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Units for Measurement
Fundamental Units
Derived Unit
Units and Dimensional Analysis: Conversion of Units
Matter
Classification of Matter
Elements
Compound
Mixture
Chemistry is the science of substances, their properties, structures and their transformation. As all objects in the universe are made of matter. Chemistry is the branch of the science which deals with the study of material object. Study of chemistry is very interesting which covers various aspects of our culture and environment. All development in any science are based on scientific approach as in chemistry too. In order to achieve correct results, one has to rely upon the various skills connected with the measurements of quantities during a physical or chemical change. The degree of accuracy is closely linked with precision of the measuring instrument as well as on the skill of the person engaged in measurement.
Physical Property: The property which can be measured without changing the chemical composition of the substance is known as physical property like mass, volume, density, refractive index etc.
Chemical Property: The property which can be evaluated at the cost of matter itself is known as chemical property. For example combustible nature of hydrogen gas can be verified by burning of hydrogen. The sweet taste of sugar by consuming it.
All physical quantities have to be measured. The value of a physical quantity is expressed as the product of the numerical value and the unit in which it is expressed.
Fundamental units are those units which can neither be derived from one another nor they can be further resolved into any other units. The seven fundamental units of measurement in S.I. system.
Quantity
Name of unit
Abbreviation
Mass
Kilogram
Kg
Length
Meter
m
Temperature
Kelvin
K
Amount of substance
Mole
Mol
Time
Second
S
Electric current
Ampere
A
Luminous intensity
Candela
Cd
Some quantities are expressed as a function of more than one fundamental units known as derived units. For example velocity, acceleration, work, energy etc.
Quantity with Symbol
Unit (S.I.)
Symbol
Velocity (v)
Metre per sec
ms-1
Area (A)
Square metre
m2
Volume (V)
Cubic metre
m3
Density (r)
Kilogram m-3
Kg m-3
Energy (E)
Joule (J)
Kg m2s-2
Force (F)
Newton (N)
Kg ms-2
Frequency (n)
Hertz
Cycle per sec
Pressure (P)
Pascal (Pa)
Nm-2
Electrical charge
Coulomb (C)
A-s (ampere – second)
The simplest way to carry out calculations that involve different units is to use dimensional analysis. In this method a quantity expressed in one unit is converted into an equivalent quantity with a different unit by using conversion factor which express the relationship between units:
Original quantity x conversion factor = equivalent quantity
(in former unit) (in other unit)
This is based on the fact that ratio of each fundamental quantity in one unit with their equivalent quantity in other unit is equal to one For example in case of mass
So 1 kg = 2.205 pond = 1000 gm
In this way any derived unit first expressed in dimension and each fundamental quantities like mass length time are converted in other system of desired unit to work out the conversion factor.
Question:
How unit of work / energy i.e. joule, in S.I. system is related with unit erg in C.G.S system
Solution:
Dimension of work = force x displacement = MLT-2´ L = ML2T-2
1 joule = 1 kg (1 metre)2´ (1see)-2
100 gm x (100)2 x1 em2 x (1 sec)-2
1000x10000 x 1 gm x1 cm2 x1 sec-1
1 joule = 107 erg
Similarly we can deduce other conversion factor for other quantity in different unit by the dimensional analysis method
Another interesting example is the conversion of litre – atmosphere to joule (the SI unit of energy) by multiplying with two successive unit factors. Thus,
Knowing that, we can write
Hence 1 L atm = 101.325 J
What is the mass of 1 L of mercury in grams and in kilograms if the density of liquid mercury is?
We know the relationship, and
Also,
We can write, mass = (volume) (density)
Therefore, the mass of 1 L of mercury is equal to
The mass in kilograms can be calculated as
(Remember, and are conversion factors with which we have to multiply for getting our answer in appropriate units).
Anything that exhibits inertia is known as matter. The quantity of matter is its mass. e.g. chalk table.
This classification of matter is based upon chemical composition of various substances. According to this matter can be further divided into two types, pure substance and mixture. Mixtures are also of two types, homogenous mixtures and heterogeneous mixtures.
The primary stuff present in all the substance is known as element, whose smallest unit is known as atom. Total 112 elements are known till date of which 92 are naturally occurring elements rest are results of artificial transmutation. There are 88 metals, 18 non metals and 6 metaloids.
A non elemental pure substance is called a compound in which more than one atom of elements are linked by chemical bonds formed due to chemical reaction. The resulting molecule is a electrically neutral particle of constant continuous composition.
Mixtures are the aggregate of more than one type of pure substance whose chemical identity remains maintained even in mixtures. Their constituent ratio may vary unlike compound. For example – sugar + water = sugar syrup, Gun-powder 75 % KNO3 10% sulphur + 15% carbon There are two types of mixture (a) homogeneous (b) heterogenous
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