C12.5 Identification of ions and gases

State charge of carbonate ions

CO^₃2-

Describe test for carbonate ions

• Add dilute acid

• Bubble gas through limewater (calcium hydroxide)

• White precipitate of calcium carbonate is formed if carbonate is present

  • Turns cloudy in presence of carbonate

  • This is because limewater is used to test for carbon dioxide, which is produced when carbonate compounds are present

Describe test for halide ions

• Acidify sample with nitric acid to remove any carbonate compounds

• Add silver nitrate solution AgNO₃

• Silver halide precipitate will form if halide ion is present — color of precipitate determines halide ion

State charge of nitrate ions

NO₃^-

Describe test for nitrate ions

• Add aqueous sodium hydroxide (NaOH) and aluminium foil

• Warm gently and test gas released, ammonia (NH₃)

• Ammonia gas will turn damp red litmus paper blue

State charge of sulfate ions

SO₄^2-

Describe test for sulfate ions

• Acidify sample with dilute nitric or hydrochloric acid

• Add a few drops of barium nitrate or chloride solution

• White precipitate of barium sulfate is formed if sulfate ion is present

State charge of ammonium ions

NH₄^-

Describe test for ammonium ions

• Ammonia can be used both as a testing agent and as a product

• Gently warm solution with sodium hydroxide solution

• This releases ammonia gas which turns red litmus paper blue

Describe the test for aqueous cations using sodium hydroxide/ammonia solutions

• Sodium hydroxide and ammonia have the same tests

• The metal hydroxide that forms will be the precipitate, written with state symbol (s)

  • If excess sodium hydroxide/ammonia is added, the precipitate may re-dissolve so it has to be added slowly

• Sodium hydroxide and ammonia test for metal ions:

  • Add few drops of sodium hydroxide/ammonia solution

  • Record color changes or precipitates formed

  • Add excess sodium hydroxide/ammonia solution

  • Record any color changes or changes to precipitate

• Why is excess sodium hydroxide/ammonia added?:

  • When you add a few drops initially, some metal ions will produce the exact same color

  • Adding excess is the only way to tell them apart, by determining whether precipitate dissolves or doesn’t dissolve

Name the color of precipitates formed from sodium hydroxide/ammonia solutions and whether they dissolve or not

Metal ion	Addition of few drops of sodium hydroxide or ammonia	Addition of excess sodium hydroxide	Addition of excess ammonia
NH4+	Ammonia gas given off
Ca2+	White precipitate forms	Precipitate does not dissolve	No precipitate forms with ammonia
Cu2+	Light blue precipitate forms	Precipitate does not dissolve	Precipitate dissolves to form a dark blue solution
Fe2+	Green precipitate forms	Precipitate does not dissolve	Precipitate does not dissolve
Fe3+	Red-brown precipitate forms	Precipitate does not dissolve	Precipitate does not dissolve
Zn2+	White precipitate forms	Precipitate dissolves to form colourless solution	Precipitate dissolves to form colourless solution

How do you determine if flame test or sodium hydroxide/ammonium solution is used for cations?

• Sodium hydroxide/ammonia solution: aqueous solution

• Flame test: solid cation (like powder or crystals)

Describe flame test for cations

• Clean unreactive metal wire (like nichrome or platinum) by dipping in concentrated HCl

• Hold it in bunsen burner flame until there is no colour change

• Moisten clean wire in HCl and dip wire into solid cation

• Place wire into flame and observe colour to determine cation

Name the colour of flame for each cation

Cation	Flame colour
Li+	Crimson
Na+	Yellow
K+	Lilac
Cu2+	Blue-green

Describe test for ammonia gas

• Gas that turns damp red litmus blue

  • Explanation: turns blue since it is highly alkaline

• Can also be identified by its strong, pungent smell

Describe test for carbon dioxide gas

• Bubbling the gas through aqueous solution of limewater (calcium hydroxide)

• If carbon dioxide is present, limewater turns cloudy white

  • Explanation: turns white because precipitate of calcium carbonate is formed

Describe test for chlorine gas

• Gas that turns damp blue litmus red, and eventually bleaches white

  • Explanation: turns red because chlorine is acidic when dissolved in water, white because it is strongly oxidizing

• Can also be identified by sharp, choking smell

Describe test for hydrogen gas

• Holding burning splint at end of test tube full with gas

• If the gas is hydrogen, it burns and releases a loud squeaky pop sound

  • Explanation: this happens due to the combustion of hydrogen and oxygen

Describe test for oxygen gas

• Place glowing splint inside test tube of gas

• If the gas is oxygen, the splint will relight

  • Explanation: this is because oxygen promotes combustion, since the wood burns more readily in oxygen compared to air