When a substance is dissolved in water, the solute can either accept or donate protons, which are also known as hydrogen ions. This process is called protonation or deprotonation and the ratio of proton acceptors to donors determines the degree of acidity or alkalinity. In an aqueous solution, the hydrogen ion concentration is determined by a number of factors such as temperature, pressure, and the concentration of other ions present in the solution.
When discussing hydrogen ions in aqueous solution, it is important to understand the concept of pH. pH is a measure of acidity or alkalinity and is expressed on a scale of 0 to 14. A pH value of 7 is neutral, values greater than 7 are alkaline and values less than 7 are acidic. The higher or lower the pH value, the greater or lesser the concentration of hydrogen ions present in the solution.
So, what substance produces hydrogen ions in aqueous solution? The answer is that any water-soluble acid can donate proton ions to the solution, thus increasing the concentration of hydrogen ions. Examples of acids include hydrochloric acid, nitric acid, sulfuric acid and carbonic acid. These substances are often referred to as proton donors as they give off hydrogen ions when dissolved in water.
It should also be noted that some substances can act as both proton acceptors and donors depending on the pH level of the solution. These substances are known as amphiprotic and include common substances such as baking soda and table salt.