Page 85 - Chemistry_Form_2

Chemistry
for Secondary Schools

A chemical reaction is either reversible or irreversible. A reversible reaction
proceeds in both forward and backward directions. In the forward reaction, the
reactants are converted into products, whereas in the backward reaction, the
products become the reactants. An irreversible reaction proceeds in one direction
from reactants to products.
FOR ONLINE READING ONLY
Chemical equations

Task 4.2

1. Use library books and reliable online resources to explore various molecular
equations.
2. Use chemistry software step by step to represent the molecular equations
identified in 1.

A chemical reaction is expressed in the form of a chemical equation. A chemical
equation is a symbolic or words representation of a chemical reaction. A chemical
equation written in words is referred to as a word equation. For example, when
calcium metal reacts with chlorine gas to form solid calcium chloride, the word
equation is written as:

A chemical equation written in symbols is referred to as a formula equation. For
example, the formula equation for a reaction between calcium metal and chlorine
gas is:

The formula equation is more useful than the word equation. However, it is
advantageous to write the equation in word form first. Formula equations
provide useful information, including compositions, amounts, formulas, and the
physical states of substances that are involved. Formula equations may also state
the conditions for the reactions to take place. The reactants and products in the
formula equations are either solids, liquids or gases. These states of matter are
represented in a chemical equation by using symbols, such as (s) for solid, (l)
for liquid, and (g) for gas. When the substances that are involved in a reaction
are dissolved in water, the word aqueous (aq) is used. These symbols should be
included when writing the chemical equations and are stated in parentheses ( ),
after the chemical symbol.
In a chemical equation, a headed arrow is used to separate the reactants and
products.

78 Student’s Book Form Two

01/08/2025 11:20:57
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 78
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 78 01/08/2025 11:20:57