ICSE Class 9 Answered

1. A.     Hit and trial method 

In this method of balancing chemical equations, trials are made to balance the equation by using the smallest whole number coefficient. Hence, this method is called the hit and trial method.

          i.    Count the number of times an element occurs on both sides.

         ii.    An element with the least frequency of occurrence is balanced first.

        iii.    When two or more elements have the same frequency, metallic elements are balanced first.

            Example: Pb(NO₃)₂  →  PbO  +  NO₂  +  O₂

In this equation,                                                 

The balanced equation is

                                                       2Pb(NO₃)₂  →  2PbO  +  4NO₂  +  O₂

1. B.     Balancing a chemical equation by the partial equation method

A chemical equation can be balanced easily by supposing a stepwise complex reaction to take place. Write equations for these steps and then add these equations. This method is known as balancing by the partial equation method.

Equation 1: Hydrogen peroxide decomposes to give water and nascent oxygen

       H₂O₂      → H₂O     +     [O]

^{Hydrogen              Water          Nascent} 

  peroxide                        Oxygen

Equation 2: Nascent oxygen obtained oxidises potassium iodide in the presence of water to give potassium hydroxide and iodine

        2KI       +    H₂O    +     [O]             →       2KOH    +    I₂  

^{Potassium                Water         Nascent                    Potassiume           Iodine}

  iodide                              Oxygen                hydroxid

Add the steps and cancel what is common on opposite sides.