Which chemical substance is known to ionize in water and produce hydrogen ions?

The correct choice is the one that identifies a substance that can donate hydrogen ions (H⁺) when dissolved in water. This property characterizes acids. When an acid ionizes in water, it releases hydrogen ions, which contribute to the acidity of the solution and can lower the pH.

For context, bases decrease the concentration of hydrogen ions in a solution by either accepting them or releasing hydroxide ions (OH⁻), leading to an increase in pH. Chlorine, while a reactive element often used for disinfection, does not exhibit the same ionization characteristic as acids do in the context of producing hydrogen ions in water. Alkaline substances are often associated with bases, contributing to higher pH levels rather than forming hydrogen ions.

Therefore, acids are the substances that ionize in water producing hydrogen ions, making this identification essential for understanding concepts related to pH, acidity, and chemical behavior in aqueous solutions.