Chemical Tests for Ions and Gases: Chemistry 0620 (Cambridge O Level / IGCSE)

A stained-glass conservator finds a small tin of pale crystals stored beside some old glass-staining chemicals in a workshop archive. She flame-tests a pinch of the crystals on a clean wire loop and observes a lilac flame.

Which ion does this observation most strongly suggest is present in the crystals?

A small leather-tanning workshop discharges its rinse water into a settling tank before further treatment. Local environmental rules require the workshop to test the rinse water for two ions it commonly contains (chloride ions, from the salt used to cure the hides, and sulfate ions, from the tanning chemicals) before the water can be released for full treatment.

(a) Describe the reagent(s) added and the observation that would confirm sulfate ions are present in a sample of the rinse water. [2]

(b) Describe the reagent(s) added and the observation that would confirm chloride ions are present in a second sample of the rinse water. [2]

(c) For part (b), a technician wants to save time by using dilute hydrochloric acid, instead of dilute nitric acid, to acidify the sample before adding the aqueous silver nitrate. Explain why this would make the chloride test unreliable. [2]

A school science technician finds two unlabelled bottles of colourless solution, P and Q, on a stock shelf. The technician's records show that the two bottles should contain aluminium sulfate solution and zinc sulfate solution, prepared for different classes, but do not say which bottle is which.

(a) The technician adds aqueous sodium hydroxide dropwise, then in excess, to a sample from bottle P. Describe what would be observed, and explain why this result alone cannot show whether bottle P contains aluminium sulfate solution or zinc sulfate solution. [3]

(b) The technician then adds aqueous ammonia dropwise, then in excess, to fresh samples from each bottle.

• In bottle P, the white precipitate formed does not dissolve when excess aqueous ammonia is added.
• In bottle Q, the white precipitate formed dissolves when excess aqueous ammonia is added, giving a colourless solution.

Identify the metal ion present in bottle P and the metal ion present in bottle Q. [2]

(c) Explain why testing with aqueous ammonia, rather than aqueous sodium hydroxide, allows the technician to tell the two solutions apart. [2]

During a stock check, a technician notices a slow gas leak from a cracked valve on a cylinder in an industrial storeroom. The leak is too faint to see any colour, but the escaping gas has a sharp, choking smell, and could be either chlorine gas or sulfur dioxide gas, since cylinders of both are kept in that part of the store. The emergency response is different for the two gases, so staff need a single quick chemical test that would confirm the leaking gas is sulfur dioxide and not chlorine.

Which test and result would confirm this?

A stream-monitoring volunteer group takes a water sample from a stream that runs past a farm. They suspect the sample may contain both carbonate ions, washed from an old lime-spreading track nearby, and nitrate ions, from fertiliser run-off. The group splits the sample into two portions to test for each ion in turn.

(a) Describe the test, including the reagent(s) added and the final observation, that would confirm carbonate ions are present in the first portion. [2]

(b) Describe the test, including the reagent(s) added and the final observation, that would confirm nitrate ions are present in the second portion. [3]

(c) Both tests in (a) and (b) work by detecting a specific gas, rather than by looking directly at the original water sample. Name the gas produced in each test. [2]

A small pottery workshop colours its glazes with metal carbonate powders. A technician finds an unlabelled tub of pale powder that workshop records say should contain either barium carbonate or copper carbonate. She places a small sample on a clean flame-test wire in a blue Bunsen flame and observes a light green colour in the flame.

Which ion does this observation confirm is present in the powder?

An electroplating workshop keeps three reagent bottles of solution for different plating baths: iron(II) sulfate solution, iron(III) chloride solution and copper(II) sulfate solution. After a stock check, the labels are found to be illegible, so a quality-control chemist takes a fresh sample from each bottle.

(a) The chemist adds aqueous sodium hydroxide dropwise, then in excess, to a sample of each solution. State the colour of the precipitate formed with each solution, and state whether each precipitate dissolves in the excess sodium hydroxide. [3]

(b) The chemist then repeats the test using aqueous ammonia instead of sodium hydroxide, adding it dropwise to a fresh sample from each bottle and then in excess. Describe how the result for the copper(II) sulfate solution differs from the results for the other two solutions. [2]

(c) Explain why the excess-ammonia test in (b) gives the chemist a second, independent check that a bottle contains copper(II) sulfate solution, beyond the precipitate colour already seen in (a). [2]

A water-treatment laboratory keeps three stock solutions, potassium chloride, potassium bromide and potassium iodide, for calibrating detection equipment. After a labelling error, three sample bottles are left marked only R, S and T, and a technician must use aqueous silver nitrate to work out which bottle contains which solution.

(a) Describe the test the technician should carry out, including how each sample should be prepared before aqueous silver nitrate is added. [2]

(b) The technician obtains the following results:

• Bottle R: cream precipitate
• Bottle S: white precipitate
• Bottle T: yellow precipitate

Identify the solution in each of bottles R, S and T. [3]

(c) Explain why the technician adds dilute nitric acid to each sample before adding aqueous silver nitrate, rather than testing the samples directly with aqueous silver nitrate. [2]

A school laboratory technician reacts a small piece of magnesium ribbon with dilute sulfuric acid in a boiling tube and collects the gas produced. To check the identity of the gas before disposing of the tube's contents, she holds a lit wooden splint to the mouth of the tube.

Which observation would confirm that the gas collected is hydrogen?

A garden centre stores two unlabelled sacks of white fertiliser granules. Records show one sack should contain ammonium sulfate and the other sodium sulfate, but both fertilisers look identical and dissolve in water to give colourless solutions. A horticultural technician needs a chemical test to work out which sack is which before restocking the shelves.

(a) Describe what would be observed when aqueous sodium hydroxide is added to a solution of each fertiliser and the mixture is warmed gently. Use your answer to explain how the technician could use this test to identify which sack contains the ammonium sulfate. [3]

(b) Suggest why simply smelling the dry granules straight from each sack, without carrying out any chemical test, would not reliably show which sack contains the ammonium sulfate. [1]

(c) State the flame colour that would confirm the presence of sodium ions in a sample from the other sack. [1]