1.4Experimental Chemistry

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

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📚 Combined Science 0653 - Chemistry Hub / Topical Revision

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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

Section 1 of 15Tests for anions

Anion	Test	Positive result
Carbonate, CO₃^2-	Add dilute acid and bubble the gas through limewater	Fizzing; limewater turns cloudy
Chloride, Cl^-	Acidify with dilute nitric acid, add aqueous silver nitrate	White precipitate
Bromide, Br^-	Acidify with dilute nitric acid, add aqueous silver nitrate	Cream precipitate
Iodide, I^-	Acidify with dilute nitric acid, add aqueous silver nitrate	Yellow precipitate
Sulfate, SO₄^2-	Acidify with dilute nitric acid, add aqueous barium nitrate	White 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.

Section 2 of 15Tests for cations with sodium hydroxide

Cation	With a few drops of NaOH	In excess NaOH
Aluminium, Al³⁺	White precipitate	Dissolves to a colourless solution
Ammonium, NH₄⁺	Ammonia gas on warming	—
Calcium, Ca²⁺	White precipitate	Insoluble
Copper(II), Cu²⁺	Light blue precipitate	Insoluble
Iron(II), Fe²⁺	Green precipitate	Insoluble; surface turns brown on standing
Iron(III), Fe³⁺	Red-brown precipitate	Insoluble
Zinc, Zn²⁺	White precipitate	Dissolves 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.

Section 3 of 15Tests for cations with aqueous ammonia

Cation	With a few drops of ammonia	In excess ammonia
Aluminium, Al³⁺	White precipitate	Insoluble
Calcium, Ca²⁺	No precipitate or very slight white	No change
Copper(II), Cu²⁺	Light blue precipitate	Dissolves to a dark blue solution
Iron(II), Fe²⁺	Green precipitate	Insoluble
Iron(III), Fe³⁺	Red-brown precipitate	Insoluble
Zinc, Zn²⁺	White precipitate	Dissolves 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.

Section 4 of 15Test 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.

Section 5 of 15Tests for gases

Gas	Test	Positive result
Ammonia, NH₃	Damp red litmus paper	Turns blue
Carbon dioxide, CO₂	Bubble through limewater	Limewater turns cloudy
Chlorine, Cl₂	Damp litmus paper	Bleached white
Hydrogen, H₂	Lighted splint at the mouth of the tube	Squeaky pop
Oxygen, O₂	Glowing splint in the gas	Splint relights

Section 6 of 15Flame tests (supplement)

Clean a nichrome wire loop in concentrated acid and heat it in a blue Bunsen flame until it gives no colour.

Dip the loop in the sample.

Hold it in the edge of the blue flame without letting it glow red.

Record the flame colour.

Cation	Flame colour
Lithium, Li⁺	Red
Sodium, Na⁺	Yellow
Potassium, K⁺	Lilac
Copper(II), Cu²⁺	Blue-green

Section 7 of 15More from the specification

Section 8 of 15Testing cations with sodium hydroxide and ammonia

Testing cations with sodium hydroxide and ammonia

Add a few drops of the aqueous alkali very slowly to a solution of the salt.

Record the colour of any precipitate.

Add excess alkali and record whether the precipitate stays or dissolves.

Section 9 of 15Distinguishing cations that give white precipitates

Three white precipitates

Al³⁺, Ca²⁺ and Zn²⁺ all give a white precipitate with sodium hydroxide.

Calcium

Calcium its precipitate does not dissolve in excess sodium hydroxide, unlike zinc and aluminium.

Zinc and aluminium

Zinc and aluminium the zinc hydroxide precipitate dissolves in excess ammonia, whereas aluminium hydroxide does not.

Slow addition

Slow addition if the alkali is added too quickly, a precipitate that dissolves in excess may dissolve as fast as it forms and be missed.

Wording

Wording colourless means a solution has lost its colour, whereas clear means you can see through it;
- a clear solution can still be coloured.

Section 10 of 15Test for ammonium ions

Test

Test warm the solution gently with sodium hydroxide solution.

Result

Result ammonia gas is released and turns damp red litmus paper blue.

Ammonium and ammonia

Ammonium and ammonia ammonium is the aqueous cation NH₄⁺; ammonia is the gas NH₃.

Section 11 of 15Flame tests

Metal ion	Flame colour
Li⁺	Red
Na⁺	Yellow
K⁺	Lilac
Ca²⁺	Orange-red
Ba²⁺	Light green
Cu²⁺	Blue-green

Section 12 of 15Tests for anions

Anion	Test	Positive result
Carbonate, CO₃^2-	Add dilute acid and bubble the gas through limewater	Fizzing; limewater turns cloudy
Chloride, Cl^-	Acidify with dilute nitric acid, add silver nitrate solution	White precipitate of AgCl
Bromide, Br^-	Acidify with dilute nitric acid, add silver nitrate solution	Cream precipitate of AgBr
Iodide, I^-	Acidify with dilute nitric acid, add silver nitrate solution	Yellow precipitate of AgI
Sulfate, SO₄^2-	Acidify with dilute nitric acid, add barium chloride or barium nitrate	White precipitate of BaSO₄
Sulfite, SO₃^2-	Add dilute acid, warm, pass gas into potassium manganate(VII)	Purple solution turns colourless
Nitrate, NO₃^-	Add aqueous sodium hydroxide and aluminium foil and warm	Ammonia gas is given off

Section 13 of 15Tests for gases

Gas	Test	Result
Ammonia, NH₃	Hold damp red litmus paper near the mouth of the tube without touching the sides	Turns blue; sharp choking smell
Carbon dioxide	Bubble through limewater (calcium hydroxide solution)	Limewater turns cloudy white
Chlorine	Damp blue litmus paper	Turns red, then bleached white; sharp choking smell
Hydrogen	Burning splint at the mouth of the tube	Squeaky pop
Oxygen	Glowing splint placed inside the tube	Splint relights
Sulfur dioxide	Bubble through acidified potassium manganate(VII)	Purple solution turns colourless

Section 14 of 15Key terms

Term	Meaning
Anion	Negatively charged ion.
Cation	Positively charged ion.
Precipitate	Insoluble solid formed in a reaction in solution.
Limewater	Calcium hydroxide solution that turns cloudy with carbon dioxide.
Flame test	Test using flame colour to identify a metal ion.
Effervescence	Fizzing as a gas is given off.

Term	Meaning
Precipitate	Insoluble solid formed in a solution during a reaction.
Flame test	Test using the colour of a flame to identify a metal ion.
Limewater	Calcium hydroxide solution that turns cloudy with carbon dioxide.
Glowing splint	Splint with the flame blown out but still glowing; relights in oxygen.
Amphoteric hydroxide	Hydroxide that dissolves in both acid and excess alkali.
Ammonium ion	NH₄⁺, the cation that releases ammonia with hot alkali.

Section 15 of 15Exam technique

Confusing ammonium (the ion) with ammonia (the gas).

Using hydrochloric acid before silver nitrate — use nitric acid.

Adding sodium hydroxide too fast and missing a precipitate that redissolves.

Confusing the lighted splint (hydrogen) with the glowing splint (oxygen).

Describing the result only when asked for the test — say what is done and what is seen.

Confusing ammonium (aqueous ion) with ammonia (gas).