What compound forms when CO₂ dissolves in water?

When carbon dioxide (CO₂) dissolves in water, it reacts with water molecules to form carbonic acid (H₂CO₃). This reaction occurs as follows: CO₂ combines with H₂O to yield H₂CO₃, which is a weak acid that partially dissociates in solution to produce bicarbonate (HCO₃⁻) and hydrogen ions (H⁺). This process is significant in various biological and environmental contexts, such as maintaining the pH of blood and in the carbon cycle.

Carbonic acid plays a critical role in buffering systems in the body, as it helps to regulate pH levels. This unique property makes the formation of carbonic acid especially important in biological systems and during processes like respiration, where CO₂ is produced as a metabolic byproduct.

The other options refer to different acids that do not form during the dissolution of CO₂ in water. For instance, hydrochloric acid is a strong acid formed from hydrogen chloride, acetic acid is the organic compound that gives vinegar its sour taste, and sulfuric acid is a highly corrosive strong acid that results from sulfur oxides reacting with water. None of these are produced when CO₂ interacts with water.