1. The original substances lose their characteristic properties. Hence
these may be products with different physical states and colours.
2. Chemical changes may be exothermic or endothermic i.e, heat may
be liberated or absorbed.
3. They may form an insoluble substance known as precipitate.
4. Gas may be evolved
In our daily life we observe variety of changes taking place around us.
In this chapter we study various types of chemical reactions and their
symbolic representation.
Chemical Equations
In activity 1, when calcium oxide reacts with water a new substance is
formed which is unlike to either calcium oxide or water. The description
of chemical reactions in a sentence in activity-1 is quite long. It can be
written in shorter form as a word equation.
The word equation of the above reaction is,
calcium oxide + water
(Reactants)---> calcium hydroxide (Product) ......................... (1)
The substances which undergo chemical change in the reaction are
called reactants and the new substances formed are called products.
A chemical reaction written in the form of word equation shows the
change of reactants to products by an arrow placed between them. The
reactants, are written on the left side of the arrow and the final substances,
or products are written on the right side of the arrow. The arrow head
point faces the products showing the direction of the reaction.
If there is more than one reactant or product involved in the reaction,
they are indicated with a plus (+) sign between them.
Writing a Chemical Equation
• Can you write a chemical reaction in any other shorter way other than
the way we discussed above?
Chemical equations can be made more precise and useful if we use
chemical formulae instead of words.
Generally, a compound is written by giving its chemical formula,
which lists the symbols of the constituent elements and uses the subscript
to indicate the number of atoms of each element present in the compound.
If no subscript os written the number is understood thus we can write