In chemistry, an acid is classified as a substance with distinctive properties. An acidic substance is sour-tasting; reacts with litmus paper, bases and metals; conducts electricity; and has a pH of less than 7. An acid can be classified as strong or weak based on its reactivity, conductivity and pH level.

The word "acid" comes from the Latin word for sour. This distinguishable property helps identify acids from other compounds such as salt and bases. Many acids can be hazardous if ingested and shouldn't be tasted. A few common acids found in grocery stores are lemon juice and vinegar.

The pH scale is used as a measure of how acidic or basic a substance is. This scale ranges from zero to 14; a pH of 7 means the substance is neutral. An acidic substance measures lower on the pH scale. A substance with a pH value of less than 7 is acidic.

Litmus paper strips are used as a general indicator to determine acidity. When dipped or rubbed on an acidic substance, blue litmus paper turns red. Because of this, litmus paper can be used to identify an acid. When a base reacts with litmus paper, there is no color change.

When an acid reacts with a base, it becomes a neutral substance. Sometimes this reaction can form water and salt. This is frequently seen when strong acids react with strong bases.

Acids react with metals to form metal salt and hydrogen. In this chemical reaction, the acid causes the metal to corrode. Not all metals react with acids, like platinum or gold. However, metals such as aluminum or zinc will corrode when they are put in contact with an acidic substance.

Acids conduct electricity. Because of this, they are used in batteries to generate electricity. An acid that strongly conducts electricity is a strong acid, and an acid that weakly conducts electricity is a weak acid.