Measurement and units

Measurement is an important part of life as well as science. In day to day life we use measurement for various purposes. When we go to a tailor, we describe length and width of cloth using a meter tape. A green-grocer uses weights for weighing vegetables and fruits and gives us our wanted quantity of them. A doctor needs a  thermometer to know the temperature of body of a patient and he advises the patient medical remedies. Similarly in chemistry, measurement has a great importance. In fact in chemistry it needs to be precise and accurate.For making pesticides and insecticides methyl bromide should be mixed with a great accuracy. If it is mixed more than its need, then it will destroy ozone layer. So measurement must be precise and accurate when reported. Let's begin with our topic.

Measurement and its unit

Measurement means comparison of physical quantity with some standard quantity. Standard quantity is an unit which has definite meaning. For example if the length of cloth is 5 m then it means 1m length can be placed fives times to the cloth. Here '5' is the quantity while 'm' (metre) is the unit. Similarly if vegetables' weight is 2 kg then it means vegetables weigh 2 times the 1 kg standard weight. Without unit, measurement is meaning less. So, unit must be mentioned with the quantity of measurement. For example 400 m race is meaningful rather than 400 race.

SI System

Suppose all nations would have their own measuring system then how difficult it would have been become for scientist s to conduct their researches and observation to scientists belonging to other nations. Therefore a need arose for such a measuring system which should be accepted globally. SI System is such a system invented by French in 1960. It has seven base units from which other units have been derived. Let's see them form table 2.1.

Measurement of :-	Name of unit	Abbreviation
Length	meter	m
Mass	kilogram	kg
Time	second	s
Temperature	kelvin	K
Amount of substance	mole	mol
Electric current	ampere	A
Luminous intensity	candela	cd

From these seven base units chemist often need only four of them. Below are some guidelines for writing the names and symbols of units.

1. Units named after a scientist's name always start with a small letter. For example SI unit of temperature is written as 'kelvin' not 'Kelvin'.
2. Symbol of units named after scientists are always written with a capital letter for example symbol of kelvin is 'K'.
3. Symbols and name of units which are not named after scientist's name are always written with small letter. For example; meter = m, kilogram= kg etc.
4. Symbol of units can not be written in plural form. For example '5 kg' is correct while '5 kgs' is wrong.
5. Full stop is not used with abbreviations of units. For example length of cloth is 5 m and width is 2m. (Note:- Full stop is used with the '2 m.' in the example due to grammatical rule.)