Synopsis

   H    Whenever a chemical change occurs, we can say that a chemical reaction has taken place.

   H   Magnesium ribbon burns with a dazzling white flame and changes into a white powder, magnesium oxide.

   H   The following observations helps us to determine whether a chemical reaction has taken place –

        1) change in state                                  2) change in colour

        3) evolution of a gas                              4) change in temperature.

   H   A word-equation shows change of reactants to products through an arrow placed between them.

   H   The reactants are written on the left-hand side (LHS) with a plus sign (+) between them.

   H   Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them.

   H   The arrow head points towards the products and shows the direction of the reaction.

   H   A chemical equation represents a chemical reaction.

   H   The equation is unbalanced when the mass is not the same on both sides of the equation.

   H   The unbalanced chemical equation is a skeletal chemical equation for a reaction.

   H   Conservation of the mass : Mass can neither be created nor destroyed in a chemical reaction.

   H   The number of atoms of each element remains the same, before and after a chemical reaction.

   H   There are four types of reactions :

        1) Combination reaction                        2) Decomposition

        3) Displacement Reaction                      4) Double displacement reaction.

   H   The decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants.

   H   Calcium oxide is called lime or quick lime. It can be thermally decomposed into quick lime and carbon dioxide.

   H   When a decomposition reaction is carried out by heating, it is called thermal decomposition.

   H   Water can be decomposed into hydrogen and oxygen through the decomposing by electrolysis.

   H   White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

   H   Zinc and lead are more reactive elements than copper. They displace
copper from its compounds.

   H   Any reaction that produces a precipitate can be called a precipitation reaction.

   H   The reactions in which one reactant gets oxidised while the other gets reduced during a reaction are called oxidation-reduction reactions or redox reactions.

   H   If a substance gains oxygen or loses hydrogen during a reaction, it is oxidised.

   H   If a substance loses oxygen or gains hydrogen during a reaction, it is reduced.

   H   When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.

   H   The black coating on silver and the green coating on copper are other examples of corrosion.

   H   When fats and oils are oxidised, they become rancid and their smell and taste change. This process is called Rancidity.