Students learn that the respiration is a burning process, that produces carbon dioxide (besides  water).

 

Experiment # 1 

Materials: 2 beakers, bromothymol blue, a drinking straw, a glass rod, a syringe, a pipette

Procedure

1. Half fill each  beaker with water
2. Add into each beaker few millilitres of bromothymol blue and stir with the glass rod.  Write down in the first column of the table below  the colour of the solutions.
3. With the syringe inject fresh air in a beaker. Write down in the second column of the table below  the colour of the solution.
4. Blow through the straw into the second beaker.using strong breaths. until you notice a change. Write down in the third column of the table below  the colour of the solution.

 

What happens?

Bromothymol blue is a pH indicator;  its colour is blue in basic or neutral solutions, but in weak acidic solutions it turns  yellow.  

With the  syringe we inject  fresh air which doesn’t change the bromothymol blue colour, because its low carbon dioxide content doesn’t affect (or only slightly affects)   the pH of the solution.

The blue solution becomes yellow when exhaled air is added to the solution, because there is more carbon dioxide in exhaled air than in the fresh air.

Exhaled carbon dioxide produces carbonic acid in the water, so the neutral solution turns acidic (changing the pH value and turning yellow the bromothymol blue colour).

The reaction is:

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