Which of the following describes a characteristic of an acid when dissolved in water?

When an acid is dissolved in water, it undergoes a process known as dissociation, where it loses protons (H⁺ ions) to form hydrogen ions in the aqueous solution. This characteristic is fundamental to the definition of acids according to the Brønsted-Lowry theory, which states that an acid is a substance that donates protons to a base. The generation of hydrogen ions is what leads to the characteristic sour taste of acids and their ability to lower the pH of a solution, making it more acidic.

In contrast, the other options do not describe characteristics of acids. The first option relates to bases, which release hydroxide ions (OH⁻) when dissolved in water. The third option suggests that the acid remains unreacted, which contradicts the behavior of acids in water where they actively participate in chemical reactions. Lastly, the fourth option implies that the acid does not have an effect on the pH of the solution, which is inaccurate since the release of hydrogen ions from acids directly decreases the pH and increases acidity. Consequently, the correct characteristic of an acid in water is its ability to lose protons, forming hydrogen ions.