Identification of common gases
Lajoy Tucker
Teacher
Introduction and Definition
Many gases are colourless and cannot be identified just by looking at them.
Chemists use simple laboratory tests to identify common gases such as hydrogen, oxygen, carbon dioxide and chlorine.
Each gas gives a distinctive, reliable result when tested.
Gas | Test | Positive Result | Why It Happens |
Hydrogen (H₂) | Hold a burning splint at the mouth of the test tube | Squeaky pop sound | Hydrogen burns rapidly in oxygen, creating a small explosion |
Gas | Test | Positive Result | Why It Happens |
Oxygen (O₂) | Insert a glowing splint into the gas | Splint relights | Oxygen supports combustion |
Gas | Test | Positive Result | Why It Happens |
Carbon dioxide (CO₂) | Bubble the gas through limewater (calcium hydroxide solution) | Limewater turns milky/cloudy | A white precipitate of calcium carbonate forms |
Gas | Test | Positive Result | Why It Happens |
Chlorine (Cl₂) | Hold damp litmus paper in the gas | Litmus paper is bleached white | Chlorine is a bleaching agent and oxidises dyes |
Practice Questions
Question 1
A student places a glowing splint into a test tube of gas. The splint relights.
Which gas is present?
Answer:
Oxygen.
Question 2
Hydrogen is tested using a burning splint. What is the positive result and why?
Answer:
A squeaky pop because hydrogen burns rapidly in oxygen.
Summary
Hydrogen: squeaky pop with a burning splint
Oxygen: relights a glowing splint
Carbon dioxide: limewater turns milky
Chlorine: damp litmus bleached white
No answer provided.