Page 113 - ICSE Chemistry 8

• It tells us about the mass of the reactant(s).
The mass of calcium carbonate is 100 u.
• It tells us about the mass of the products.

The mass of calcium oxide is 56 u and the mass of carbon dioxide is 44 u.

LIMITATIONS OF A CHEMICAL EQUATION

A chemical equa on provides us with a lot of informa on about a chemical reac on that takes place.
However, a chemical equa on has some limita ons too. This informa on can be added to an equa on to
remove these limita ons.

Physical States of Reactants and Products

A chemical equa on does not tell us about the physical state of the substance. A chemical equa on
should indicate the physical states of the reactants and products: ‘s’ for solid, ‘l’ for liquid, ‘g’ for gas and
‘aq’ for aqueous.

Pure State or Solution

Some reac ons occur only when they are present in a solu on form. For instance, sodium iodide solu on
is added to silver nitrate solu on and a reac on takes place. However, this reac on will not take place if
silver nitrate is in solid state and sodium nitrate is in a solu on form. Therefore, an equa on must indicate
the informa on about pure state and solu on forms of reactants and products.

Heat and Temperature

There are some reac ons that occur at high temperatures and others that occur at low temperatures. A
chemical equa on should indicate if heat is necessary or not.

Light

Some chemical reac ons take place in the presence of light. Photosynthesis is a chemical reac on that occurs
in green plants in the presence of sunlight. The chemical equa on should indicate the necessity of light, if
any.

Electricity

Some chemical reac ons need electricity to take place. Decomposi on of water into hydrogen and oxygen
is one such example. The chemical equa on should indicate that electricity is necessary or not.

Pressure

Some chemical reac ons in which one or more reactants are in the gaseous state occur when the pressure
is applied. The chemical equa on should indicate pressure as a necessity or not.

Catalyst

A substance that alters the rate of a chemical reac on without itself undergoing any change is known
as a catalyst. Diff erent reac ons use diff erent catalysts because one catalyst cannot be same for all the
reac ons.

99