Question

Pure water is kept in a vessel and it remains exposed to atmospheric ${\mathrm{CO}}_2$ which is absorbed. Then the $\mathrm{pII}$ will be (1) greater than 7 (2) less than 7 (3) 7 (4) depends on ionic product of water

Short answer

The pH will be less than 7.

Step-by-step solution

Understanding the Absorption of ${\text{CO}}_2$

${\text{CO}}_2$ from the atmosphere dissolves into the water.

Forming Carbonic Acid

${\text{CO}}_2$ reacts with water to form carbonic acid: $${\text{CO}}_2+{\text{H}}_2\text{O}\to {\text{H}}_2{\text{CO}}_3$$

Dissociation of Carbonic Acid

Carbonic acid (${\text{H}}_2{\text{CO}}_3$) dissociates partially into hydrogen ions (${\text{H}}^{+}$) and bicarbonate ions (${\text{HCO}}_3^{-}$): $${\text{H}}_2{\text{CO}}_3\rightleftharpoons {\text{H}}^{+}+{\text{HCO}}_3^{-}$$ This increases the concentration of ${\text{H}}^{+}$ ions in the water.

Effect on pH

An increase in ${\text{H}}^{+}$ ions lowers the pH of the solution.

Conclusion

Since the pH decreases with the absorption of ${\text{CO}}_2$, the pH will be less than 7.

Key concepts

carbonic acid formation

When atmospheric CO2 comes into contact with water, it dissolves in it. This dissolution forms a new compound known as carbonic acid. The chemical reaction for this process is: CO2 + H2O → H2CO3 Carbonic acid (H2CO3) is weak and breaks down less readily into its components compared to strong acids. However, its formation marks the initial step in the series of reactions that will alter the pH of the water.

acid-base equilibrium

Once carbonic acid forms, it doesn’t stay as a single molecule. It partially dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). This chemical reaction is: H2CO3 ⇌ H+ + HCO3- This reaction is an example of acid-base equilibrium. The dissociation is not complete, meaning only a fraction of the H2CO3 splits. The presence of H+ ions is crucial as it directly influences the pH of the solution. Conditions such as temperature and the initial concentration of carbonic acid can affect the balance of this dissociation.

atmospheric CO2 absorption

Atmospheric CO2 is continuously absorbed by water surfaces. This absorption increases as the concentration of CO2 in the air rises. Oceans and lakes are prime examples where such CO2 absorption takes place. When CO2 is taken up by water, it initiates the sequence of chemical reactions leading to carbonic acid formation. Factors like wind, temperature, and water movement can impact the rate of CO2 absorption. Once CO2 dissolves, it can change the chemical environment of the water significantly.

water pH alteration

The final outcome of the absorbed atmospheric CO2 and the subsequent formation and dissociation of carbonic acid is the alteration of water pH. The increase in hydrogen ions (H+) from dissociated carbonic acid reduces the pH, making the water more acidic. The pH scale ranges from 0 to 14, with 7 being neutral. When H+ concentration increases, pH falls below 7, indicating an acidic environment. Key points to remember:

• Pure water has a neutral pH of 7.
• CO2 absorption leads to carbonic acid formation and H+ release.
• Increased H+ concentration means lower pH (pH < 7).

This explains why water exposed to atmospheric CO2 will have a pH less than 7.