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part of the In a Nutshell series on Adrian’s Study Club

Table of Contents

It is important to familiarise yourself with word and chemical equations. You should also practice balancing word and chemical equations.

An equation has two parts: the reactant on the left, and the products on the right. Plus signs + denote the separation between different substances and arrow signs –> separate reactant (left) and product (right) sides of the equation.

There are three things you have to consider:

  1. Valency
  2. Balancing
  3. The Rule

The third option is especially true if there’s something above the arrow (e.g photosynthesis) or if it’s an acid/base reaction.


Take this neutralisation reaction of an acid and a base

CuCO₃ + 2HNO₃ –> Cu(NO₃)₂ + H₂O + CO₂

It’s already balanced (which is looked at in the next section), but more importantly, it respects the valency. As copper(II) has a valency of 2+ and nitrate has an overall valency of 1- we can use the crossover method and determine that we need two nitrate molecules to balance one copper atom.


Take this wonderful nice equation of photosynthesis.

carbon dioxide + water –––––––––––> glucose + oxygen

Without balancing, this would be

CO₂ + H₂O –> C₆H₁₂O₆ + O₂

But of course, this is impossible, because there is 1 carbon on the reactant side while there is 6 on the carbon side, and 3 oxygen on reactant side, and 8 oxygen on the product side, and 2 hydrogen on the reactant side, and 12 hydrogen on the product side.

We can use our knowledge from Maths and find a number that would multiply the atoms so the equation becomes balanced.

Now how about we place a 6 in front of all of the substances except for glucose…

6CO₂ + 6H₂O –> C₆H₁₂O₆ + 6O₂

Now we have on both sides:

  • 6 carbon
  • 18 oxygen
  • 12 hydrogen

Yay! This is known as the law of conservation of mass.


The final thing to take consideration of is the rule. For acid/base reactions, you can test yourself on Quizlet here:

Test yourself on Quizlet

Difference between subscript and coefficient

The subscript shows the number of atoms in a molecule. The coefficient shows the number of molecules. They both represent quantity.


H₂SO₄ means there are 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atom in a molecule/substance 3H₂SO₄ means there are 3 molecules of sulfuric acid (also known as dihydrogen sulfur quadoxide) with 2 hydrogen, 1 sulfur and 4 oxygen in each. Therefore in total there is 6 hydrogen, 3 sulfur and 12 oxygen.

Balancing Chemical Equations

Footnotes and Other Notes

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