Identification of Ions & Gases (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Chemistry): Flashcards
Exam code: 0654 & 0973
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What reagents are added to test for carbonate ions, and what observation confirms a positive result?
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What reagents are added to test for carbonate ions, and what observation confirms a positive result?
Dilute acid is added to the sample, then the gas produced is bubbled through limewater.
A positive result is confirmed when the limewater turns milky (cloudy), showing carbon dioxide has been produced.
To test for halide ions, the sample is first acidified with dilute ______ acid, then aqueous ______ nitrate is added.
To test for halide ions, the sample is first acidified with dilute nitric acid, then aqueous silver nitrate is added.
What reagents are used to test for nitrate ions, and what observation confirms a positive result?
Aqueous sodium hydroxide and aluminium foil are added to the sample, which is then warmed carefully.
A positive result is confirmed when ammonia gas is produced, turning damp red litmus paper blue.
True or False?
Chloride, bromide and iodide ions all produce a white precipitate when silver nitrate solution is added.
False.
Chloride gives a white precipitate, bromide gives a cream precipitate and iodide gives a yellow precipitate.
Why must hydrochloric acid not be used in the acidification step of the halide ion test?
Hydrochloric acid must not be used because it contains chloride ions, which would react with silver nitrate to form a white precipitate and interfere with the results.
To test for sulfate ions, the sample is acidified with dilute nitric acid and aqueous ______ nitrate is added. A ______ precipitate of barium sulfate confirms the presence of SO42-.
To test for sulfate ions, the sample is acidified with dilute nitric acid and aqueous barium nitrate is added. A white precipitate of barium sulfate confirms the presence of SO42-.
Why must nitric acid be added before barium nitrate in the sulfate ion test?
Nitric acid is added first to remove any carbonate ions that may be present. Carbonates would also form a precipitate with barium nitrate, which would interfere with the results.
How do you test for ammonium ions (NH4+) in solution?
Add aqueous sodium hydroxide solution and warm gently. A positive result is confirmed when ammonia gas is released, turning damp red litmus paper blue.
Adding a few drops of sodium hydroxide to Cu2+ ions produces a ______ blue precipitate, which is ______ in excess sodium hydroxide.
Adding a few drops of sodium hydroxide to Cu2+ ions produces a light blue precipitate, which is insoluble in excess sodium hydroxide.
What colour precipitate forms when sodium hydroxide or ammonia solution is added to iron(II) ions, and what happens to this precipitate on standing?
A green precipitate forms with iron(II) ions (Fe2+).
On standing, the precipitate turns brown near the surface as the iron(II) hydroxide is oxidised.
True or False?
Both calcium ions and zinc ions form a white precipitate with sodium hydroxide, but they can be distinguished because the zinc hydroxide dissolves in excess sodium hydroxide.
True.
Both Ca2+ and Zn2+ form white precipitates with sodium hydroxide, but the zinc hydroxide precipitate dissolves in excess to give a colourless solution, while the calcium hydroxide precipitate does not dissolve.
Why must a blue (roaring) Bunsen flame be used when carrying out a flame test?
A blue Bunsen flame must be used because a yellow (safety) flame would mask the colour produced by the metal ion, making it impossible to identify the cation accurately.
What result is observed when excess aqueous ammonia is added to a solution containing copper(II) ions?
When excess aqueous ammonia is added to Cu2+ ions, the initial light blue precipitate dissolves to form a dark blue solution.
In a flame test, lithium produces a ______ flame, potassium produces a ______ flame and copper produces a ______ flame.
In a flame test, lithium produces a red flame, potassium produces a lilac flame and copper produces a blue-green flame.
What colour precipitate is formed when sodium hydroxide solution is added to iron(III) ions?
A red-brown precipitate is formed when sodium hydroxide solution is added to iron(III) ions (Fe3+). This precipitate is insoluble in excess sodium hydroxide.
What test is used to identify hydrogen gas, and what observation confirms a positive result?
A lighted splint is held at the mouth of the test tube.
A positive result is confirmed by a loud squeaky pop sound as the hydrogen burns rapidly with oxygen.
Carbon dioxide turns limewater ______, while chlorine ______ damp litmus paper.
Carbon dioxide turns limewater milky, while chlorine bleaches damp litmus paper.
What is the test for ammonia gas, and what observation is seen with a positive result?
Damp red litmus paper is held near the mouth of the test tube. A positive result is when the damp red litmus paper turns blue.
True or False?
A glowing splint is used to test for hydrogen gas.
False.
A lighted (burning) splint is used to test for hydrogen, producing a squeaky pop. A glowing splint is used to test for oxygen, which relights the splint.
Why is extinguishing a burning splint not a reliable test for carbon dioxide?
Extinguishing a burning splint is not reliable because other gases, such as nitrogen, will also put out a flame.
The definitive test for carbon dioxide is that it turns limewater milky.
Oxygen gas is identified by placing a ______ splint into the test tube. A positive result is when the splint ______.
Oxygen gas is identified by placing a glowing splint into the test tube. A positive result is when the splint relights.
What two observations are seen when chlorine gas is tested with damp litmus paper?
Damp blue litmus paper first turns red (as chlorine dissolves in water to form an acidic solution) and then bleaches to white. The bleaching is the key confirming observation.
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