Question

A student breathes into BTB solution and gets a yellowish coloured solution. What conclusions can you draw about:-

i) The intensity of the activity the student may have been doing.

ii) Explain the reason for the change in the BTB solution.

Answer 1

Explanation :-

Bromomethyl Blue i.e. BTB is a synthetic indicator. It is blue in colour which changes into yellow in acidic medium, blue in basic and green in a neutral medium.

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(i) When the student breathes out, CO₂ (carbon-dioxide) gas is given out which changes the colour of BTB from blue to yellow. The more intensely he breathes, the more yellow the solution becomes.

(ii) The reason behind it is, the exhaled CO₂ dissolves in water to form carbonic acid which lowers pH of the solution below 7.

As the solution turns acidic, the colour gradually changes from blue to yellow.

CO₂ + H₂O ⇌ H₂CO₃