Nitrate, sulfate and sulfite: distinguish three similar names
Use the required aluminium/alkali, barium-nitrate and manganate(VII) tests, with gas confirmation and contamination checks.
Section C11.1 · Carbonates
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Read NO₃⁻, SO₄²⁻ and SO₃²⁻ as different ions.
- Recognise ammonia's damp-red-litmus test and the meaning of a precipitate.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which formula represents sulfite rather than sulfate?
What you will learn
- Describe the required nitrate, sulfate and sulfite tests.
- Distinguish precipitate, gas and decolourisation evidence and explain a misleading reagent choice.
Nitrate, sulfate and sulfite comparison reference
Try recalling the result, then open this comparison. Read the test and observation together. Actual testing belongs to an appropriate supervised laboratory method.
Open the comparison reference
Nitrate · NO₃⁻
- Test
- Aqueous sodium hydroxide, then aluminium foil; careful warming in the supervised method.
- Observation
- Ammonia is produced; confirm with damp red litmus turning blue.
Sulfate · SO₄²⁻
- Test
- Dilute nitric acid, then aqueous barium nitrate.
- Observation
- White precipitate.
Sulfite · SO₃²⁻
- Test
- A small volume of acidified aqueous potassium manganate(VII).
- Observation
- Purple reagent becomes colourless.
Use the supplied candidate set and conditions. One observation does not establish every ion in a mixture or certify purity.
Nitrate gives an identified gas in the stated reduction test
For nitrate NO₃⁻, the required test uses aqueous sodium hydroxide, then aluminium foil, with careful warming in the supervised method. Ammonia is produced in the stated positive test and turns damp red litmus paper blue. Aluminium and alkali are part of the required nitrate test; adding silver nitrate to the sample does not replace them.
Do not identify ammonia by deliberately smelling an unknown gas. Use the specified litmus observation in the paper exercise and appropriate laboratory handling in real work. An ammonium-containing sample can produce ammonia on warming with sodium hydroxide alone. If that is a possible candidate, compare a separate suitable portion before aluminium is added rather than automatically assigning every ammonia result to nitrate.
Sulfate precipitates; sulfite decolourises
For sulfate SO₄²⁻, acidify with dilute nitric acid then add aqueous barium nitrate, Ba(NO₃)₂. A white precipitate supports sulfate in the supplied method. Sulfuric acid is unsuitable as this pretreatment because it introduces sulfate itself. The word nitrate in barium nitrate names the reagent; it does not mean the test is for nitrate in the unknown.
For sulfite SO₃²⁻, add a small volume of acidified aqueous potassium manganate(VII), KMnO₄, in the stated test. The purple reagent becomes colourless in the positive observation. Sulfate and sulfite differ in formula and required test. Decolourisation is not the same response as a white precipitate, and an arbitrary purple substance disappearing under unrelated conditions does not identify sulfite without the specified method and candidate context.
Sources for this lesson
Pause and explain
Why would sulfuric-acid pretreatment undermine a sulfate test?
Worked example
Three separate supplied solutions give: ammonia after NaOH/aluminium and warming; a white precipitate after nitric acid/barium nitrate; and purple-to-colourless acidified manganate(VII). Assign the required candidates.
Show the worked solution
- Ammonia confirmed by damp red litmus turning blue supports nitrate in the stated reduction test.
- The acidified barium-nitrate white precipitate supports sulfate.
- The specified manganate(VII) decolourisation supports sulfite.
Answer Nitrate NO₃⁻; sulfate SO₄²⁻; sulfite SO₃²⁻, in the stated order.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the nitrate test and identify the resulting gas using the required paper observation.
Give me a hint
Name the alkali, aluminium and gas confirmation rather than smell.
Compare my reasoning
- The stated test adds aqueous NaOH then aluminium foil and warms carefully in a supervised method.
- A positive nitrate result produces ammonia.
- The gas turns damp red litmus blue; do not use deliberate smelling as identification.
NaOH(aq), aluminium foil and supervised careful warming; ammonia turns damp red litmus blue.
Look for these in your work
- The required test components appear.
- The gas is supported by the specified litmus observation.
Compare sulfate and sulfite formulas, reagents and positive observations.
Give me a hint
Keep the white precipitate distinct from purple-to-colourless reagent.
Compare my reasoning
- Sulfate is SO₄²⁻: dilute nitric acid then aqueous Ba(NO₃)₂ gives white precipitate.
- Sulfite is SO₃²⁻: a small volume of acidified aqueous KMnO₄ changes from purple to colourless.
- Neither test should be inferred merely from a similar-sounding name.
SO₄²⁻: nitric acid/Ba(NO₃)₂, white precipitate. SO₃²⁻: acidified KMnO₄, purple to colourless.
Look for these in your work
- Both formulas and reagent sequences are correct.
- The two types of observation are clearly distinguished.
An unknown gives ammonia when warmed with NaOH before any aluminium is added. A student calls this proof of nitrate. Evaluate it using the required ammonium/nitrate tests.
Give me a hint
Ask whether the gas could come from a different supplied candidate before the full nitrate test.
Compare my reasoning
- Ammonium can release ammonia with NaOH and warming alone.
- The observed gas appeared before the aluminium part of the nitrate reduction test.
- The result supports investigating ammonium and does not by itself prove nitrate; use separate controlled portions and the specified tests.
Ammonia before aluminium is not proof of nitrate; pre-existing ammonium is a relevant alternative in the supplied candidate set.
Look for these in your work
- Timing and reagent sequence inform the inference.
- A possible interfering ion is considered without inventing a confirmed identity.
Common mistakes
- Confusing the nitrate reagent name with the ion identified in the sulfate test.
- Using sulfuric acid before testing for sulfate.
- Swapping sulfate and sulfite formulas or observations.
- Treating any ammonia result as nitrate despite possible pre-existing ammonium.
What can you explain now?
Which required test distinguishes sulfate here?
Compare with the explanation
Dilute nitric acid, then aqueous barium nitrate gives a white precipitate.
The specified pretreatment avoids adding sulfate from the acid itself.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Write the three ion formulas and describe their distinct gas, precipitate and decolourisation tests, including the pretreatment interference.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.