RP4 - Testing for cations and anions

An overview of RP4 - Testing for cations and anions

7 min

Use a fume cupboard: Always perform tests involving ammonia gas and the reaction of halides with sulfuric acid in a fume cupboard to avoid inhaling harmful gases.
Wear gloves: As well as goggles, wear gloves when handling concentrated sulfuric acid, as it is corrosive.
Dispose of properly: Dispose of all chemicals in the designated waste containers, especially sulfuric acid and ammonium solutions. Do not pour them down the drain.
Protect skin and eyes: In case of skin contact, rinse with water immediately. For eye contact, flush with water for at least 15 minutes and seek medical help.

Experimental method for AQA Required practical 4: Testing for cations and anions

Part 1 observations adding sodium hydroxide to group 2 compounds

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 4 - PART 1. 1. Add 10 drops of a Group 2 halide solution into a clean test tube. Halide salts are typically soluble and provide a good source of Group 2 ions. Group 2 in aqueous solution. 2. Add 10 drops of NaOH and record any precipitate formed. Add slowly to observe the initial reaction accurately. 10 drops of dilute NaOH (aq). 3. Continue adding NaOH dropwise until in excess and note changes. Dropwise addition helps distinguish between soluble and insoluble hydroxides. 4. Repeat for each solution and note the comparison. Use consistent volumes / concentrations of solutions to compare observations fairly. Mg(OH)2 Solubility increases Ca(OH)2 Sr(OH)2 Alkalinity increases Ba(OH)2.

Experimental method for AQA Required practical 4: Testing for cations and anions

Part 2 observations adding sulfuric acid to Group 2 compounds.

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 4 - PART 2. 1. Add 10 drops of a Group 2 halide solution into a clean test tube. Halide salts are typically soluble and provide a good source of Group 2 ions. 2. Add 10 drops of dilute sulfuric acid and observe for a white precipitate. Precipitate forms when the Group 2 sulfate is insoluble (e.g. BaSO4). 3. Record which ions form a precipitate and which remain clear: Mg2+: no visible change (MgSO4 is soluble), Ca2+: slight white precipitate (CaSO4 is sparingly soluble), Ba2+: thick white precipitate (BaSO4 is insoluble). This shows the trend in decreasing solubility of Group 2 sulfates down the group. Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2. Solubility decreases.

Experimental method for AQA Required practical 4: Testing for cations and anions

Part 3 testing for ammonium ions.

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 4 - PART 3. 1. Add 10 drops of the ammonium containing test solution to a clean test tube. Solution containing ammonium. 2. Add 10 drops of NaOH and swirl gently to mix. Swirling ensures thorough mixing and even reaction. 10 drops of dilute NaOH (aq). 3. Warm gently in a water bath. Avoid direct flame to prevent splashing and uneven heating. 4. Hold damp red litmus paper near the mouth of the tube – a blue colour confirms ammonia. Ensure the paper is damp to detect the alkaline gas.

Experimental method for AQA Required practical 4: Testing for cations and anions

Part 4 testing for halide ions.

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 4 - PART 4. 1. Add the halide containing test solution to a clean test tube. 2. Add a few drops of dilute nitric acid. This removes carbonate or hydroxide impurities. 3. Add a few drops of silver nitrate and observe the colour of any precipitate: • White: AgCl • Cream: AgBr • Yellow: AgI. 4. Add dilute ammonia solution to the precipitate and observe solubility: • AgCl: precipitate dissolves in dilute NH3 • AgBr: precipitate dissolves in concentrated NH3 • AgI: does not dissolve in ammonia.

Experimental method for AQA Required practical 4: Testing for cations and anions

Part 5 testing for sulfate ions.

Experimental method AQA required practical 4 - part 5. 1. Add the sulfate containing test solution to a clean test tube. Metal sulfate solution. 2. Add a few drops of dilute HCl followed by a few drops of BaCl2(aq). The HCl removes carbonate ions that could form a white precipitate. Few drops of HCl followed by BaCl2(aq). 3. Observe for a white precipitate. Barium sulfate forms only if sulfate ions are present. Milky white precipitate of barium sulfate.

Experimental method for AQA Required practical 4: Testing for cations and anions

Part 6 testing for carbonate ions.

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 4 - PART 6. 1. Add the carbonate containing test solution to a clean test tube. 2. Add dilute HCl and observe for effervescence (CO2 release). Effervescence confirms gas evolution. 3. Bubble the gas through limewater – a cloudy result confirms CO2. Use fresh limewater and bubble gently for clarity.

Experimental method for AQA Required practical 4: Testing for cations and anions

Part 7 testing for halide ions in solid salts.

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 4 - PART 7. 1. Place a small amount of solid halide salt in a dry test tube. Use only a small sample – the reaction is vigorous. 2. Add a few drops of concentrated sulfuric acid down the side of the tube and observe changes. Use in a fume cupboard – toxic gases may be released. 3. Record the observations and identify the gas evolved: NaCl: steamy fumes of HCl which turn damp blue litmus red; NaBr: steamy fumes, orange vapour of Br2; NaI: steamy fumes, purple vapour of I2 and rotten egg smell (H2S).

Experimental method for AQA Required practical 4: Testing for cations and anions

Part 8 testing for hydroxide ions in aqueous solutions.

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 4 - PART 8. 1. Add a small amount of the hydroxide test solution to a clean test tube. NaOH. 2. Dip a strip of red litmus paper into the solution. Use red litmus to confirm alkalinity. 3. Observe the colour change: • If OH- is present, the red litmus turns blue. Confirms alkaline conditions due to hydroxide ions. Alternatively, you can use universal indicator – the solution should turn blue to purple. Gives a more detailed pH range estimate (typically pH 11–14).

Wearing the appropriate personal protective equipment is essential to avoid direct contact with harmful chemicals and to ensure safety throughout the experiment.

A simple illustration of a pair of goggles and gloves.

Wear goggles and gloves when handling any chemicals, particularly concentrated acids, ammonia, and metal salts, as they can be corrosive or irritating to the skin and eyes.

A person wearing no goggles or gloves whilst handling chemicals.

Don't

Skip protective equipment just because ‘it’s only a few drops’.

Ensuring proper ventilation during chemical testing is critical to avoid inhaling harmful fumes, such as ammonia or sulfuric acid vapours, which can irritate the respiratory system.

A simple illustration of a display cabinet featuring a transparent upper section with a blue background and a solid lower section with two doors.

Conduct all tests in a well-ventilated area or under a fume cupboard to avoid inhaling harmful fumes or gases, such as ammonia or sulfuric acid vapours.

A bottle pouring blue liquid into a glass, with steam rising from the glass.

Don't

Perform the tests in an enclosed or poorly ventilated space, as this can lead to the accumulation of dangerous gases.

It is crucial to use clean and dry glassware when performing analytical tests.

Five test tubes arranged in a row, with varying heights and shapes.

Ensure all glassware is clean and dry before use to avoid contamination and ensure accurate results.

A test tube with a white body and brownish stains or markings on its surface.

Don't

Reuse glassware that has not been properly cleaned and dried, as this could lead to inaccurate readings or unintended chemical reactions.

Proper disposal of chemicals is essential to avoid environmental damage and ensure safe laboratory practices. Always follow disposal guidelines.

A diagram showing a chemically compatible container with screw-on cap labeled 'HAZARDOUS WASTE' placed inside a secondary container. There are labels indicating 'Labelling according to the local and national regulations'.

Dispose of chemicals safely after testing by placing them in the designated waste containers. Always follow your teacher’s instructions for disposal.

A funnel pouring blue liquid into a drain.

Don't

Pour chemicals down the drain unless specifically directed to do so, as some chemicals may cause harmful reactions in the plumbing or be harmful to aquatic organisms.

The tables below show the positive test results for all ions present in Required practical 4. These are the observations that you should have noted for each.

Below is a summary of the positive cation tests from Required Practical 4:

A table titled 'Cation test' with the header 'Observations with cation'. Columns list cations: Magnesium, Calcium, Strontium, Barium, Ammonium. Rows list reagents: Sodium hydroxide, Sulfuric acid. Sodium hydroxide with Magnesium: 'Forms a white precipitate', Calcium: 'Forms a white precipitate', Strontium: 'No precipitate blue', Barium: 'No precipitate blue', Ammonium: 'Damp red litmus over a warming test tube will turn blue'. Sulfuric acid with Magnesium: 'No precipitate', Calcium: 'No precipitate', Strontium: 'Forms a white precipitate', Barium: 'Forms a white precipitate'.

Below is a summary of all halide anion tests from Required Practical 4:

A table with two columns labeled 'Anion test' and 'Observations with halide ion,' which is further divided into three sub-columns: 'Chloride,' 'Bromide,' and 'Iodide.' The first row under 'Anion test' states, 'Nitric acid and silver nitrate followed by dilute ammonia.' Under 'Chloride,' it reads, 'White precipitate is formed. Precipitate dissolves in ammonia.' Under 'Bromide,' it reads, 'Cream precipitate is formed. Precipitate dissolves in ammonia.' Under 'Iodide,' it reads, 'Yellow precipitate is formed. No change with ammonia.' The second row under 'Anion test' states, 'Sulfuric acid (When halide is in solid salt).' Under 'Chloride,' it reads, 'White fumes of hydrogen chloride are observed.' Under 'Bromide,' it reads, 'Red-brown bromine gas is observed.' Under 'Iodide,' it reads, 'Forms solid yellow sulfur, purple vapour of iodine and release a strong smell of hydrogen sulfide.'

Below is a summary of all other negative anion tests from Required Practical 4:

A table titled 'Anion test' with columns labeled 'Observations with anions' and rows labeled 'Sulfate', 'Carbonate', and 'Hydroxide'. The first row under 'Anion test' reads 'Dilute HCl and BaCl2'. Under 'Sulfate', it says 'White precipitate is formed.' The 'Carbonate' and 'Hydroxide' columns are empty. The second row under 'Anion test' reads 'HCl followed by limewater test'. Under 'Carbonate', it states 'Effervescence will be observed. Bubbling the gas through limewater will turn it cloudy.' The 'Sulfate' and 'Hydroxide' columns are empty. The third row under 'Anion test' reads 'Damp red litmus paper or universal indicator'. Under 'Hydroxide', it says 'Turns red litmus paper blue. Observe colour change to blue/purple with universal indicator.' The 'Sulfate' and 'Carbonate' columns are empty.

RP4 - Testing for cations and anions

On this page

Experimental aim

Safety

Method

Practical pitfalls

Analysis and evaluation