Question

A fresh sample of soda had a pH of 2.92. The soda was placed on a magnetic stirrer and made to go flat. The pH is measured again. Should the pH of the flat soda be higher, lower or the same as the pH of the fresh soda?

Answer 1

The pH of flat soda should be higher, as CO₂ left the solution. This is due to, as the solution is stirred vigorously, kinetic energy from magnetic stirrer warms up the solution and also the solution comes into more contact with the air surrounding it. As a result, since the concentration of CO₂ is higher in the solution than in the surrounding environment, the CO₂ will exit the solution and the pH will go up. Warming up any dissolved gases solution makes the dissolved gas escapes.

This means as CO₂ is removed, it is not available to act like a buffer and bind with H⁺, hence the free H⁺ decreases.

CO₂ + H₂O <==> H₂CO₃ (pH < 7)

H₂CO₃ + H₂O <==> HCO₃^- + H₃O⁺

HCO₃^- + H₂O <==> CO₃^2- + H₃O⁺

If CO₂ escapes only H₂O remains ( pH ~ 7)

Answer: The pH of flat soda should be higher.

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