Measurement: Standard Units

In science, measurement is the assignment of a numerical value to a characteristic, like the mass, size, and temperature of the matter.

Every measurement provides two kinds of information. The first is the number – the size or magnitude of the measurement. The second is the unit – a standard of comparison for the measurement.

So, when water boils at 100 °C, the number 100 represents the magnitude of measurement, and °C represents the unit. Together, they convey the relative size of the property of the matter in question – the boiling point.

Without units, a number can be meaningless or confusing. For example, if the height of the Eiffel Tower is reported to be 300, without providing units the context of this 300 number is unclear. This means the Eiffel tower could be relatively tall, or very short, but without the units to compare, this makes that difficult to assess.

Sometimes, a third form of information, error, is also provided. These data are an indication of the uncertainty of the measurement.

Thus, in order to measure and report the various properties of matter accurately, common systems of measurement have been developed. The International System of Units or the SI system is the one used by scientists universally.

Under the SI system, measurement units for 7 fundamental properties — length, mass, time, temperature, electric current, amount of substance, and luminous intensity — have been fixed. These are called the Standard Units, or the base units. Of these, the base units for the first four properties are the most commonly used in chemistry.

The standard unit of length in the SI system is the meter, represented by the unit symbol “m”. Longer distances are often reported in kilometers, or “km”, where 1 kilometer is equal to 1000 meters. Conversely, shorter distances can be reported in centimeters, or “cm”, where 1 centimeter is equal to 0.01 meters; or millimeters, “mm”, where 1 millimeter is equal to 0.001 meters.

Similarly, the standard unit of mass in the SI system is the kilogram, represented by the unit symbol “kg”. A kilogram is equal to 1000 grams.

Temperature is another fundamental property, with the SI unit kelvin, and the unit symbol “K” in uppercase. This is displayed without the word “degree” or the symbol degree, to distinguish it from degrees Celsius and degrees Fahrenheit. For example, water boils at 373.15 K, and normal human body temperature is approximately 310 K.

The fourth most commonly used base unit is second, the SI unit of time, with the unit symbol “s”. While shorter time intervals can be expressed as milliseconds, where 1 millisecond is equal to 0.001 seconds, longer time intervals can be expressed as megaseconds where 1 megasecond is equal to 1 million seconds. Alternatively, non-SI units like hours, days, and years are also used.