An acid is a molecular compound that consists of at least one hydrogen atom and when it is dissolved in water, it produces hydrogen ions. Since acids are molecular, in their pure state, their structure consists of individual molecules. However, when dissolved in water, the chemical bond shared between the hydrogen atom and the remainder of the molecules break. This leaves a hydrogen ion that is positively charged and an anion. Acids are essential to many chemical processes. Reactions involving acids are determined by the transfer of protons or single H+ ions. The acidity of a solution depends upon the concentration of hydrogen ions.