Home › Subjects › O-Level Pure Chemistry › Qualitative Analysis
O-Level Qualitative Analysis
identifying cations, anions and gases through chemical tests
What the syllabus expects
- With aqueous sodium hydroxide and/or aqueous ammonia, pick out these aqueous cations from any precipitate that appears and whether it later redissolves: aluminium, ammonium (ammonia is given off when warmed), calcium, copper(II), iron(II), iron(III) and zinc.
Scope: Formulae of complex ions are not required. - Describe tests for the anions carbonate (dilute acid, then limewater), chloride and iodide (acidify with nitric acid, then add aqueous silver nitrate), nitrate (add aluminium to the alkaline solution of aqueous sodium hydroxide, reducing it to ammonia, then check with damp red litmus) and sulfate (acidify with nitric acid, then add aqueous barium nitrate).
- Describe how to test for the gases ammonia (damp red litmus paper), carbon dioxide (limewater), chlorine (damp litmus paper), hydrogen (a burning splint), oxygen (a glowing splint) and sulfur dioxide (acidified potassium manganate(VII)).
How it's examined
Questions on this topic most often ask you to state, outline. About 2% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (2 marks)
Iron powder and zinc powder are mixed together and added to an excess of sulfuric acid. Outline a chemical test you could perform on the reacting mixture to demonstrate that only the zinc has reacted. [2] [Total: 5]
Show the worked answer
Both zinc and iron react with sulfuric acid, but the claim is that only zinc has reacted while the iron has not. Zinc reacts to give colourless Zn2+ ions (ZnSO4), whereas if iron had reacted it would give Fe2+ ions (green FeSO4). Test the reacting mixture for iron(II) ions: take a sample and add aqueous sodium hydroxide. If only zinc has reacted, adding NaOH gives a white precipitate (Zn(OH)2, soluble in excess NaOH) and NO green precipitate; the absence of a green precipitate of iron(II) hydroxide shows iron has not gone into solution, confirming only the zinc reacted.
Example 2 (2 marks)
You have S in a boiling tube and solution T in a vial. When heated strongly, S breaks down to give an oxide. Take the stopper out of the boiling tube containing S and record the combined mass of the tube and its contents. Heat S strongly for 3 minutes until nothing further changes; while heating, draw off several gas samples with a teat pipette (avoiding the sides of the tube) and bubble each one through limewater. Allow the boiling tube to cool back to room temperature, then weigh it with its contents again and record the mass. Write down your observations. [2]
Show the worked answer
S is a carbonate that decomposes on heating to an oxide plus carbon dioxide gas. Bubbling the evolved gas through limewater turns the limewater milky/cloudy, confirming carbon dioxide. Because gas is lost from the tube, the combined mass of the tube and its contents is lower after heating than before.
Example 3 (2 marks)
Write a balanced chemical equation, including state symbols, for gas B being bubbled through aqueous calcium hydroxide to produce a white precipitate.
Show the worked answer
A gas that gives a white precipitate when bubbled through aqueous calcium hydroxide (limewater) is carbon dioxide; the white precipitate is insoluble calcium carbonate. The balanced equation with state symbols is: CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l). Atoms balance (1 Ca, 1 C, 3 O on each side plus the water).
More O-Level Pure Chemistry topics
Experimental Chemistry · The Particulate Nature of Matter · Chemical Bonding and Structure · Chemical Calculations · Acid-Base Chemistry · Redox Chemistry · all of O-Level Pure Chemistry