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4.2.2Chemical Analysis

Oxygen

5 min read 5 parts Lesson 8 of 21 Unit quiz

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Section 1 of 5Carrying Out the Test

4.2.2 Oxygen

The glowing splint is prepared first, then placed directly into the gas being tested.

1
Light a wooden splint, then blow out the flame so that the tip of the splint still glows red.
2
Place the glowing splint inside the test tube of gas, so that the tip is in the gas.
3
If the gas is oxygen, the glowing splint relights and bursts back into flame.
4
Record the result as the splint relighting, which is the accepted evidence for oxygen.

Section 2 of 5Why the Splint Relights

Supports combustion
  • Oxygen is needed for combustion, so a sample with more oxygen makes a burning material burn more vigorously.
Glowing tip
  • The tip of the splint is already hot, so only a small extra supply of oxygen is needed to set it alight again.
Air is not enough
  • Air contains only about 21% oxygen, so a glowing splint does not relight in air and needs a richer sample.
Brightness
  • A splint that glows very brightly without fully relighting also shows oxygen, although relighting is the clearer result.

Section 3 of 5Recognising Oxygen

Recognising Oxygen
Positive result
  • A glowing splint that relights when placed inside the gas confirms that the gas is oxygen.
Test choice
  • A glowing splint is used for oxygen and a lit splint for hydrogen, so the two tests must not be confused.
Method detail
  • The splint is placed inside the tube, because the test depends on the gas being in contact with the glowing tip.
Role in air
  • Oxygen makes up about one fifth of air, and it is the part of air that allows fuels to burn.

Section 4 of 5Worked Example

Question

  • A test tube contains an unknown gas.
  • Describe how you would test it for oxygen and state the result that would confirm oxygen.

  1. Light a splint and blow out the flame so that the splint glows red at its tip.
  2. Insert the glowing splint into the test tube of the gas.
  3. Observe whether the splint relights.

Answer If the glowing splint relights, the gas is oxygen.

Section 5 of 5Exam Checklist & Tips

4.2.2 – Key Points for the Exam

  • Supports combustion A gas that allows other substances to burn more easily, such as oxygen.
  • Glowing splint A wooden splint blown out after lighting, so that only its tip is still red hot.
  • Relight To set a glowing splint alight again when it is placed in a gas that contains oxygen.
  • Combustion A reaction in which a substance reacts rapidly with oxygen, releasing energy.

Common Mistakes

  • Holding the glowing splint only at the mouth of the tube instead of placing it inside the gas.
  • Using a splint that is still flaming, which tests for hydrogen and gives a different result.
  • Claiming that a glowing splint relights in air, since air contains only about 21% oxygen.
  • Describing the splint as going out as the positive result, when relighting is the correct observation.
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