Gases in our breath in Respiration In Organisms – Class 7 Science Experiment

Chapter Name: Respiration In Organisms

Activity Name: Gases in our breath in Respiration In Organisms

Activity Description:

Gases in our breath in Respiration In Organisms - Class 7 Science Experiment
Gases in our breath in Respiration In Organisms - Class 7 Science Experiment

In this experiment, we will investigate the gases present in our breath by using an apparatus with two boiling tubes filled with phenolphthalein solution. By repeatedly blowing in and sucking out air through the apparatus, we will observe changes in the indicator solution and analyze the flow of air.

Required Items:

  1. Apparatus as shown in the Figure (not provided)
  2. Phenolphthalein solution
  3. Glass tubes
  4. Cork
  5. Teacher’s assistance

Step by Step Procedure:

  1. Set up the apparatus as shown in the Figure, making sure to handle the glass tube with care while inserting it into the cork.
  2. Fill both boiling tubes to one fourth of their volume with phenolphthalein solution and label them as A and B.
  3. Repeatedly blow air into the apparatus and then suck it out. Observe the movements of the indicator solution in the boiling tubes during the process.

Experiment Observations:

  • When you suck in air, the air flows into the apparatus through one of the boiling tubes (let’s say tube A). You can find this out by observing the movement of the indicator solution in both tubes – the level of solution in tube A will rise while in tube B, it will remain relatively stable or may even decrease slightly.
  • When you blow air out, the air flows out of the apparatus through the other boiling tube (tube B). The reason the air does not go out through the other boiling tube (tube A) is due to the pathway created by the apparatus and the setup of the glass tubes.
  • The color of the indicator solution changes in the boiling tube (tube B) through which the air flows out of the apparatus. This indicates a change in the composition of the air during expiration.
  • Inspired air and expired air are not entirely similar. There are differences between them, which might include variations in the concentration of gases like oxygen, carbon dioxide, and possibly other trace gases.


  1. Handle the glass tube with care to avoid breakage.
  2. Conduct the experiment under the guidance of a teacher or responsible adult.
  3. Dispose of the phenolphthalein solution properly after the experiment.

Lesson Learnt from Experiment:

This experiment helps students understand the concept of respiration and the exchange of gases during breathing. It demonstrates how the composition of inhaled and exhaled air can differ, particularly in terms of oxygen and carbon dioxide levels.

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