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1.4Experimental Chemistry

Identification of ions and gases — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 15 parts Lesson 5 of 10 1 quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Lesson summary

1.4 Identification of ions and gases — COMBINED SCIENCE (CHEMISTRY) notes: Describe the tests for common anions, cations and gases.AnionTestPositive resultCarbonate, CO32-Add dilute acid and bubble the gas through limewaterFizzing; limewater turns cloudyChloride, Cl-Acidify with …

  • Nitric acid
  • Acidifying for sulfate
  • Zinc and aluminium
  • Ammonium and ammonia
  • Precipitate
  • Flame test

Part 1 of 7Tests for anions

AnionTestPositive result
Carbonate, CO32-Add dilute acid and bubble the gas through limewaterFizzing; limewater turns cloudy
Chloride, Cl-Acidify with dilute nitric acid, add aqueous silver nitrateWhite precipitate
Bromide, Br-Acidify with dilute nitric acid, add aqueous silver nitrateCream precipitate
Iodide, I-Acidify with dilute nitric acid, add aqueous silver nitrateYellow precipitate
Sulfate, SO42-Acidify with dilute nitric acid, add aqueous barium nitrateWhite precipitate
Nitric acid
  • Nitric acid is used for acidifying because hydrochloric acid contains chloride ions that would interfere.
Acidifying for sulfate
  • Acidifying for sulfate removes any carbonate that would also give a precipitate.

Part 2 of 7Tests for cations with sodium hydroxide

CationWith a few drops of NaOHIn excess NaOH
Aluminium, Al3+White precipitateDissolves to a colourless solution
Ammonium, NH4+Ammonia gas on warming—
Calcium, Ca2+White precipitateInsoluble
Copper(II), Cu2+Light blue precipitateInsoluble
Iron(II), Fe2+Green precipitateInsoluble; surface turns brown on standing
Iron(III), Fe3+Red-brown precipitateInsoluble
Zinc, Zn2+White precipitateDissolves to a colourless solution
  • Technique
    • add the alkali slowly, a few drops at a time, or a precipitate that dissolves in excess may be missed.

Part 3 of 7Tests for cations with aqueous ammonia

CationWith a few drops of ammoniaIn excess ammonia
Aluminium, Al3+White precipitateInsoluble
Calcium, Ca2+No precipitate or very slight whiteNo change
Copper(II), Cu2+Light blue precipitateDissolves to a dark blue solution
Iron(II), Fe2+Green precipitateInsoluble
Iron(III), Fe3+Red-brown precipitateInsoluble
Zinc, Zn2+White precipitateDissolves to a colourless solution
  • Zinc and aluminium
    • both give white precipitates that dissolve in excess sodium hydroxide, but only zinc hydroxide dissolves in excess ammonia.

Part 4 of 7Test for ammonium ions

Test for ammonium ions
Warm the solution gently with sodium hydroxide solution.
Hold damp red litmus paper at the mouth of the tube without touching the sides.
Ammonia gas is released and turns the litmus paper blue.
  • Ammonium and ammonia
    • ammonium is the aqueous ion NH4+ and ammonia is the gas NH3.

Part 5 of 7Tests for gases

GasTestPositive result
Ammonia, NH3Damp red litmus paperTurns blue
Carbon dioxide, CO2Bubble through limewaterLimewater turns cloudy
Chlorine, Cl2Damp litmus paperBleached white
Hydrogen, H2Lighted splint at the mouth of the tubeSqueaky pop
Oxygen, O2Glowing splint in the gasSplint relights

Part 6 of 7Flame tests (supplement)

1
Clean a nichrome wire loop in concentrated acid and heat it in a blue Bunsen flame until it gives no colour.
2
Dip the loop in the sample.
3
Hold it in the edge of the blue flame without letting it glow red.
4
Record the flame colour.
CationFlame colour
Lithium, Li+Red
Sodium, Na+Yellow
Potassium, K+Lilac
Copper(II), Cu2+Blue-green

Part 7 of 7More from the specification

More from the specification
Precipitation of insoluble metal hydroxide
  • Precipitation of insoluble metal hydroxide adding aqueous sodium hydroxide to unknown solutions precipitates characteristic colored transition metal hydroxides for cation identification.
Halide
  • A negative ion formed when a halogen gains one electron.
Sulfur dioxide
  • An acidic gas made when sulfur-containing fuels burn.

Key terms

TermMeaning
AnionNegatively charged ion.
CationPositively charged ion.
PrecipitateInsoluble solid formed in a reaction in solution.
LimewaterCalcium hydroxide solution that turns cloudy with carbon dioxide.
Flame testTest using flame colour to identify a metal ion.
EffervescenceFizzing as a gas is given off.

Avoid these mistakes

1
Confusing ammonium (the ion) with ammonia (the gas).
2
Using hydrochloric acid before silver nitrate — use nitric acid.
3
Adding sodium hydroxide too fast and missing a precipitate that redissolves.
4
Confusing the lighted splint (hydrogen) with the glowing splint (oxygen).
5
Describing the result only when asked for the test — say what is done and what is seen.

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