base

Four different numerical systems showing the numbers 1–16, with so, some examples of greater numbers. In the hexadecimal (base 16) system, all numbers up to 15 must be represented by a single character. To achieve this the decimal values 10–15 are represented by the letters A–F.

In mathematics, the number of different single-digit symbols used in a particular number system. In our usual (decimal) counting system of numbers (with symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, 9) the base is 10. In the binary number system, which has only the symbols 1 and 0, the base is two. A base is also a number that, when raised to a particular power (that is, when multiplied by itself a particular number of times as in 10² = 10 x 10 = 100), has a logarithm equal to the power. For example, the logarithm of 100 to the base ten is 2.

In geometry, the term is used to denote the line or area on which a polygon or solid stands.

base

In chemistry, a substance that accepts hydrogen ions, or protons. A base reacts with an acid, neutralizing it to form a salt: acid + base → salt + water. Metal oxides and metal hydroxides are bases; examples include copper oxide and sodium hydroxide. Bases can contain negative ions such as the hydroxide ion (OH⁻), which is the strongest base, or be molecules such as ammonia (NH₃). Ammonia is a weak base, as only some of its molecules accept protons.

OH⁻ + H⁺_(aq) → H₂O_(l)

NH₃ + H₂O → NH₄⁺ + OH⁻

Bases that dissolve in water are called alkalis.

Inorganic bases are usually oxides or hydroxides of metals, which react with dilute acids to form a salt and water. Many carbonates also react with dilute acids, additionally giving off carbon dioxide.