Some basic concepts of chemistry : Some basic concepts of chemistry Contents
Base quantities and their units
Definition of SI base units
Prefixes used in SI system
Mass and weight
Uncertainty in measurements
Scientific notation Base quantities and their units : Base quantities and their units Definitions of SI Base quantities and their units : Definitions of SI Base quantities and their units Length : Length Definition : To measure the distance between any two points in space we use the term length and to specify the magnitude we use the unit of length
Base Unit of length-metre:
In 1799, legal standard of length became metre which was defined as one tenth-millionth the distance from the equator to the north pole.
In 1960, the length of metre was the distance between two lines on a specific platinum iridium stored condition.
Recently the metre was defined as 1650,763.73 wavelength of orange red light emitted from a krypton-86.however in October 1983, the metre was redefined as the distance travelled by light in vacuum during a time of 1/299792458 second. Mass : Mass Definition : mass of a substance is the amount of matter present in it. It is a basic property of a substance.
Base unit of mass-kilogram:
The kilogram is equal to the mass of the international prototype of the kilogram(a platinum-iridium alloy cylinder) kept at international bureau of Weights and measures at Sevres, near Paris, France.
This mass standard was established in 1887,and there has been no change since that time because platinum-iridium is an unusually stable alloy.
A duplicate is kept at the National Institute of Standards and Technology in Gaithersburg. Electric current : Electric current Definition: Current, flow of electric charge. The electric charge in a current is carried by minute particles called electrons that orbit the nuclei of atoms. Each electron carries a small electric charge. When a stream of electrons moves from atom to atom—for example, inside a copper wire—the flow of the charge they carry is called electric current. Batteries and generators are devices that produce electric current to power lights and other appliances. Electric currents also occur in nature—lightning being a dramatic example
Base unit of Electric current-Ampere:
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length ,of negligible circular cross-section ,and placed 1meter apart in vacuum ,would produce between these conductor a force equal to 2 x 10⁻⁷Newton per metre of length. Time : Time Definition: Time, conscious experience of duration, the period during which an action or event occurs. Time is also a dimension representing a succession of such actions or events
Base unit of time-second:
Before 1960, second was defined as (1/60)(1/60)(1/24) of a mean solar day.
In 1967, the definition of second was modified using the characteristics frequency of a particular kind of caesium atom as the “reference clock”.
The second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of ground state of the caesium -133 atom. Thermodynamic temperature : Thermodynamic temperature Definition: Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Temperature, in physics, property of systems that determines whether they are in thermal equilibrium. Thermodynamic temperature is an “absolute” scale because it is the measure of the fundamental property underlying temperature: its null or zero point, absolute zero, is the temperature at which the particle constituents of matter have minimal motion and can be no colder.
Base unit of thermodynamics-kelvin:
The kelvin unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of triple point of water Amount of substance : Amount of substance Base unit-mole:
The mole is the amount of substance of a system which contains as many elementary entities as there are in 0.012 kilogram of carbon-12;its symbol is “mol”.
When the mole is used, the elementary entities must be specified and may be atoms ,molecules, ions, electrons, other particles, or specified group of such particles.
The first usage in English dates from 1897, in a work translated from German. The names gram-atom and gram-molecule have also been used in the same sense as "mole", but these names are now obsolete.
A mole has 6.0221415×1023 atoms or molecules of the pure substance being measured. A mole will possess mass approximately equal to the substance's molecular/atomic weight in grams. Because of this, one can measure the number of moles in a pure substance by weighing it and comparing the result to its molecular/atomic weight. Luminous Intensity : Luminous Intensity Definition: amount of light in particular direction; the amount of light emitted by a source in a particular direction.Symbol : IV
Base unit of luminous intensity- Candela:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 X 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The candela (symbol: cd) is the power emitted by a light source in a particular direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelengths, also known as the luminous efficiency function[). A common candela emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.
The definition describes how to produce a light source that (by definition) emits one candela. Such a source could then be used to calibrate instruments designed to measure luminous intensity.
The candela is sometimes still called by the old name candle , such as in foot-candle and the modern definition of candlepower. Prefixes used in SI system : Prefixes used in SI system Contd. : Contd. Mass and Weight : Mass and Weight In practical or everyday applications, weight means the same as mass as that term is used in physics. In modern scientific usage, however, weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, whereas weight is a force that results from the action of gravity on matter: it measures how strongly gravity pulls on that matter.
However, the recognition of this difference is historically a relatively recent development and in many everyday situations the word "weight" continues to be used when, strictly, "mass" is meant. For example, most people would say that an object "weighs one kilogram", even though the kilogram is a unit of mass
The mass of a substance is constant whereas the weight may differ form place to place due to the change in gravity.
The distinction between mass and weight is unimportant for many practical purposes because the strength of gravity is very similar everywhere on the surface of the Earth. In such a constant gravitational field, the gravitational force exerted on an object (its weight) is directly proportional to its mass. For example, object A weighs 10 times as much as object B, so therefore the mass of object A is 10 times greater than that of object B. This means that an object's mass can be measured indirectly by its weight Volume : Volume The volume of any solid, liquid, gas, plasma, theoretical object, or vacuum is how much three-dimensional space it occupies, often quantified numerically. One-dimensional figures (such as lines) and two-dimensional shapes (such as squares) are assigned zero volume in the three-dimensional space. Volume is commonly presented in units such as cubic meters, cubic centimeters, litres, or millilitres.
The SI unit for volume is cubic metre. A common unit which is used for measurements of liquids
Volume can be measured with the help of:
buoyant weight (solids)
overflow trough (solids)
Measuring cup (grained solids, liquids)
Flow measurement devices (liquids)
Graduated cylinder (liquids)
Eudiometer, pneumatic trough (gases) Density : Density The density of a material is defined as its mass per unit volume. The symbol of density is ρ (the Greek letter rho).
where ρ (rho) is the density, m is the mass, V is the volume.
Different materials usually have different densities, so density is an important concept regarding buoyancy, metal purity and packaging.
In some cases density is expressed as the dimensionless quantities specific gravity (SG) or relative density (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas. Temperature : Temperature In physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature
Temperature changes have to be measured in terms of other property changes of a substance. Thus, the conventional mercury thermometer measures the expansion of a mercury column in a glass capillary, the change in length of the column being related to the temperature change.
Celsius temperature scale , temperature scale according to which the temperature difference between the reference temperatures of the freezing and boiling points of water is divided into 100 degrees. The freezing point is taken as 0 degrees Celsius and the boiling point as 100 degrees Celsius. The Celsius scale is widely known as the centigrade scale because it is divided into 100 degrees. It is named for the Swedish astronomer Anders Celsius , who established the scale in 1742. William Thomson Kelvin used it as the basis of his absolute temperature scale, now known as the Kelvin temperature scale , in 1848 .Temperatures on the Celsius scale can be converted to equivalent temperatures on the Fahrenheit temperature scale by multiplying the Celsius temperature by 9/5 and adding 32° to the result, according to the formula 9 C /5+32= F.
it is interesting to note that temperature below zero degree Celsius(i.e negative values )are not possible Uncertainty in measurements : Uncertainty in measurements In metrology, measurement uncertainty describes a region about an observed value of a physical quantity which is likely to enclose the true value of that quantity. Assessing and reporting measurement uncertainty is fundamental in engineering, and experimental sciences such as physics.
Measurement uncertainty may be denoted by error bars on a graph, or by the following notations:
measured value ± uncertainty
Measurement uncertainty is related with both the systematic and random error of a measurement, and depends on both the accuracy and precision of the measurement instrument. The lower the accuracy and precision of a measurement instrument are, the larger the measurement uncertainty is. Notice that both precision and measurement uncertainty are often determined as the standard deviation of the repeated measures of a given value. However, this is correct only when the instrument is accurate. When it is inaccurate, the uncertainty is larger than the standard deviation of the repeated measures. Scientific notation : Scientific notation Scientific notation, also known as standard form or as exponential notation, is a way of writing numbers that accommodates values too large or small to be conveniently written in standard decimal notation. Scientific notation has a number of useful properties and is often favored by scientists, mathematicians and engineers, who work with such numbers.
In scientific notation all numbers are written like this:
a × 10b
("a times ten to the power of b"), where the exponent b is an integer, and the coefficient a is any real number (but see normalized notation below), called the significand or mantissa (though the term "mantissa" may cause confusion as it can also refer to the fractional part of the common logarithm). If the number is negative then a minus sign precedes a (as in ordinary decimal notation). Thank you : Thank you Made by: Abhishek Chatterjee
Class : XI-A
Roll number: 02
Assigned by: Dr. R.P Chand
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