One likes to give away protons and the other likes to snatch them up

If a chemist tells you soapy water is basic, she isn't calling it simple. She's referring to the sodium hydroxide used to make soap; it is an alkaline (AL-kuh-lin) substance. Basic — or alkaline — describes properties of certain molecules in a solution. These substances are the opposite of acids — such as the citric, ascorbic and malic acids that give lemon juice its puckering sourness.

Throughout history, chemists have created different definitions of acids and bases. Today, many people use the Brønsted-Lowry version. It describes an acid as a molecule that will give away a proton — a type of subatomic particle, sometimes called a hydrogen ion — from one of its hydrogen atoms. At a minimum, that tells us that all Brønsted-Lowry acids must contain hydrogen as one of their building blocks.

Hydrogen, the simplest atom, is made up of one proton and one electron. When an acid gives away its proton, it hangs on to the hydrogen atom's electron. This is why scientists sometimes call acids proton donors. Acids will taste sour. The type in vinegar is known as acetic (Uh-SEE-tik) acid.

Its chemical formula can be written as either C2H4O2 or CH3COOH. Citric (SIT-rik) acid is what makes orange juice sour. Its chemical formula is a bit more complicated and is written as C6H8O7 or CH2COOH-C(OH)COOH-CH2COOH or C6H5O7(3−).

Brønsted-Lowry bases, in contrast, are good at stealing protons, and they'll gladly take them from acids. One example of a base is ammonia. Its chemical formula is NH3. You can find it in many window-cleaning products.

Scientists sometimes use another scheme — the Lewis system — to define acids and bases. Instead of protons, this Lewis definition describes what molecules do with their electrons. In fact, a Lewis acid doesn't need to contain any hydrogen atoms at all. Lewis acids only need to be able to accept electron pairs.

Different definitions are useful for different situations, explains Jennifer Roizen. She is a chemist at Duke University in Durham, N.C. "We use both definitions in my lab," Roizen says. "Most people use both. But a given application," she says, "may rely on one."

Water (H2O) is chemically neutral. That means it is neither an acid nor a base. But mix an acid with water and the water molecules will act as bases. They'll snag hydrogen protons from the acid. The altered water molecules are now called hydronium (Hy-DROHN-ee-um).

Mix water with a base and that water will play the part of the acid. Now the water molecules give up their own protons to the base and become what are known as hydroxide (Hy-DROX-ide) molecules.

To identify acids from bases, and the relative strength of each, chemists tend to use a pH scale. Seven is neutral. Anything with a pH below 7 is acidic. Anything with a pH above 7 is basic. One of the earliest tests to determine acids from bases was the litmus test. A chemical patch turned red for acids, blue for bases. Today chemists can also use pH indicator paper, which turns every color of the rainbow to indicate how strong or weak an acid or base is.

Acid: A chemical term for sour materials that have a pH below 7.0 (on a 14-point scale). Acids often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place. Chemically, acids are known for having the ability to either donate a proton (hydrogen ion) to another compound or to accept a pair of electrons. The first is known as a Brønsted-Lowry acid. Chemists refer to the second type as a Lewis acid.

Alkaline: An adjective that describes a chemical that can accept a proton from another molecule, or donate a pair of electrons. Alkaline solutions are also referred to as basic — as in the opposite of acidic — and have a pH above 7.

Ammonia: A colorless gas with a nasty smell. Ammonia is a compound made from the elements nitrogen and hydrogen. It is used to make food and applied to farm fields as a fertilizer. Secreted by the kidneys, ammonia gives urine its characteristic odor. The chemical also occurs in the atmosphere and throughout the universe.

Application: A particular use or function of something.

Atom : The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

Base : (in chemistry) A chemical that can accept protons from hydrogen molecules, or donate a pair of electrons. Basic solutions are also referred to as alkaline.

Chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

Electron: A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

Hydrogen : The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It's an integral part of many fuels, fats and chemicals that make up living tissues. It's made of a single proton (which serves as its nucleus) orbited by a single electron.

Hydrogen ion: A positively charged nucleus of a hydrogen atom. It consists of a single proton.

Hydronium: A hydrogen ion (H+) bound to a molecule of water (H2O). It becomes H3O+, and is the form in which hydrogen ions exist in water. It's also known as a hydrated hydrogen ion.

Hydroxide: Also known as hydroxide ion (OH-), it's a negatively charged ion. It's made from an atom of hydrogen bound to an atom of oxygen.

Ion: (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.

Litmus test: A simple chemical test used to determine whether some solution is an acid or base. Blue litmus paper turns red for acids and red litmus paper turns blue for bases. It was initially developed in the 1300s using compounds isolated from lichens.

Molecule: An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2); water is made of two hydrogen atoms and one oxygen atom (H2O).

Particle: A minute amount of something.

pH: A measure of a solution's acidity or alkalinity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, also called bases, have a pH higher than 7; the farther above 7, the stronger the base.

Proton: A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

Sodium hydroxide: A chemical that is used in the production of paper and soap. It is used to make solutions more basic (or alkaline).

Subatomic: Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

Taste: One of the basic properties the body uses to sense its environment, especially foods, using receptors (taste buds) on the tongue (and some other organs).

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