To understand what an acid is, it’s important to know what protons and electrons are.

Atoms are made of positively charged protons and negatively charged electrons. Something is an acid if when you add it to water, it has a “loose” proton which is peeled off by the water molecules. For a substance that’s really acidic like hydrochloric acid, the water molecules pretty much take of its loose protons. When another chemical comes into contact with water that has an extra proton, it may take the proton back in a chemical reaction. When something rusty is dipped into water with an acid in it, the excess protons on the water will basically pull the rust off the metal. Acids are also really important in biology, for instance, in your stomach is really acidic which helps pull apart the food you eat into smaller parts the body can use. Acids can also be dangerous like if you get them on your skin, they will react with the molecules that make up your skin and damage it.

Substances can be classified as acidic, basic or neutral depending on their position on the pH scale.
The pH scale is 0-14.
A pH of 0-7 is acidic and a pH of 7-14 is basic.
A pH of exactly 7 is neutral.

Most acids contain the element, hydrogen. This is not a strict rule in chemistry, but it is true for most substances that we call acids. The pH scale is a measure of the “power of hydrogen,” and it describes the relative acidity of difference substances.

Acids are “corrosive,” which means they dissolve metal and also can burn your skin or eyes if it is strong enough (very low pH). Our bodies handle small amounts of weak acids (moderate pH) like vinegar or lemon juice. These foods have a sour taste, which is characteristic of acids.

An acid is any chemical compound that can dissociate into an anion (a molecule with a negative charge) and a positively-charged proton, when dissolved in a solvent (like water). Basically, it's a salt, but the cation is a proton instead of a larger cation.

An acid is a molecule that in solution will produce hydrogen ions (H+). In contrast, a base is a molecule that will produce hydroxide ions (OH-). An acid can be identified by it's physical and chemical properties. For example, acids taste sour, they have a pH less than 7, and they produce hydrogen gas under a chemical reaction with a metal. If you had orange juice this morning, you drank an acid. Other acids include lemons and vinegar.