The fresh distilled water has a pH of 7. After it was left standing for a short time the pH was observed to have dropped to below 7. Which equation best explains the phenomenon ?
a) HCl-----------> H + +. Cl -
b) H2O -------------> H+ +. OH-
c) CO2 + H2O-----------------> CO3 2- + 2H+
d) NH3 + H2O -----------------> NH4 + + OH-

The fresh distilled water has a pH of 7 which is represented by equation (b).
After it was left standing for a short time the pH was observed to have dropped to below 7. Means pH becomes acidic after sometime. It is best explained by equation (c).
Equation (c) increases the concentration of H+ ions on reaction with atmospheric CO2. That lowers the pH of water below 7.

 

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