Acids are chemical compounds that release hydrogen ions (H+) when placed in water. For example, when hydrogen chloride is placed in water, it releases its hydrogen ions and the solution becomes hydrochloric acid.
Bases are chemical compounds that attract hydrogen atoms when they are placed in water. An example of a base is sodium hydroxide (NaOH). When this substance is placed in water, it attracts hydrogen ions, and a basic (or alkaline) solution results as hydroxyl (–OH) ions accumulate.