EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.5.1 Word & Chemical Equations

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.5.1 Word & Chemical Equations

Writing Equations


Nothing created - nothing destroyed

  • New substances are made during chemical reactions
    • However, the same atoms are always present before and after reaction
    • They have just joined up in different ways
    • Atoms cannot be created or destroyed, so if they exist in the reactants then they absolutely must be in the products!


  • Because of this the total mass of reactants is always equal to the total mass of products
  • This idea is known as the Law of Conservation of Mass

Conservation of Mass

  • The Law of Conservation of Mass enables us to balance chemical equations, since no atoms can be lost or created
  • You should be able to:
    • Write word equations for reactions outlined in these notes
    • Write formulae and balanced chemical equations for the reactions in these notes


Word Equations

  • These show the reactants and products of a chemical reaction using their full chemical names
  • The reactants are those substances on the left-hand side of the arrow and can be thought of as the chemical ingredients of the reaction
  • They react with each other and form new substances
  • The products are the new substances which are on the right-hand side of the arrow
  • The arrow (which is spoken as “goes to” or “produces”) implies the conversion of reactants into products
  • Reaction conditions or the name of a catalyst (a substance added to make a reaction go faster) can be written above the arrow
  • An example is the reaction of sodium hydroxide (a base) and hydrochloric acid produces sodium chloride (common table salt) and water:


sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water


Representing reactions as equations

  • Chemical equations use the chemical symbols of each reactant and product
  • When balancing equations, there has to be the same number of atoms of each element on either side of the equation in accordance with the Law of Conservation of Mass
  • A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction
  • A symbol equation must be balanced to give the correct ratio of reactants and products:   

S + O2 → SO2


  • This equation shows that one atom of sulfur (S) reacts with one  molecule of oxygen (O2) to make one molecule of sulfur dioxide (SO2)
  • The following non-metals must be written as molecules: H2, N2, O2, F2, Cl2, Br2 and I2
  • To balance an equation you work across the equation from left to right, checking one element after another
  • If there is a group of atoms, for example a nitrate group (NO3) that has not changed from one side to the other, then count the whole group as one entity rather than counting the individual atoms
  • Examples of chemical equations:
    • Acid-base neutralisation reaction:


NaOH (aq) + HCl (aq)  ⟶ NaCl (aq) + H2O (l) 

    • Redox reaction:


2Fe2O(aq) + 3C (s) ⟶ 4Fe (s) + 3CO2 (g)


  • In each equation there are equal numbers of each atom on either side of the reaction arrow so the equations are balanced


  • Don't forget to add state symbols when writing balanced equations:

Balancing Equations

The best approach is to practice lot of examples of balancing equations

  • By trial and error change the coefficients (multipliers) in front of the formulae, one by one checking the result on the other side
  • Balance elements that appear on their own, last in the process


Worked Example

Example 1Balance the following equation:


aluminium + copper(II)oxide ⟶ aluminium oxide + copper 


Unbalanced symbol equation:

Al + CuO ⟶ Al2O3 + Cu







Worked Example

Example 2

Balance the following equation:


magnesium oxide + nitric acid ⟶ magnesium nitrate + water 


Unbalanced symbol equation:


MgO + HNO3 ⟶ Mg(NO3)2 + H2O







Exam Tip

Chemical equations do not contain an equals sign between the left and right-hand sides but are written with an arrow instead. The arrow means that the reactants have reacted together and formed the product(s).