Identification of Ions and GasesCambridge IGCSE Chemistry: Revision notes
Section 1
How do we identify anions using chemical tests?
Anions are negatively charged ions that can be identified using specific reagents and observations. Each anion requires a distinct test procedure:
Carbonate ions (CO₃²⁻)
- Add dilute acid (e.g. dilute hydrochloric acid)
- Observation: Effervescence (fizzing) occurs
- Test the gas produced with limewater: the limewater turns white/cloudy, confirming carbon dioxide (CO₂)
Chloride (Cl⁻), Bromide (Br⁻), and Iodide (I⁻) ions
- Acidify the solution with dilute nitric acid (this removes any interfering ions)
- Add aqueous silver nitrate
- Observations:
- Chloride: white precipitate of silver chloride
- Bromide: cream precipitate of silver bromide
- Iodide: yellow precipitate of silver iodide
- Note: The order is white → cream → yellow in terms of increasing colour
Nitrate ions (NO₃⁻)
- Add aqueous sodium hydroxide
- Add aluminium foil
- Warm the mixture
- Observation: Ammonia gas is produced (characteristic pungent smell; damp red litmus paper turns blue)
- This indicates nitrogen has been reduced from the nitrate ion
Sulfate ions (SO₄²⁻)
- Acidify with dilute nitric acid
- Add aqueous barium nitrate (or barium chloride)
- Observation: White precipitate of barium sulfate forms
- This precipitate is insoluble even in dilute acids
Sulfite ions (SO₃²⁻)
- Add acidified aqueous potassium manganate(VII)
- Observation: The purple colour of the manganate(VII) is discharged (fades to colourless)
- The sulfite is oxidised and the manganate(VII) is reduced, causing decolourisation
Always acidify chloride, bromide and iodide solutions with dilute nitric acid (not hydrochloric acid) before adding silver nitrate – this prevents silver chloride precipitate forming from the acid itself, which would give a false positive.
Test for sulfate: Add dilute nitric acid to the unknown solution, then add barium nitrate solution. A white precipitate forms immediately. This white precipitate is insoluble in dilute acids, confirming sulfate ions are present.
Section 2
What do cations form with sodium hydroxide and ammonia?
Cations are positively charged ions. Two reagents are used to identify them based on precipitation and colour changes:
| Cation | NaOH (aqueous) | NH₃ (aqueous) |
|---|---|---|
| Aluminium (Al³⁺) | White precipitate → dissolves in excess NaOH to form colourless solution | White precipitate (insoluble in excess) |
| Ammonium (NH₄⁺) | No precipitate; ammonia gas produced on heating (damp red litmus turns blue) | No visible reaction at room temperature |
| Calcium (Ca²⁺) | White precipitate (insoluble in excess NaOH) | White precipitate (insoluble in excess) |
| Chromium(III) (Cr³⁺) | Green precipitate → dissolves in excess NaOH to form green solution | Green precipitate (insoluble in excess) |
| Copper(II) (Cu²⁺) | Blue precipitate (insoluble in excess NaOH) | Blue precipitate → dissolves in excess NH₃ to form deep blue solution |
| Iron(II) (Fe²⁺) | Green/white precipitate that turns brown on standing (oxidised by air) | Green/white precipitate (turns brown) |
| Iron(III) (Fe³⁺) | Red/brown precipitate (insoluble in excess NaOH) | Red/brown precipitate (insoluble in excess) |
| Zinc (Zn²⁺) | White precipitate → dissolves in excess NaOH to form colourless solution | White precipitate → dissolves in excess NH₃ to form colourless solution |
Key observation principle: When NaOH or NH₃ is added dropwise, precipitates form. When excess reagent is added, some cations' precipitates dissolve whilst others remain insoluble. This difference helps distinguish between cations.
The colour and behaviour in excess of each reagent are critical. Always describe what happens in both limited and excess NaOH/NH₃ – this is what examiners mark. Write 'white precipitate forms' first, then describe whether it dissolves in excess.
Students often confuse copper(II) with iron(III). Remember: copper(II) is blue and dissolves in excess ammonia (turning deep blue); iron(III) is red/brown and does NOT dissolve in excess ammonia.
Think of NaOH and NH₃ as 'selective filters': some cations' precipitates dissolve in excess (amphoteric hydroxides like Al(OH)₃ and Zn(OH)₂), while others stay solid. Copper(II) with ammonia is unique – it redissolves due to forming a complex ion.
Section 3
How are gases identified using simple chemical tests?
Gas identification relies on characteristic observations when specific reagents contact the gas:
| Gas | Test | Positive Result |
|---|---|---|
| Ammonia (NH₃) | Hold damp red litmus paper near the gas source | Red litmus paper turns blue (alkaline gas); pungent smell |
| Carbon dioxide (CO₂) | Bubble gas through limewater | Limewater turns white/cloudy (calcium carbonate precipitate forms) |
| Chlorine (Cl₂) | Hold damp litmus paper near the gas | Litmus paper is bleached (turns white); acrid smell |
| Hydrogen (H₂) | Insert lighted splint into gas | 'Pop' sound as hydrogen burns in oxygen |
| Oxygen (O₂) | Insert glowing splint into gas | Glowing splint reignites/burns brightly |
| Sulphur dioxide (SO₂) | Add acidified aqueous potassium manganate(VII) | Purple colour is discharged (fades to colourless) |
Important procedural notes:
- For ammonia and chlorine, the damp litmus paper must be held near (not in) the gas stream to avoid contamination
- For hydrogen and oxygen tests, use a burning or glowing splint introduced into a test tube containing the gas
- Gas tests must be performed safely; never directly inhale gases except to detect smell from a distance
Examiners mark both the test procedure and the observation. Write 'damp red litmus paper turns blue' for ammonia, not just 'litmus turns blue' – the initial colour of the litmus and the type of damp paper matter.
Test for hydrogen: Collect hydrogen gas in a test tube by displacement of water. Remove the test tube and hold a lighted splint to the mouth of the tube. A characteristic 'pop' sound is heard as hydrogen combusts with oxygen in air (2H₂ + O₂ → 2H₂O).
Section 4
How does flame testing identify metal cations?
Flame testing is a rapid, simple test for identifying certain metal cations based on the colour they produce in a flame. The metal ions absorb heat energy and emit light at characteristic wavelengths, producing distinctive flame colours.
Procedure:
- Dip a clean nichrome or platinum wire into dilute hydrochloric acid (to clean it)
- Heat the wire in the hottest part of a Bunsen flame until colourless
- Dip the clean wire into the solution containing the unknown cation
- Hold the wire in the hottest part of the flame (usually the blue region)
- Observe the colour of the flame
- Repeat the procedure to confirm the colour (metal ions may leave traces)
Flame colours for each cation:
| Cation | Flame Colour |
|---|---|
| Lithium (Li⁺) | Red |
| Sodium (Na⁺) | Yellow |
| Potassium (K⁺) | Lilac (purple) |
| Calcium (Ca²⁺) | Orange-red |
| Barium (Ba²⁺) | Light green |
| Copper(II) (Cu²⁺) | Blue-green |
Important practical notes:
- Sodium contamination is common in laboratory glassware; even trace amounts produce a strong yellow flame. Always use a clean wire and clean it thoroughly between tests
- If a strong yellow flame appears, it may mask other colours; wait for the yellow to fade and observe for other colours
- Potassium (lilac) can be difficult to see if sodium is present; look for the lilac colour in the cooler parts of the flame
- Some ions (e.g. magnesium, zinc) do not produce distinctive flame colours and require other identification methods
Examiners expect you to state exactly which colour you observed, not just 'coloured flame'. For example, write 'orange-red flame' for calcium, not just 'red'. Also mention that you cleaned the wire in dilute acid and heated it until colourless first – this shows good practical understanding.
Students often confuse lithium (red) with calcium (orange-red). Remember: lithium is pure red, while calcium is orange-red (more orange than red). Also, don't confuse potassium (lilac) with sodium (yellow) – they are very different colours.
Section 5
What is the systematic approach to identifying unknown ions in solutions?
When faced with an unknown solution containing one or more ions, a systematic approach maximises your chances of correct identification:
Step 1: Visual observations
- Note any colour (e.g. blue suggests Cu²⁺, green suggests Fe²⁺ or Cr³⁺, brown suggests Fe³⁺)
- Check for cloudiness or precipitates already present
Step 2: Test for gases by adding dilute acid
- If effervescence occurs, suspect carbonate (CO₃²⁻)
- Collect the gas and test with limewater for CO₂ confirmation
Step 3: Add aqueous sodium hydroxide in stages
- Add a few drops and observe any precipitate forming and its colour
- Add excess NaOH and note whether the precipitate dissolves
- This identifies most cations (Al³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Zn²⁺, Ca²⁺, Cr³⁺, NH₄⁺)
Step 4: If NaOH test is unclear, add aqueous ammonia
- Add ammonia in stages and observe precipitate behaviour
- Some cations give different results with ammonia (e.g. Cu²⁺ precipitate dissolves in excess)
- This provides additional confirmation
Step 5: For anions, add relevant reagents
- Test for halides (Cl⁻, Br⁻, I⁻): acidify with dilute HNO₃, add AgNO₃, observe precipitate colour
- Test for sulfate (SO₄²⁻): acidify with dilute HNO₃, add Ba(NO₃)₂, observe white precipitate
- Test for sulfite (SO₃²⁻): add acidified manganate(VII) solution, observe decolourisation
- Test for nitrate (NO₃⁻): add NaOH and Al foil, warm, test for ammonia
Step 6: Use flame testing if cations remain unclear
- Clean the nichrome wire thoroughly
- Perform flame test on the original solution to identify Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺, or Cu²⁺
Record observations methodically: Always write down what you observe (colour, precipitate form, etc.) and what this indicates about the identity of the ion.
In exam questions, examiners want to see your reasoning. Write 'A blue precipitate forms when NaOH is added, which is insoluble in excess NaOH, indicating copper(II) ions' – don't just state 'Cu²⁺ present'. Show your thought process by linking observations to conclusions.
Unknown solution procedure: The solution is colourless. Adding dilute acid produces no gas. Adding NaOH produces a white precipitate that dissolves in excess NaOH. Adding ammonia produces a white precipitate that dissolves in excess ammonia to form a colourless solution. This indicates zinc ions (Zn²⁺) – the key distinguishing feature is that both precipitates dissolve in excess reagents.
Must Know
- Anion tests: Carbonate + dilute acid → CO₂ gas (test with limewater = white/cloudy); Halides + dilute HNO₃ + AgNO₃ → white/cream/yellow precipitate; Nitrate + NaOH + Al foil + heat → ammonia (damp red litmus turns blue); Sulfate + dilute HNO₃ + Ba(NO₃)₂ → white precipitate; Sulfite + acidified manganate(VII) → purple colour discharged
- Cation tests with NaOH: Al³⁺ (white ppt dissolves in excess), Cu²⁺ (blue ppt insoluble in excess), Fe³⁺ (red-brown ppt insoluble), Fe²⁺ (green/white ppt turning brown), Zn²⁺ (white ppt dissolves in excess), Ca²⁺ (white ppt insoluble), Cr³⁺ (green ppt), NH₄⁺ (gas on heating)
- Cation tests with ammonia: Cu²⁺ blue ppt dissolves in excess to form deep blue solution (unique feature); Zn²⁺ white ppt dissolves in excess; others either insoluble or no reaction
- Gas tests: NH₃ (damp red litmus → blue), CO₂ (limewater → white/cloudy), Cl₂ (damp litmus → bleached/white), H₂ (lighted splint → pop), O₂ (glowing splint → reignites), SO₂ (acidified manganate(VII) → decolourised)
- Flame colours: Li⁺ (red), Na⁺ (yellow), K⁺ (lilac), Ca²⁺ (orange-red), Ba²⁺ (light green), Cu²⁺ (blue-green) – always clean the wire first and note exact colour
- Systematic approach: Observe colour → test for gases with acid → add NaOH/ammonia in stages → test anions with specific reagents → use flame test if needed – always link observations to conclusions in exam answers
That's the notes covered.
Carry on to the next subtopic.