Ch 5 · Identification of Ions and Gases · IGCSE Chemistry 0620
Topic 5 of 5 in Experimental Techniques and Chemical Analysis — IGCSE Chemistry 0620 — 48 lessons.
Introduction
Qualitative analysis is detective work with a fixed set of clues. You add specified reagents to an unknown and match the observations to the tables in the syllabus. Every test is a reagent plus an observation, and the mark scheme wants both. This lesson gives the complete set of anion, cation, flame and gas tests and shows how to combine them.
Key ideas — Tests for anions
The nitric acid in the halide and sulfate tests removes carbonate ions, which would otherwise give a false white precipitate. The precipitates are the silver halides and barium sulfate:
🧑🏫 Teacher's explanation
Identifying ions is like being a detective at a crime scene. You can't just look at a clear liquid and know what's in it; you have to add a 'reagent' (a chemical clue) to see how it reacts. Some reactions produce a color change, some create a solid (precipitate) that makes the liquid cloudy, and others release a gas that you can test for.
📖 Definitions
🔍 Worked example — Easy
A student is given an unknown solution. They add dilute nitric acid and then aqueous silver nitrate. A white precipitate forms. Identify the anion present.
🔍 Worked example — Standard
A solution contains sulfate ions. Describe the test and the observation.
🔍 Worked example — Exam-style
A sample contains nitrate ions. Describe the chemical test and the observation made using litmus paper.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Identify the anion present in Solution A, Solution B, and Solution C. [3] [3 marks]
Key ideas — Tests for cations with sodium hydroxide and ammonia
Add aqueous sodium hydroxide dropwise, then to excess; repeat with aqueous ammonia on a fresh sample. The precipitate colour and its behaviour in excess identify the cation.
🧑🏫 Teacher's explanation
When we add sodium hydroxide (NaOH) or ammonia (NH_3) to a solution containing metal ions, we are essentially adding hydroxide ions (OH^-). These ions react with the metal cations to form insoluble metal hydroxides, which appear as colorful 'clouds' or precipitates in the water. Think of it like adding a tiny bit of milk to a glass of water; the water turns cloudy because the milk particles are too large to stay dissolved.
📖 Definitions
🔍 Worked example — Easy
A solution contains Zn^2+ ions. Describe the observation when aqueous sodium hydroxide is added dropwise and then in excess.
🔍 Worked example — Standard
A student is testing an unknown cation. They add aqueous ammonia and observe a light blue precipitate. Upon adding excess ammonia, the precipitate dissolves to form a dark blue solution. Identify the cation.
🔍 Worked example — Exam-style
A solution contains Al^3+ ions. Compare the results of testing with aqueous NaOH versus aqueous NH_3.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Solution X produces a green precipitate with aqueous sodium hydroxide, which is insoluble in excess. Solution Y produces a white precipitate with aqueous sodium hydroxide, which dissolves in excess. Solution Z produces a white precipitate with aqueous sodium hydroxide, which is insoluble in excess, and also produces a white precipitate with aqueous ammonia, which is insoluble in excess. Identify the metal ions present in solutions X, Y, and Z. [3 marks]
Key ideas — Flame tests
Dip a clean nichrome wire in concentrated hydrochloric acid, then in the sample, and hold it in a roaring Bunsen flame.
🧑🏫 Teacher's explanation
Imagine you are looking at a neon sign. The bright colors you see aren't just random; they are caused by electrons inside the atoms jumping between different energy levels. When we put a metal salt into a hot Bunsen flame, the heat provides energy that 'excites' the electrons. As these electrons fall back down to their original, lower energy state, they release that extra energy as light of a specific color.
📖 Definitions
🔍 Worked example — Easy
A student performs a flame test on an unknown white salt. The flame turns a distinct lilac color. Identify the cation present.
🔍 Worked example — Standard
A sample contains a mixture of Na^+ and Cu^2+. Predict the observed flame color.
🔍 Worked example — Exam-style
An unknown metal salt is dissolved in water. When a nichrome wire dipped in the solution is placed in a Bunsen flame, the flame turns orange-red. Identify the cation.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the metal cation present. [1] (b) Describe the procedure the student should follow to ensure the results are accurate and not contaminated. [2] [3 marks]
Key ideas — Tests for gases
Litmus must be damp for ammonia and chlorine because the gases must dissolve to act. Chlorine may turn blue litmus red before bleaching it; the bleaching is the identifying observation. Hydrogen needs a lighted splint, oxygen a glowing one.
🧑🏫 Teacher's explanation
Identifying gases is like being a chemical detective. You cannot just look at a gas; you have to interact with it using a specific 'reagent' (a chemical tool) to see how it changes. Some gases are reactive and change the color of indicators, while others are highly flammable and react violently with fire.
📖 Definitions
🔍 Worked example — Easy
A gas is bubbled through damp red litmus paper. The paper turns blue. Identify the gas.
🔍 Worked example — Standard
A student performs a test on an unknown gas. A glowing splint is placed in the gas jar, and the splint relights. Identify the gas.
🔍 Worked example — Exam-style
A gas is passed through acidified aqueous potassium manganate(VII). The purple solution turns colorless. Identify the gas and state the color change of the reagent.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) A lighted splint is held at the mouth of the tube. A 'pop' sound is heard. (b) The gas is bubbled through acidified potassium manganate(VII).
Worked example
Question. Solid S is a green compound. When dilute hydrochloric acid is added, S fizzes and the gas turns limewater milky. When S is dissolved in water and aqueous sodium hydroxide is added, a green precipitate forms that does not dissolve in excess. The same result is obtained with aqueous ammonia. (a) Identify the anion in S and the gas produced. [2] (b) Identify the cation in S, explaining how the sodium hydroxide result rules out chromium(III). [3] (c) Name S. [1]
🔍 Worked example — Easy
A sample of gas is bubbled through limewater. The limewater turns milky. Identify the gas.
🔍 Worked example — Standard
A solution containing aqueous Cu^2+ ions is added to aqueous NaOH. What is the observation?
🔍 Worked example — Exam-style
A solid X reacts with dilute HCl to produce a gas that turns damp red litmus paper blue. A solution of X reacts with aqueous NH_3 to form a white precipitate that dissolves in excess. Identify the anion and cation in X.
⚠️ Common errors
Common mistakes
Practice
Answers Acidify with dilute nitric acid and add aqueous silver nitrate [B1]; a yellow precipitate forms [B1]. Aluminium, mathrmAl^3+. [B1] Add aqueous sodium hydroxide (or aqueous ammonia) to each [B1]; iron(II) gives a green precipitate [B1]; iron(III) gives a red-brown precipitate [B1]. Potassium lilac [B1]; barium light green [B1]; copper(II) blue-green [B1]. Hydrogen: a lighted splint gives a pop [B1]; oxygen: relights a glowing splint [B1]; sulfur dioxide: turns acidified aqueous potassium manganate(VII) from purple to colourless [B1].