Question

Which of the following chemical species will dominantly exist in water having a pH of less than 5?

• A. HCO₃⁻
• B. CO₃²⁻
• C. H₂CO₃
• D. Free CO₂

Hint:

The pH level determines the dominant form of carbon species in water: H2CO3 at low pH, HCO^− 3 at neutral pH, and CO2− 3 at high pH.

Answer 1

The Correct Option is C

At a pH of less than 5, the water is acidic. Under these conditions, carbonic acid (H₂CO₃) is the dominant species because the equilibrium shifts towards the protonated form of carbon dioxide in water:

CO₂ + H₂O ⇌ H₂CO₃

Bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) are predominant only at higher pH levels.