Examples of acids include vinegar, citrus fruits, tomato juice, black coffee, bananas, milk, sulfuric acid, battery acid and hydrochloric acid, which is a digestive compound found in the stomach. Acids are ionic compounds made up of positively and negatively charged ions that separate in water to form hydrogen ions.
The concentration of hydrogen ions in an acidic solution determines its strength. The more hydrogen ions present, the stronger the acid. Acids have some similar characteristics and behaviors. They typically taste sour, and strong acids are highly reactive with metals.
The strength of an acid, or its level of acidity, is measured by using the pH scale. This is a method used to determine the concentration of hydrogen atoms relative to pure water. The scale ranges from 0 to 14, and each number represents a ten-fold change in hydrogen ion concentration. Pure water has a pH of 7, meaning it is neither acidic or basic. Any substance that has a pH below 7 is considered an acid. A basic substance is one that absorbs hydrogen ions instead of losing them like acids. When a base is dissolved in water, it is called an alkaline solution. Any substance with a pH measurement above 7 is considered a base.