SME
Grade 7 Science · Physical Science
Chemical reactions and balancing basics
Use this for “Balance H₂ + O₂ → H₂O,” and check that the atoms on each side match.
Diagram
4 H₂ + 2 O₂ → 4 H₂O
Hydrogen: 4 × 2 = 8 before, 4 × 2 = 8 after
Oxygen: 2 × 2 = 4 before, 4 × 1 = 4 after
Balanced: the same atoms on both sides, only rearranged.
This is an example. Choose your own values to start a new problem.
a
H₂
+
b
O₂
→
c
H₂O
h₁
Hydrogen atoms before
worked out (not typed)
o₁
Oxygen atoms before
worked out (not typed)
h₂
Hydrogen atoms after
worked out (not typed)
o₂
Oxygen atoms after
worked out (not typed)
Formulas
a
=
c
(hydrogen
molecules
used
=
water
molecules
made)
4 = 4
b
=
c
÷
2
(oxygen
molecules
used
=
water
molecules
made
÷
2)
2 = 4 ÷ 2
h₁
=
2
×
a
(hydrogen
atoms
before
=
2
×
hydrogen
molecules
used)
8 = 2 × 4
o₁
=
2
×
b
(oxygen
atoms
before
=
2
×
oxygen
molecules
used)
4 = 2 × 2
h₂
=
2
×
c
(hydrogen
atoms
after
=
2
×
water
molecules
made)
8 = 2 × 4
o₂
=
c
(oxygen
atoms
after
=
water
molecules
made)
4 = 4
Assumptions
•
In a chemical reaction atoms are not made or destroyed. They let go of each other and join in new ways.
•
Balanced means each element has as many atoms after the arrow as before it: 2 H₂ + O₂ → 2 H₂O.
•
The new substance has its own properties: water is a liquid that puts fires out, made from a gas that burns and a gas that helps things burn.
•
Water molecules come in pairs here, because each O₂ brings two oxygen atoms.
Step-by-step
WE KNOW
c = 4 (water molecules made)
FIND
hydrogen molecules used (a), oxygen molecules used (b), hydrogen atoms before (h₁), oxygen atoms before (o₁), hydrogen atoms after (h₂), oxygen atoms after (o₂)
Step 1 · Find hydrogen molecules used (a)
Use a = c (hydrogen molecules used = water molecules made)
One hydrogen molecule for every water molecule: each H₂ gives its 2 hydrogen atoms to one H₂O.
a
=
4
Step 2 · Find oxygen molecules used (b)
Use b = c ÷ 2 (oxygen molecules used = water molecules made ÷ 2)
Half as many oxygen molecules as water molecules: each O₂ has 2 atoms and each H₂O needs 1.
b
=
4
2
b
=
2
Step 3 · Find hydrogen atoms before (h₁)
Use h₁ = 2 × a (hydrogen atoms before = 2 × hydrogen molecules used)
Each H₂ molecule has 2 hydrogen atoms.
h₁
=
2
×
4
h₁
=
8
Step 4 · Find oxygen atoms before (o₁)
Use o₁ = 2 × b (oxygen atoms before = 2 × oxygen molecules used)
Each O₂ molecule has 2 oxygen atoms.
o₁
=
2
×
2
o₁
=
4
Step 5 · Find hydrogen atoms after (h₂)
Use h₂ = 2 × c (hydrogen atoms after = 2 × water molecules made)
Each H₂O molecule has 2 hydrogen atoms.
h₂
=
2
×
4
h₂
=
8
Step 6 · Find oxygen atoms after (o₂)
Use o₂ = c (oxygen atoms after = water molecules made)
Each H₂O molecule has 1 oxygen atom.
o₂
=
4
Check
Put the numbers back in. Both sides should match.
4 = 4 ✓
2 = 4 ÷ 2 ✓
8 = 2 × 4 ✓
4 = 2 × 2 ✓
8 = 2 × 4 ✓
4 = 4 ✓
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