Ions are atoms, or a group of atoms, that have either gained or lost one or more electrons. Positively-charged ions are called cations, while negatively-charged ions are called anions. Being able to identify different types of ions is a key part of the GCSE chemistry syllabus, so it’s important to have a good foundation of knowledge.
In this article, we recap some of the basics of ions and include some useful revision tips and sample questions to help you prepare for your GCSE chemistry exam.
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What are ions?
An ion is formed when an atom or group of atoms gains or loses electrons. If an atom(s) loses electrons, it becomes positively charged (a cation). Conversely, when an atom or group of atoms gains electrons, it becomes negatively charged (an anion). These charged particles can either be composed of one or multiple types of atoms.
Ionic compounds, acids, and bases all have constituents that dissociate into anions and cations when they’re dissolved in water. Even pure water exhibits spontaneous ionisation into hydronium and hydroxide ions.
However, ions aren’t just formed through the dissociation of ionic, acidic, or basic compounds in an aqueous solution. They can also form when atoms are subjected to an electrical current (such as in the case of electrolysis) or high energy radiation like ultraviolet rays and X-rays.
You can identify types of ions based on their atomic constituents. Elements produce certain wavelengths of electromagnetic waves when they’re ionised and subjected to particular tests. A flame test is one of the easiest ways to determine which ion is present, as ions produce different colour flames when they’re burned. Some also transition from one colour to another when they react with other substances to form complexes and precipitates.
Here are some examples of ions you may encounter in your GCSE chemistry studies:
- Al+3 aluminium ion – colourless
- Cr+3 chromium ion – green
- Fe+2 iron ion – orange red
- Fe+3 iron ion – yellow green
- K+ potassium ion – colourless
- Li+ lithium ion – colourless
- Mg magnesium ion – colourless
- Na+ sodium ion – colourless
- Mn+2 manganese ion – pink orange-brown
- Pb+2 lead ion – colourless
- Pb+3 lead ion – blue-green
- Pb+4 lead ion – colourless
- Ti+4 titanium ion – white
Techniques for identifying ions
Flame test
As we’ve already mentioned, the flame test is probably the simplest technique you can use to identify different types of ions. The test can easily be performed in a school chemistry laboratory using a Bunsen burner and a sample compound.
To carry out a flame test, set your Bunsen burner to a high heat by adjusting the air regulator. Next, put your sample compound into a test loop to hold it in place. Light the burner and moderate the flame. Subject the test sample to heat and wait for a few seconds until a distinctive colour is produced by the flame as the sample burns. Refer to a table of colours to determine the specific type of ion that’s present (see our list of common examples above).
Precipitation reactions
Certain ionic compounds form complexes and precipitate when they react with acids, bases, and other chemicals. When a negatively charged ion reacts with a positively charged metal ion in an ionic solution, the two form a salt that can be distinguished according to the colour of the precipitate.
This test is effective for determining the type of cations or negatively charged ions for a known anion. For instance, the anion SO42- can be used to distinguish between a compound that contains Ba2+ and one that contains Cu2+. The reactions form BaSO4(s) and CuSO4(aq) respectively (the former is white and the latter is blue).
Using litmus and universal indicator
Litmus paper and a universal indicator are both used to test the acidity and basicity of an aqueous solution. Specifically, they test for the presence of hydrogen (also hydronium) ions or hydroxide ions. The presence of hydrogen ions makes a solution acidic, causing blue litmus paper to turn red/ pink. On the other hand, the presence of hydroxide ions makes the solution alkaline or basic. These types of solutions turn red litmus paper blue.
A universal indicator tends to be more precise than litmus paper. It can change to different colours based on the pH level of the solution (within certain limits). A universal indicator is a solution mixture that’s mainly composed of thymol blue, methyl red, bromothymol blue, and phenolphthalein.
Solubility rules
The solubility of ionic compounds in water follows certain general patterns. When a compound is insoluble in water, it becomes a precipitate. Some of the rules summarised below may appear to contradict each other. If this is the case, then the preceding rule takes precedence.
- Salt compounds that contain Group I elements are generally soluble in water, although there are a few exceptions to this rule. Salts that contain ammonium ions (NH4+) are also soluble in water.
- Salt compounds that have nitrate ions (NO3–) are mostly soluble.
- Generally, salts that have chlorine, bromine, or iodine ions are soluble. However, there are some important exceptions. These include halide salts of Ag+, Pb2+, and (Hg2)2+.
- Salts formed with silver generally aren’t soluble in water and they form precipitates. There are a few exceptions, such as silver nitrate [AgNO3] and silver acetate [Ag(C2H3O2)].
- Sulphate salts are mostly soluble, except calcium sulphate (CaSO4), barium sulphate (BaSO4), lead sulphate (PbSO4), silver sulphate (Ag2SO4), and strontium sulphate (SrSO4).
- The majority of hydroxide salts, including those of Group II elements, are only slightly soluble in water. The main exceptions to this are the hydroxide salts of Group I elements, which are very soluble, and the hydroxide salts of transition metals and Al3+ (these aren’t soluble).
- Transition metal sulphides such as CdS, FeS, ZnS, and Ag2S are highly insoluble, as are the sulphides of arsenic, antimony, bismuth, and lead.
- Generally, carbonate compounds are insoluble. This includes Group II carbonates, as well as FeCO3 and PbCO3.
- Typically, chromates like PbCrO4 and BaCrO4 are not soluble.
- In most cases, phosphates such as Ca3(PO4)2 and Ag3PO4 aren’t soluble.
- Generally, fluorides are not soluble.
Revision tips for studying ions in GCSE chemistry
As with any subject, chemistry revision is about strengthening your understanding of the fundamental principles and concepts. There’s no ‘best’ way to revise and what works for one person might not be suited to another.
However, here are some general tips you might find useful when preparing for your GCSE chemistry exam:
- Create a revision schedule – think about when and where you’ll study – and for how long. Make sure you give yourself plenty of time and are realistic about what you can achieve
- Customise your notes – you might want to create flashcards, experiment with colour coding or incorporate other visual elements – whatever works best for you. Try and rewrite information in your own words rather than simply copying from a textbook
- Get a study buddy – ask a friend or family member to be your study buddy. This can help break the monotony of revising solo – just be careful not to get distracted!
- Take regular short breaks – studying for hours on end without a break will leave you tired and stressed. A good rule of thumb is to take a 15 or 30-minute break after one hour of revision
- Look at past exam papers – study previous GCSE chemistry exams to get an idea of the types of ion-related questions that might come up
Refer to our article, Top Five Tips For Revising GCSE Chemistry, for more information and advice.
Sample questions & answers
Answering practice GCSE chemistry questions about ions can help you improve your knowledge and understanding of the topic. Here are some examples of common GCSE chemistry questions about ions that you may encounter in the exam.
Examples of multiple choice questions
- What colour precipitate does Calcium (Ca2+) form when mixed with NaOH?
- White
- Blue
- Green
- Brown
- White at first, but then redissolves with excess NaOH to form a colourless solution
- A metal atom becomes an ion when it ____________ the electrons in its outer shell.
- Loses
- Gains
- Li+
- Mg2+
Example of short answer questions
Question: When iron reacts with chlorine it forms iron chloride, or FeCl3. Iron atoms lose three electrons each to form ions. In this case, what would be the charge on the ions?
Example of practical/application questions
The flame test is a quick and easy way to identify certain types of metal ions in a sample. It’s a good analytical method that you can use to determine the presence of certain metals. How can you distinguish between copper and barium?
Additional resources
Here are some additional resources you might find helpful when recapping your knowledge on identifying ions:
- LibreTexts Chemistry – flame test
- BBC Bitesize – precipitate test
- ThoughtCo – litmus test
- Science Notes – universal indicator test
- Chemistry Talk – solubility rules
- MMERevise – identifying ions
You might also want to read our round-up of the top GCSE chemistry revision resources here.
Conclusion
In chemistry, an ion is an atom or group of atoms that has a positive or electric charge. The flame test is one of the simplest techniques you can use to identify different ions. If you’re revising for your GCSE chemistry exam, you’ll need to have a clear understanding of the various types of ions and how they’re formed. Look at past exam papers to get an idea of the types of questions you might be asked.
Visit the GCSE chemistry resources section on our website for more help and support.
