# Chemical Tests

> Chemistry · Edexcel IGCSE 4CH1
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-chemical-tests/

This guide covers all required qualitative chemical tests for Edexcel IGCSE Chemistry (4CH1), including gas tests, flame tests, cation/anion precipitate tests, and tests for water and water purity, aligned directly to specification points 2.44–2.50.

**Prerequisites:** [Knowledge of common ion formulae and precipitation reactions](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-ionic-compounds/); [Understanding of pure substances and melting/boiling points](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-states-of-matter/)

## Learning objectives

- Recall and carry out tests for 5 common specified gases
- Perform flame tests and recall flame colours for 5 listed cations
- Carry out and interpret sodium hydroxide cation tests for $NH_4^+$, $Cu^{2+}$, $Fe^{2+}$, $Fe^{3+}$
- Recall and apply anion tests for halides, sulfate, and carbonate ions
- Test for the presence of water and confirm water purity via melting/boiling point measurements

## Tests for Common Gases

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, $CO_2$ | Bubble gas through limewater | Limewater turns milky/cloudy white |
| Ammonia, $NH_3$ | Hold damp red litmus near the gas outlet | Damp red litmus paper turns blue |
| Chlorine, $Cl_2$ | 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 ($NH_3$)

> **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.

## Flame Tests for Cations

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.

**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.

1. 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.
2. Dip the clean loop into the solid or aqueous sample.
3. Hold the loop in the edge of the roaring blue flame and observe the flame colour.

| Cation | Flame Colour |
| --- | --- |
| Lithium, $Li^+$ | Red |
| Sodium, $Na^+$ | Yellow |
| Potassium, $K^+$ | Lilac |
| Calcium, $Ca^{2+}$ | Orange-red |
| Copper(II), $Cu^{2+}$ | 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.

## Sodium Hydroxide Tests for Cations

Dilute sodium hydroxide ($NaOH$) 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, $NH_4^+$ | Warm sample with dilute $NaOH$, test any gas with damp red litmus | Damp red litmus turns blue (ammonia gas released) |
| Copper(II), $Cu^{2+}$ | Add a few drops of dilute $NaOH$ to the sample | Blue precipitate of copper(II) hydroxide forms |
| Iron(II), $Fe^{2+}$ | Add a few drops of dilute $NaOH$ to the sample | Green precipitate of iron(II) hydroxide forms |
| Iron(III), $Fe^{3+}$ | Add a few drops of dilute $NaOH$ 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, $Fe^{2+}$

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

## Tests for Anions

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, $Cl^-$ | Add dilute nitric acid, then silver nitrate solution | White precipitate of silver chloride forms |
| Bromide, $Br^-$ | 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, $SO_4^{2-}$ | Add dilute hydrochloric acid, then barium chloride solution | White precipitate of barium sulfate forms |
| Carbonate, $CO_3^{2-}$ | 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'.

## Tests for Water and Water Purity

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.

**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:* $CuSO_4 \text{ (white)} + 5H_2O \rightarrow CuSO_4 \cdot 5H_2O \text{ (blue)}$

- **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.

## Common pitfalls

- **Wrong:** Writing 'litmus paper' without specifying colour or dampness for ammonia/chlorine tests
  - Why it fails: 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: Always write 'damp red litmus paper' for ammonia tests, and 'damp blue litmus paper' for chlorine tests
- **Wrong:** Writing 'purple' instead of 'lilac' for potassium flame test colour
  - Why it fails: The Edexcel mark scheme explicitly only accepts 'lilac' as the correct answer for potassium flame colour
  - Correct: Use the mnemonic 'Potassium = Lilac' to remember the exact required term
- **Wrong:** Omitting 'acidified' when describing silver nitrate or barium chloride tests for anions
  - Why it fails: 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: Always specify 'acidified silver nitrate' for halide tests, and 'acidified barium chloride' for sulfate tests
- **Wrong:** Confusing $Fe^{2+}$ and $Fe^{3+}$ precipitate colours with sodium hydroxide
  - Why it fails: These are commonly mixed up, and mark schemes only accept green for $Fe^{2+}$ and brown for $Fe^{3+}$
  - Correct: Use the mnemonic 'Fe2 Green, Fe3 Brown' to remember the correct precipitate colours
- **Wrong:** Using anhydrous copper(II) sulfate to test for pure water
  - Why it fails: Anhydrous copper(II) sulfate only confirms water is present, not that it has no dissolved impurities
  - Correct: 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

## Cheatsheet

| Test Type | Target Substance | Reagent/Method | Positive Observation |
| --- | --- | --- | --- |
| Gas | $H_2$ | Lighted splint | Squeaky pop |
| Gas | $O_2$ | Glowing splint | Splint relights |
| Gas | $CO_2$ | Limewater | Turns milky |
| Gas | $NH_3$ | Damp red litmus | Turns blue |
| Gas | $Cl_2$ | Damp blue litmus | Turns red then bleaches white |
| Flame test | $Li^+$ | Heat in blue flame | Red flame |
| Flame test | $Na^+$ | Heat in blue flame | Yellow flame |
| Flame test | $K^+$ | Heat in blue flame | Lilac flame |
| Flame test | $Ca^{2+}$ | Heat in blue flame | Orange-red flame |
| Flame test | $Cu^{2+}$ | Heat in blue flame | Blue-green flame |
| Cation | $NH_4^+$ | Warm with $NaOH$ | $NH_3$ gas released (damp red litmus turns blue) |
| Cation | $Cu^{2+}$ | $NaOH$ solution | Blue precipitate |
| Cation | $Fe^{2+}$ | $NaOH$ solution | Green precipitate |
| Cation | $Fe^{3+}$ | $NaOH$ solution | Brown precipitate |
| Anion | $Cl^-$ | Acidified $AgNO_3$ | White precipitate |
| Anion | $Br^-$ | Acidified $AgNO_3$ | Cream precipitate |
| Anion | $I^-$ | Acidified $AgNO_3$ | Yellow precipitate |
| Anion | $SO_4^{2-}$ | Acidified $BaCl_2$ | White precipitate |
| Anion | $CO_3^{2-}$ | Dilute $HCl$ + limewater | Limewater turns milky |
| Water | Presence of $H_2O$ | Anhydrous $CuSO_4$ | White solid turns blue |
| Water | Pure $H_2O$ | Measure boiling point | Boils at exactly 100°C at 1 atm |

## 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.

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