IGCSE Chemistry 0620 · Common mistakes
Common mistakes with ion tests, gas tests and salts
Updated · by the Fahmazing team
Tests and salt preparation turn up on the theory paper and the practical paper. The chemistry is simple; the marks are lost on exact colours, the right reagent and the order of steps.
Mixing up iron(II) and iron(III)
With aqueous sodium hydroxide, iron(II) gives a green precipitate and iron(III) gives a red-brown one. Copper(II) gives light blue.
A solution gives a red-brown precipitate with sodium hydroxide. Which ion is present?
✗ Wrong: Iron(II), Fe²⁺
✓ Right: Iron(III), Fe³⁺
Free lesson: Identification of ions and gases
Stopping after the sodium hydroxide test
Aluminium and zinc ions both give a white precipitate that dissolves in excess sodium hydroxide. Aqueous ammonia tells them apart: the zinc precipitate dissolves in excess ammonia, the aluminium one does not.
Both give a white precipitate that dissolves in excess NaOH. How do you tell Al³⁺ from Zn²⁺?
✗ Wrong: Add more sodium hydroxide.
✓ Right: Add aqueous ammonia in excess. Zn²⁺: the precipitate dissolves. Al³⁺: it stays.
Free lesson: Identification of ions and gases
Using the wrong acid for halide tests
Acidify with dilute nitric acid before adding silver nitrate. Hydrochloric acid would add its own chloride ions and give a false result.
Testing for chloride ions
✗ Wrong: Add dilute hydrochloric acid, then silver nitrate.
✓ Right: Add dilute nitric acid, then aqueous silver nitrate: a white precipitate shows chloride.
Free lesson: Identification of ions and gases
Confusing which white precipitate means what
A white precipitate with silver nitrate means chloride. A white precipitate with barium nitrate means sulfate. Check which reagent was used. Bromide is cream and iodide is yellow with silver nitrate.
Acidified barium nitrate gives a white precipitate.
✗ Wrong: Chloride ions are present.
✓ Right: Sulfate ions are present.
Free lesson: Identification of ions and gases
Mixing up the splints
Lighted splint: hydrogen gives a squeaky pop. Glowing splint: oxygen relights it. They are two different tests.
Test for oxygen
✗ Wrong: Put a lighted splint in; it pops.
✓ Right: Put a glowing splint in; it relights.
Free lesson: Tests for gases
Giving the conclusion, not the observation
Observations are what you see. "Carbon dioxide is made" is a conclusion.
What do you see when dilute acid is added to a carbonate?
✗ Wrong: Carbon dioxide is given off.
✓ Right: Fizzing (effervescence). The gas turns limewater milky.
Free lesson: Tests for gases
Heating a salt solution to dryness
To make crystals of a soluble salt, heat the filtrate only to the crystallising point, then let it cool. Heating to dryness can break down the crystals.
Making copper(II) sulfate crystals from copper(II) oxide and sulfuric acid
✗ Wrong: Add excess oxide, filter, then evaporate all the water.
✓ Right: Add excess oxide, filter, heat the filtrate to the crystallising point, cool, then filter off and dry the crystals.
Free lesson: Preparation of salts
Keeping the wrong part after filtering
When making a soluble salt from an insoluble solid, the unreacted solid is the residue. The salt you want is in the filtrate.
After filtering off excess zinc oxide, where is the zinc sulfate?
✗ Wrong: In the residue on the filter paper
✓ Right: Dissolved in the filtrate
Free lesson: Preparation of salts
Using precipitation for a soluble salt
Precipitation only makes insoluble salts. Remember the exceptions: silver and lead chlorides, and barium, calcium and lead sulfates, do not dissolve.
How would you make barium sulfate?
✗ Wrong: React barium carbonate with excess sulfuric acid, filter and crystallise.
✓ Right: Mix solutions of a soluble barium salt and a soluble sulfate. Filter, wash and dry the precipitate.
Free lesson: Preparation of salts
Questions students ask
Why is nitric acid used in the halide test?
It removes carbonate ions that could also give a precipitate, and it adds no halide ions of its own, unlike hydrochloric acid.
How do I test for ammonium ions?
Warm the substance with aqueous sodium hydroxide. Ammonia is given off, which turns damp red litmus paper blue.
What flame colours do I need for IGCSE Chemistry?
Lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green and copper(II) blue-green.