Chemical Tests

IGCSE Edexcel Chemistry
2.44–2.50 Tests for gases, ions and water
Key Concept: Different substances are identified by their characteristic reactions: gases by splint/litmus tests; metal ions by flame colours or NaOH precipitates; anions by silver nitrate or barium chloride; water by colour changes of anhydrous copper sulfate or cobalt chloride paper.

Section A — Tests for Gases

1. Complete the table for the gas tests. [10]
Gas Test Positive result
Hydrogen
Oxygen
Carbon dioxide
Ammonia
Chlorine

Section B — Flame Tests

2. Describe the correct method for carrying out a flame test. [4]
3. Complete the table for flame test colours. [5]
Metal ion Flame colour
Lithium (Li⁺)
Sodium (Na⁺)
Potassium (K⁺)
Calcium (Ca²⁺)
Copper (Cu²⁺)
4. Why must the nichrome wire loop be cleaned with hydrochloric acid before each test? [2]

Section C — Tests for Cations

5. Sodium hydroxide solution is added to three separate solutions. Complete the table. [6]
Ion present Observation with NaOH
Cu²⁺
Fe²⁺
Fe³⁺
NH₄⁺
6. A green precipitate forms when NaOH is added to a solution. On standing in air, the precipitate turns brown. Explain these observations. [2]

Section D — Tests for Anions

7. Describe the test for each anion and state the positive result. [8]
Anion Test Positive result
Cl⁻ (chloride)
Br⁻ (bromide)
I⁻ (iodide)
SO₄²⁻ (sulfate)
8. Describe the test for carbonate ions and explain how you would confirm the identity of the gas produced. [3]

Section E — Tests for Water and Purity

9. Describe two different tests for the presence of water. For each, state the reagent used, the observation and what it indicates. [4]
10. Describe how you would show that a sample of water is pure using a physical test. What result would you expect for pure water? [2]
11. A sample of liquid boils over the range 96–103°C. What does this suggest about the sample? [1]

Total marks: 47

Mark Scheme

1. Hydrogen: lighted splint → squeaky pop; Oxygen: glowing splint → relights; CO₂: bubble through limewater → turns milky/cloudy; Ammonia: damp red litmus paper → turns blue; Chlorine: damp blue litmus paper → turns red, then bleached white [10 — 2 per gas]
2. Clean nichrome wire with concentrated HCl and heat until no colour; dip wire in the sample; hold in the blue/non-luminous part of a Bunsen flame; observe flame colour [4]
3. Li⁺: red/crimson; Na⁺: yellow/orange; K⁺: lilac/violet; Ca²⁺: brick red/orange-red; Cu²⁺: blue-green/green [5]
4. Removes any contaminants from previous tests; ensures the colour observed is only from the test sample and not impurities [2]
5. Cu²⁺: blue precipitate; Fe²⁺: green precipitate; Fe³⁺: brown/rust precipitate; NH₄⁺: no precipitate — ammonia gas produced on warming, turns damp red litmus blue [6]
6. Green precipitate = Fe(OH)₂ from Fe²⁺ ions; on standing, Fe²⁺ is oxidised to Fe³⁺ by oxygen in air; brown precipitate = Fe(OH)₃ [2]
7. Cl⁻: add dilute nitric acid then silver nitrate → white precipitate (AgCl); Br⁻: add dilute nitric acid then silver nitrate → cream precipitate (AgBr); I⁻: add dilute nitric acid then silver nitrate → yellow precipitate (AgI); SO₄²⁻: add dilute hydrochloric acid then barium chloride → white precipitate (BaSO₄) [8 — 2 per anion]
8. Add dilute acid to the sample; effervescence/bubbling occurs; bubble the gas through limewater; limewater turns milky/cloudy confirming CO₂/carbonate [3]
9. Test 1: add anhydrous copper(II) sulfate (white solid); turns blue in presence of water; Test 2: apply cobalt chloride paper (blue); turns pink in presence of water [4]
10. Measure the boiling point; pure water boils at a sharp, fixed temperature of 100°C (at atmospheric pressure) [2]
11. The sample is impure / a mixture — pure substances have sharp, fixed boiling points [1]