Chemistry is based on observation and experimentation, trying to obtain a series of results that in many cases can be expressed numerically, allowing their comparison with those obtained in other experiences.

In all observation and experimentation we look at those properties that are susceptible of comparison and that, therefore, we can measure. These properties are called magnitudes.

There is a small group of magnitudes, called fundamental, from which all the others can be obtained. These magnitudes are: length, mass, time, current, temperature, amount of substance and luminous intensity.

The international system of units assigns a unit to each magnitude. The meter is the unit of length. If by measuring a distance, we obtain 20m means that the length is 20 times the unit of the international system.

The definitions of the fundamental units in the international system, in accordance with the corresponding resolutions of the General Conference of Weights and Measures (CGMP), are the following:

**1.- Unit of length. The meter**is the distance that travels the light in the vacuum in 1 / 299.792.486 seconds. Approximately 39.37 inches. A meter is divided into 100 centimeters and 1000 millimeters.

**2.- Unit of mass. The kilogram**is the mass of a cylinder and platinum and iridium stored in Sevres, near Paris, France. The pound has a mass of 0.4536 kilograms. The kilogram is divided into 1000 grams.

**3.- Unit of time. The second**is the duration of 9,192,631,770 times the period of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom 133.

**4.- Unit of current of electrical current. The ampere**is the intensity of a constant current which, held between two parallel, rectilinear conductors of infinite length, of negligible circular cross-section and placed at a distance of one meter from each other, in the vacuum, produces between these conductors a force of 2 10

^{-7}newton per meter in length.

**5.- Unit of thermodynamic temperature. The Kelvin**is the fraction 1 / 273.16 of the thermodynamic temperature of the triple point of water.

**6. Unit of amount of substance. The mole**is the amount of substance in a system containing as many elemental entities as there are atoms in 12 grams of carbon 12.

**7.- Unit of light intensity. The candela**is the luminous intensity, in the perpendicular direction, of an area of 1 / 600,000 square meters of a black body at freezing temperature of platinum under the pressure of 101,325 newtons per square meter.

Some magnitudes such as electric intensity, luminous intensity, are little used in chemistry.