• A physical change is one in which the substance undergoing the change is not destroyed.
  It is the change in the form of matter but not in its chemical identity.
• A chemical change (or chemical reaction) is the change in which one or more kinds of matter are transformed into a new kind of matter or several new kinds of matter.
  The chemical nature of the original substance is changed and new substance or new substances with different chemical nature are formed. 
• Chemical reaction is a process in which some known substances are changed into new substance or new substances.
  The properties of new substances are different form the properties of the starting substances.
• Chemical Equation:
  A chemical equation is defined as a shorthand notation of an actual chemical reaction in terms of the symbols and formulae along with the number of atoms and molecules of its reactants and products.

While writing a chemical equation, the following points are kept in mind:

• 1. All the reactants and products are written in terms of symbols and formulae.
  2. The various reactants and products are separated by means of an arrowhead mark which starts from the reactants and points towards products.
  3. Each reactant or the product in turn is separated from one another by using a plus sign (+) in between them. The (+) sign in between the reactants implies as 'reacts with' while a similar sign in between the products means 'in addition to' or 'and'.
  
• An equation in which the number of atoms of all similar elements on both sides of a chemical equation may not be equal is known as skeleton equation.
• EXOTHERMIC REACTIONS: A change is called exothermic when heat is given out during that change. The term exothermic is taken form Greek word which means heat goes out (exo = out, therm = heat). In an exothermic process the heat is given to the surroundings so that the temperature increases.
• ENDOTHERMIC REACTIONS
  A change is called endothermic when heat is absorbed by the system (Endo = in, therm = heat). In an endothermic process heat is taken from the environment so that a decrease in temperature of the environment is observed.
• Types of reactions:
  Combination Reactions:
  Reactions in which two or more substances combine to form a single substance are called combination reactions.
• Displacement Reactions
  In these reactions, an atom or group of atoms present in a molecule is replaced by another atom or group of atoms.
  Double displacement reactions are those in which two compounds exchange ions to form two new compoundso  Decomposition ReactionsThe reactions in which a compound breaks up into two or more simple substances are known as decomposition reactions. The decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Thus all decomposition reactions are endothermic reactions. 
• Oxidation reduction (Redox) Reactions
  Oxidation-reduction reactions are called redox reactions. These two half reactions take place simultaneously. In redox reaction one substance is reduced while other is oxidized.