Chemical Tests

Chemistry· 2.44–2.50 (Section 2(h))· 15 min read

1. Tests for Common Gases★★☆☆☆⏱ 3 min

Gas tests are among the most frequently asked practical questions in your exam. You must recall both the test method and exact observation for each gas to score full marks.

Gas	Test Method	Observation
Hydrogen, $H_{2}$	Hold a lighted splint at the test tube mouth	Squeaky pop sound as hydrogen burns
Oxygen, $O_{2}$	Insert a glowing splint into the test tube	Glowing splint relights immediately
Carbon dioxide, $C O_{2}$	Bubble gas through limewater	Limewater turns milky/cloudy white
Ammonia, $N H_{3}$	Hold damp red litmus near the gas outlet	Damp red litmus paper turns blue
Chlorine, $C l_{2}$	Hold damp blue litmus near the gas outlet	Damp blue litmus turns red, then bleaches white

📐 Worked Example

A student collects a colourless gas that turns damp red litmus paper blue. Identify the gas.

1
Step 1: Match the observation to the gas test table. The only alkaline gas in the specification is ammonia.
2
Step 2: No confirmatory test is required as this observation is unique to ammonia.
3
Answer: Ammonia ( $N H_{3}$ )

Exam tip:

Never write 'litmus paper' without specifying its initial colour and whether it is damp for ammonia and chlorine tests, as dry litmus will not change colour with these gases.

2. Flame Tests for Cations★★☆☆☆⏱ 3 min

Flame tests identify metal cations in solid or aqueous samples. You must recall the exact flame colour for each listed cation to score full marks.

📘 Definition

Flame test

A qualitative analytical method that uses the characteristic colour emitted by a metal cation when heated in a Bunsen flame to identify the cation present.

Example:

Sodium ions emit a bright yellow flame when heated.

Clean a nichrome/platinum wire loop by dipping it in concentrated hydrochloric acid, then holding it in a roaring blue Bunsen flame until no colour is emitted.
Dip the clean loop into the solid or aqueous sample.
Hold the loop in the edge of the roaring blue flame and observe the flame colour.

Cation	Flame Colour
Lithium, $L i^{+}$	Red
Sodium, $N a^{+}$	Yellow
Potassium, $K^{+}$	Lilac
Calcium, $C a^{2 +}$	Orange-red
Copper(II), $C u^{2 +}$	Blue-green

📐 Worked Example

A student performs a flame test on an unknown solid and observes a lilac flame. Identify the cation present.

1
Step 1: Match the flame colour to the flame test results table.
2
Step 2: Lilac is the unique flame colour for potassium cations.
3
Answer: Potassium ion, $K^{+}$

Exam tip:

Do not write 'purple' for potassium's flame colour: the Edexcel mark scheme only accepts 'lilac' as a correct answer.

3. Sodium Hydroxide Tests for Cations★★★☆☆⏱ 3 min

Dilute sodium hydroxide ( $N a O H$ ) solution is used to test for ammonium, copper(II), iron(II) and iron(III) cations via precipitation or gas release reactions. You must recall the exact precipitate colour for each cation.

Cation	Test Method	Observation
Ammonium, $N H_{4}^{+}$	Warm sample with dilute $N a O H$ , test any gas with damp red litmus	Damp red litmus turns blue (ammonia gas released)
Copper(II), $C u^{2 +}$	Add a few drops of dilute $N a O H$ to the sample	Blue precipitate of copper(II) hydroxide forms
Iron(II), $F e^{2 +}$	Add a few drops of dilute $N a O H$ to the sample	Green precipitate of iron(II) hydroxide forms
Iron(III), $F e^{3 +}$	Add a few drops of dilute $N a O H$ to the sample	Brown precipitate of iron(III) hydroxide forms

📐 Worked Example

Adding dilute sodium hydroxide solution to an unknown solution produces a green precipitate. Identify the cation present.

1
Step 1: Match the precipitate colour to the sodium hydroxide test results table.
2
Step 2: A green precipitate is unique to iron(II) cations when reacted with sodium hydroxide.
3
Answer: Iron(II) ion, $F e^{2 +}$

Exam tip:

Do not confuse iron precipitate colours: $F e^{2 +}$ gives green, $F e^{3 +}$ gives brown. Mark schemes do not accept 'orange' or 'red' for $F e^{3 +}$ .

4. Tests for Anions★★★☆☆⏱ 3 min

Anion tests use precipitation or gas release reactions to identify negatively charged ions. You must specify that silver nitrate and barium chloride are acidified to get full marks, as the acid removes interfering carbonate ions that would produce false positive results.

Anion	Test Method	Observation
Chloride, $C l^{-}$	Add dilute nitric acid, then silver nitrate solution	White precipitate of silver chloride forms
Bromide, $B r^{-}$	Add dilute nitric acid, then silver nitrate solution	Cream precipitate of silver bromide forms
Iodide, $I^{-}$	Add dilute nitric acid, then silver nitrate solution	Yellow precipitate of silver iodide forms
Sulfate, $S O_{4}^{2 -}$	Add dilute hydrochloric acid, then barium chloride solution	White precipitate of barium sulfate forms
Carbonate, $C O_{3}^{2 -}$	Add dilute hydrochloric acid, bubble gas through limewater	Limewater turns milky (carbon dioxide released)

📐 Worked Example

Acidified silver nitrate is added to an unknown solution, producing a yellow precipitate. Identify the anion present.

1
Step 1: Match the precipitate colour to the halide test results table.
2
Step 2: A yellow precipitate with acidified silver nitrate confirms the presence of iodide ions.
3
Answer: Iodide ion, $I^{-}$

Exam tip:

Always state the acid used first before the reagent for halide and sulfate tests, e.g. 'add dilute nitric acid, then silver nitrate' not just 'add silver nitrate'.

5. Tests for Water and Water Purity★★☆☆☆⏱ 2 min

You need to know two separate tests for water: one to confirm the presence of water, and one to confirm that a water sample is pure.

📘 Definition

Anhydrous copper(II) sulfate

White crystalline copper(II) sulfate that contains no water of crystallisation, which turns blue when it forms hydrated copper(II) sulfate on contact with water.

Example:

$C u S O_{4} (white) + 5 H_{2} O \to C u S O_{4} \cdot 5 H_{2} O (blue)$

Test for presence of water: Add a small amount of anhydrous copper(II) sulfate to the sample. If water is present, the white solid turns blue.
Test for purity of water: Measure the boiling point of the sample at 1 atm pressure. Pure water boils at exactly 100°C. Impure water will boil at a temperature higher than 100°C, over a range of temperatures. You can also measure melting point: pure water freezes at exactly 0°C.

📐 Worked Example

A student tests a clear liquid with anhydrous copper(II) sulfate, which turns blue. They measure its boiling point as 102°C. State what this tells you about the liquid.

1
Step 1: The anhydrous copper(II) sulfate turning blue confirms water is present in the liquid.
2
Step 2: The boiling point of 102°C is higher than the boiling point of pure water (100°C at 1 atm), so the water is impure, containing dissolved solids.
3
Answer: The liquid is impure water.

Exam tip:

Do not confuse the test for presence of water with the test for purity: anhydrous copper(II) sulfate only shows water is present, not that it is pure.

6. Common Pitfalls

Wrong move:

Writing 'litmus paper' without specifying colour or dampness for ammonia/chlorine tests

Why:

Dry litmus does not react with ammonia or chlorine gases, so the observation is only valid for damp litmus of the correct starting colour

Correct move:

Always write 'damp red litmus paper' for ammonia tests, and 'damp blue litmus paper' for chlorine tests

Wrong move:

Writing 'purple' instead of 'lilac' for potassium flame test colour

Why:

The Edexcel mark scheme explicitly only accepts 'lilac' as the correct answer for potassium flame colour

Correct move:

Use the mnemonic 'Potassium = Lilac' to remember the exact required term

Wrong move:

Omitting 'acidified' when describing silver nitrate or barium chloride tests for anions

Why:

Interfering ions like carbonate will produce false positive precipitates if the reagent is not acidified, so this is a required part of the test to score full marks

Correct move:

Always specify 'acidified silver nitrate' for halide tests, and 'acidified barium chloride' for sulfate tests

Wrong move:

Confusing $F e^{2 +}$ and $F e^{3 +}$ precipitate colours with sodium hydroxide

Why:

These are commonly mixed up, and mark schemes only accept green for $F e^{2 +}$ and brown for $F e^{3 +}$

Correct move:

Use the mnemonic 'Fe2 Green, Fe3 Brown' to remember the correct precipitate colours

Wrong move:

Using anhydrous copper(II) sulfate to test for pure water

Why:

Anhydrous copper(II) sulfate only confirms water is present, not that it has no dissolved impurities

Correct move:

Use boiling point or melting point measurement to confirm water purity: pure water boils at exactly 100°C and freezes at 0°C at 1 atm

7. Quick Reference Cheatsheet

Test Type	Target Substance	Reagent/Method	Positive Observation
Gas	$H_{2}$	Lighted splint	Squeaky pop
Gas	$O_{2}$	Glowing splint	Splint relights
Gas	$C O_{2}$	Limewater	Turns milky
Gas	$N H_{3}$	Damp red litmus	Turns blue
Gas	$C l_{2}$	Damp blue litmus	Turns red then bleaches white
Flame test	$L i^{+}$	Heat in blue flame	Red flame
Flame test	$N a^{+}$	Heat in blue flame	Yellow flame
Flame test	$K^{+}$	Heat in blue flame	Lilac flame
Flame test	$C a^{2 +}$	Heat in blue flame	Orange-red flame
Flame test	$C u^{2 +}$	Heat in blue flame	Blue-green flame
Cation	$N H_{4}^{+}$	Warm with $N a O H$	$N H_{3}$ gas released (damp red litmus turns blue)
Cation	$C u^{2 +}$	$N a O H$ solution	Blue precipitate
Cation	$F e^{2 +}$	$N a O H$ solution	Green precipitate
Cation	$F e^{3 +}$	$N a O H$ solution	Brown precipitate
Anion	$C l^{-}$	Acidified $A g N O_{3}$	White precipitate
Anion	$B r^{-}$	Acidified $A g N O_{3}$	Cream precipitate
Anion	$I^{-}$	Acidified $A g N O_{3}$	Yellow precipitate
Anion	$S O_{4}^{2 -}$	Acidified $B a C l_{2}$	White precipitate
Anion	$C O_{3}^{2 -}$	Dilute $H C l$ + limewater	Limewater turns milky
Water	Presence of $H_{2} O$	Anhydrous $C u S O_{4}$	White solid turns blue
Water	Pure $H_{2} O$	Measure boiling point	Boils at exactly 100°C at 1 atm

8. Frequently Asked

Do I need to state that silver nitrate is acidified for halide tests?

Yes! You must specify 'acidified silver nitrate' to get full marks, as the acid removes interfering carbonate ions that would also form a white precipitate.

What colour is the flame for $K^{+}$ ions?

Lilac, not purple. The Edexcel exam board only accepts 'lilac' as the correct answer for potassium flame tests.

How do I test if water is pure, not just present?

To test for presence, use anhydrous copper(II) sulfate (turns blue). To confirm purity, measure its boiling point: pure water boils at exactly 100°C at 1 atm pressure. Impure water will boil at a higher temperature.

Going deeper

What's Next

Now that you have mastered all required chemical tests for Edexcel IGCSE Chemistry, you can apply this knowledge to practical problem-solving questions where you identify unknown ionic compounds from test results. This skill is heavily tested in both Paper 1 and Paper 2, especially in 6-mark extended response questions that ask you to design an experimental procedure to identify an unknown substance. You should also practice linking test results to ionic compound formulae and balanced chemical equations for precipitation reactions to score full marks on longer questions.