Balance It!
Make the atoms equal on both sides of the arrow.
What this lesson covers
Why it matters
A ribbon of magnesium bursts into a blinding white flame. It joins with oxygen from the air — but the atoms have to add up exactly on both sides, like a balanced see-saw.
Predict first
For Mg + O₂ → MgO, the oxygen atoms do not match. What must we do to balance it?
Put a 2 in front of Mg and MgO: 2Mg + O₂ → 2MgO. Now there are 2 Mg and 2 O on each side. We balance by changing coefficients, never the formulae.
- Change O₂ to O so the formula fits
- Add coefficients: 2Mg + O₂ → 2MgO — correct
- Leave it — equations do not need to balance
- Remove an oxygen atom from the left side
What you do
Balance the burning of magnesium: 2Mg + O₂ → 2MgO. Add coefficients until the atoms on both sides of the see-saw are equal.
Check yourself
Why must the number of atoms be the same on both sides of an equation?
In a chemical reaction atoms are only rearranged, not made or lost. This is the law of conservation of mass, so each element must have equal atoms on both sides.
Which statement describes a correctly balanced equation?
A balanced equation has the same number of atoms of every element on the left and the right. The number of molecules does not have to match — only the atoms.
How do you balance an equation correctly?
You balance only by adjusting coefficients — the big numbers in front of each formula. Never change subscripts, because that would turn the chemical into a completely different substance.
- Because atoms are never created or destroyed — the law of conservation of mass — correct
- Because atoms disappear during a reaction
- Because the arrow points to the right
- Because gases weigh nothing
- It has equal atoms of each element on both sides — correct
- It has more atoms on the product side
- It has the same number of molecules on each side
- It has no numbers in front of any formula
- By changing the big numbers (coefficients) in front of formulae — correct
- By changing the small subscript numbers inside a formula
- By adding brand-new elements that were not there
- By rubbing out atoms until the sides look equal