Testing for Oxygen and Carbon Dioxide

Introduction

Oxygen and carbon dioxide are both colourless, odourless gases. A test tube full of one looks exactly like a test tube full of the other, or like a tube of ordinary air. So how can a chemist tell which gas has been collected? The answer is a chemical test : a quick reaction that gives a clear, visible result for one gas and not for others. Two of the most famous tests in school chemistry are the glowing splint test for oxygen and the limewater test for carbon dioxide.

Core explanation

Why we need tests. Our senses cannot identify most gases. Colour, smell and appearance do not help with oxygen or carbon dioxide, so chemists use a property that shows up clearly and belongs to only one gas.

The test for oxygen. A wooden splint is lit, allowed to burn briefly and then blown out so that the tip is still glowing red. The glowing tip is placed into the mouth of a test tube of the gas.

- Positive result: the glowing splint relights — it bursts back into flame. - Why it works: burning is a reaction with oxygen. Air contains only about 21% oxygen, which is not enough to restart a flame from a glowing ember. Pure or nearly pure oxygen is about five times more concentrated, so the hot wood reacts fast enough to flare up again.

The test for carbon dioxide. The gas is bubbled through limewater , a clear, colourless solution of calcium hydroxide, Ca(OH)₂.

- Positive result: the limewater turns milky (cloudy white). - Why it works: carbon dioxide reacts with calcium hydroxide to make calcium carbonate, which is insoluble in water. Tiny particles of solid calcium carbonate form a precipitate , scattering light and making the liquid look milky.

calcium hydroxide + carbon dioxide → calcium carbonate + water

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Do not confuse the tests. Carbon dioxide puts out a lighted splint, but so do nitrogen, argon and many other gases. Putting out a flame is therefore not a test for carbon dioxide — only limewater is specific enough. Similarly, a lighted (flaming) splint that gives a squeaky pop is the test for hydrogen , not oxygen. The oxygen test uses a glowing splint.

Safety at a conceptual level. These tests are carried out with small amounts of gas, eye protection and care with the flame. Limewater is an alkali and irritates the eyes, and oxygen makes anything burn more fiercely, so it is kept away from oils and flammable materials.

Formulae

Limewater test: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). The state symbol (s) on calcium carbonate shows it is the solid precipitate that causes the milkiness.

Step-by-step reasoning

To identify an unknown colourless gas that might be oxygen or carbon dioxide:

1. Collect two small samples of the gas. 2. Place a glowing splint in the first sample. If it relights, the gas is oxygen. 3. Bubble the second sample through limewater. If the limewater turns milky, the gas is carbon dioxide. 4. If neither test is positive, the gas is something else, and further tests are needed.

Visual explanation

Imagine two side-by-side pictures. On the left, a smouldering orange splint tip enters a test tube and suddenly a bright yellow flame leaps up. On the right, gas bubbles rise through a tube of clear liquid which gradually turns white and cloudy, like watered-down milk.

Real-world analogy

A glowing ember in a campfire looks almost dead, but if you blow on it gently it glows brighter and may burst into flame. Blowing supplies extra air, and so extra oxygen. Pure oxygen is like blowing with an extremely rich supply, so the splint catches light at once.

Real-world example

You can show that you breathe out carbon dioxide by gently blowing through a straw into limewater: after a short time it turns milky. The air you breathe in contains only about 0.04% carbon dioxide, while exhaled air contains roughly 4%, so exhaled air turns limewater milky much more quickly.

Why?

Why does a glowing splint relight in oxygen but not in air? Air is about 78% nitrogen, which does not support burning and dilutes the oxygen. In pure oxygen, far more oxygen molecules collide with the hot wood every second, so the reaction speeds up enough to produce a flame.

Common misconception

"Any gas that puts out a flame must be carbon dioxide." Many gases, including nitrogen and argon, put out flames because they do not support burning. Only a limewater test confirms carbon dioxide.

Worked example

Question: A student heats a solid and collects a colourless gas. A glowing splint does not relight in the gas, but when the gas is bubbled through limewater the limewater turns milky. Identify the gas and write a word equation for the test.

Reasoning: The glowing splint result rules out oxygen. The limewater result is the positive test for carbon dioxide.

Answer: The gas is carbon dioxide. Calcium hydroxide + carbon dioxide → calcium carbonate + water.

Quick check

1. What is the positive result of the test for oxygen? Answer: A glowing splint relights (bursts back into flame).

Exam focus

Always name the splint correctly: glowing splint relights for oxygen, lighted splint gives a squeaky pop for hydrogen. For carbon dioxide write "limewater turns milky (or cloudy)", not simply "changes colour". Marks are often given for naming calcium carbonate as the precipitate.

Advanced insight

If carbon dioxide is bubbled through limewater for a long time, the milkiness disappears again. Excess carbon dioxide reacts with the calcium carbonate and water to form calcium hydrogencarbonate, Ca(HCO₃)₂, which is soluble. This same reaction happens naturally when rainwater containing dissolved carbon dioxide passes through limestone, and it is one cause of hard water.

Summary

Oxygen and carbon dioxide cannot be identified by sight or smell, so chemical tests are used. Oxygen relights a glowing splint because it is concentrated enough to restart burning. Carbon dioxide turns limewater milky because it forms a precipitate of insoluble calcium carbonate. Putting out a flame is not a test for carbon dioxide, and the lighted splint pop test is for hydrogen.

Practice questions

1. Describe how you would show that a gas is oxygen, including the result. Answer: Insert a glowing splint into the gas; if the splint relights, the gas is oxygen. 2. Name the chemical in limewater and the solid that makes it turn milky. Answer: Limewater contains calcium hydroxide; the milkiness is caused by a precipitate of calcium carbonate. 3. Why is "it puts out a lighted splint" not a good test for carbon dioxide? Answer: Other gases, such as nitrogen and argon, also put out a flame, so the result is not specific to carbon dioxide. 4. Explain why exhaled air turns limewater milky faster than inhaled air. Answer: Exhaled air contains about 4% carbon dioxide compared with about 0.04% in inhaled air, so more carbon dioxide reaches the limewater each second. 5. Write a balanced symbol equation for the limewater test. Answer: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).