Page 20 - Big Ideas Year 10 sciPAD 2025

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20 SCIPAD - BIG IDEAS - BOOK 2
NATURE OF THE ATOM
Balancing Symbol Equations
Nearly 2 500 years ago, Greek philosophers (early scientists) came up with the theory that “nothing
comes from nothing”. Over time many scientists have further refined this theory to suggest that
“matter can not be created or destroyed” - a statement that scientists refer to as the ‘Law of
Conservation of Mass’. In simple terms, this means if you weighed the ingredients of a cake,
then baked it in a closed system, the mass of the cake at the end would equal the mass of the
ingredients. The same is true for chemical reactions. The number of atoms in the reactants will
always equal the number of atoms in the products. Scientists show this by writing balanced symbol
equations. For example, consider hydrogen gas reacting with oxygen gas to create water.
2 atoms of hydrogen 2 atoms of oxygen 2 atoms of hydrogen
and 1 atom of oxygen
1. Balance the following symbol equations.
The equation above is unbalanced. There are 4 atoms in total on the left-hand side of the equation
and only 3 atoms on the right-hand side - an oxygen atom is missing!
In order to balance an equation there are a couple of rules you must stick to:
•
You can never add small numbers to a compound - e.g: H2O cannot become H2O2.
•
You can only add numbers to the front of a compound. This number multiplies all the
atoms in that compound.
So to fix the example above we need to add a ‘2’ in front of the hydrogen and another 2 in front
of the water, so...
H H H
H
O
O O
+
H2 O2
+ H2O
Graphically this reaction would look like this.
H H
H H
O O H H
O
H H
O
+
Graphically this reaction would look like this.
As a symbol equation is would look like this.
2 x 2 = 4 H’s 2 x 1 = 2 O’s 2 x 2 = 4 H’s
2 x 1 = 2 O’s
6 atoms in total 6 atoms in total
2H2 O2
+ 2H2O
As a symbol equation is would look like this.
(a) C
+
H2 CH4
(b) Zn
+
O2 ZnO
(c) H2
+
Cl2 HCl
(d) Li
+
O2 Li2O
(e) Mg
+
Cl2 MgCl2
(f) Na
+
O2 Na2O
(g) K
+
I2 KI
(h) H2
+
F2 HF
(i) Li
+
Cl2 LiCl
(j) Cu
+
O2 CuO
(k) K
+
O2 K2O
(l) Fe
+
O2 Fe2O2
(m) Na
+
Cl2 NaCl
(n) P4
+
O2 P4O10
(o) Al
+
O2 Al2O3
(p) V
+
Cl2 VCl3
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