solutions to observe if there will be precipitates. For reactions that don’t form precipitates, you may also use indicator papers to test the acidity or alkalinity of the solutions.

Observations and results

This experiment is primarily based on qualitative observations. For both the flame test and precipitation test, carefully record your observations in a table. 

Use a periodic table as a reference to determine the charges and reactivity of elements. You might want to take note of these in your observations. 

The results of this practical will largely depend on the chemicals you’ve used. You can record your results in a table like the one below or you may want to create your own version.

Real-world applications of cation and anion identification

Cation and anion identification is a qualitative method that enables certain types of ions to be ruled out or identified based on the colour change observed in flame and precipitation tests. These are simple tests that can easily be performed in a secondary school chemistry laboratory.

However, cation and anion identification isn’t just useful for academic purposes – it also has applications in the real world. For example, these experiments are often used in forensic chemistry to identify unknown substances that may contaminate a crime scene. The test is simple enough to be performed onsite using certain types of reagents to rule out or identify particular substances. 

It’s also very useful in environmental science because it enables experts to identify different pollutants. In the manufacturing industry, cation and anion identification experiments can sometimes form part of the quality control process.

Conclusion

You can identify cations and anions by performing simple techniques such as the flame test and the precipitation test. These methods produce predictable flame colours and precipitates that indicate certain types of ions. Cation and anion identification has a wide range of applications and is often used in sectors like forensics and manufacturing.

Looking for more A level chemistry resources? Check out our revision guides and other articles in our chemistry education hub here.