Chemistry > Basic Concepts of Chemistry

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  • Matter is anything that has mass and occupies space. It exists in three states- solid, liquids and gases.
  • Solids have definite shape and definite volume. Liquids have definite volume but not a definite shape. Gases neither have definite shape nor definite volume.
  • Inter-convertibility-Solids on heating changes to a liquid and liquids on heating changes to gaseous state. On cooling, gases change to liquid and liquids change to solid state.
  • Matter can be classified into mixtures and pure substances. Mixtures are further subdivided into heterogeneous and homogeneous and pure substances as compounds and elements.
  • Mixtures-Two or more substances which have been combined such that each substance retains its own chemical identity. Can be separated by simple physical methods.
  • Homogeneous refers to a substance that is consistent or uniform throughout its volume.
  • Heterogeneous mixtures- Having a non-uniform composition. Examples: A mixture of sand and water is heterogeneous.
  • A Pure substance is a sample of matter with both definite and constant composition with distinct chemical properties. Cannot be separated by simple physical methods.
  • A chemical element is a substance that cannot be broken down by chemical means. It consists of only one type of particles.
  • When two or more atoms of different elements combine, the molecule of a compound is obtained. The constituents of a compound cannot be separated into simpler substances by physical methods. They can be separated by chemical methods.
  • Physical properties are those properties which can be measured or observed without changing the identity or the composition of the substance. Ex-boiling point, melting point etc.
  • Chemical properties require a chemical change to occur. Ex-acidity, basicity.
  • The SI system has seven base units. These units pertain to the seven fundamental scientific quantities .The other physical quantities such as speed, volume; density etc. can be derived from these quantities.
  • Basic Physical Quantities and their Units.

Basic Physical Quantity

Symbol for Quantity

Name of SI unit

Symbol for SI unit













Electric current








Amount of substance




Luminous intensity




  • Mass of a substance is the amount of matter present in it while weight is the force exerted by gravity on an object. The mass of a substance is constant whereas its weight may vary from one place to another due to change in gravity.
  • Density is defined as mass per unit volume.
  • There are three common scales to measure temperature — °C (degree celsius), °F (degree fahrenheit) and K (kelvin). Here, K is the SI unit.
  • F = (  ) * C + 3.
  • K = C + 273.1
  • Precision refers to the closeness of various measurements for the same quantity. However, accuracy is the agreement of a particular value to the true value of the result.
  • Significant figures are meaningful digits which are known with certainty. The uncertainty is indicated by writing the certain digits and the last uncertain digit.
  • There are certain rules for determining the number of significant figures-
    • All non-zero digits are significant. For example in 285 cm, there are three significant figures and in 0.25 mL, there are two significant figures.
    • Zeros preceding to first non-zero digit are not significant. Such zero indicates the position of decimal point. Thus, 0.03 has one significant figure and 0.0052 has two significant figures.
    • Zeros between two non-zero digits are significant. Thus, 2.005 have four significant figures.
    • Zeros at the end or right of a number are significant provided they are on the right side of the decimal point. For example, 0.200 g has three significant figures.
    • Exact numbers have infinite zeros at the end or right of a number is significant provided they are on the right side of the number of significant figures.

             Often while calculating, there is a need to convert units from one system to other. The method used to accomplish this is called factor label method or unit factor method or dimensional analysis.

             Law of Conservation of Mass-It states that matter can neither be created nor destroyed. This law was put forth by Antoine Lavoisierin 1789.

             Law of Definite Proportions-This law was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight.

             Law of Multiple Proportions-This law was proposed by Dalton in 1803.According to this law, if two elements can combine to form more than one compound, themasses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.

             Gay Lussac's Law of Gaseous Volumes-This law was given by GayLussac in 1808. He observed that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.

             Avogadro Law-In 1811, Avogadro proposed that equal volumes of gases at the same temperature and pressure should contain equal number of molecules.

             Dalton's Atomic Theory-Matter consists of indivisible atoms. All the atoms of a given element have identical properties including identical mass. Atoms of different elements differ in mass. Compounds are formed when atoms of different elements combine in a fixed ratio. Chemical reactions involve reorganization of atoms. These are neither created nor destroyed in a chemical reaction. Dalton's theory could explain the laws of chemical combination.

             One atomic mass unit (amu) is defined as a mass exactly equal to onetwelfth the mass of one carbon-12 atom.

    And 1 amu = 1.66056×10-24 g.

             Molecular mass is the sum of atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass ofeach element by the number of its atoms andadding them together.

             One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012kg) of the 12C isotope.

             The mass of one mole of a substance in grams is called its molar mass. The molar mass in grams is numerically equal to atomic/molecular/formula mass in u.

             An empirical formula represents the simplest whole number ratio of various atoms present in a compound whereas the molecular formula shows the exact number of different types of atoms present in a molecule of a compound.

             Stoichiometry is the study of the relationships or ratios between two or more substances undergoing a physical or chemical change

             According to the law of conservation of mass, a balanced chemical equation has the same number of atoms of each element on both sides of the equation.

             Thereactant which gets consumed, limits the amount of product formed is called the limiting reagent.

             Mole fraction-It is the ratio of number of moles of a particular component to the total number of moles of the solution. If a substance 'A' dissolves in substance 'B' and their number of moles arena and nb respectively; then the mole fractions of A and B are given as.

             Molarity=No of moles/Volume of Solution.

             Molality-It is defined as the number of moles of solute present in 1 kg of solvent. It is denoted by m.


Sample Examples



Calculate molecular mass of glucose () molecule.


Molecular mass of glucose () =

=  = 180.162 u



The density of 3 M solution of NaCl is1.25 g mL–1. Calculate molality of the solution.


M = 3 mol L–1

Mass of NaClin 1 L solution = 3 × 58.5 = 175.5 g

Mass of1L solution = 1000 × 1.25 = 1250 g (since density = 1.25 g mL–1)

Mass of water in solution = 1250 –175.5 = 1074.5 g

Molality is defined as the number of moles of solute present in 1 kg of solvent.

Molality = 3 mol / 1.0745 kg = 2.79 m



Determine the empirical formula of a compound with the followingcomposition by mass : 8.04 percent lithium, 91.96 percent bromide.




Determine the empirical formula of a compound with the following composition by mass :  29.4 percent calcium, 23.5 percent sulfur, 47.1 percent oxygen.


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