When hydrogen fluoride is dissolved in water, the amount of hydrogen ion in the solution increases. Any substance that increases the H+ content of an aqueous solution is called an acid. An aqueous solution of HF molecules is known as hydrofluoric acid. Just as hydroxide ions can assist in chemical reactions by attacking molecules where they have a slight excess of positive charge, so hydrogen ions can attack local negative regions of molecules. Acids and bases both can speed up reactions that would take place very slowly or not at all in a neutral solution. Acids have a sharp taste, familiar in the acetic acid of vinegar and citric acid of lemons. It is not advisable ever to taste laboratory acids such as hydrofluoric, hydrochloric, or sulfuric acids, for they are so strong as to be dangerous. The most common laboratory acid-base indicator, litmus paper, is turned red by acids.