Nothing is lost
Mass is the same before and after.
Weigh it
24 g of magnesium burns completely in 16 g of oxygen and makes magnesium oxide.
How many grams of magnesium oxide will you get?
What this lesson covers
The idea
Mass can neither be created nor destroyed in a chemical reaction, so the number of atoms of each element remains the same before and after it; hence chemical equations must always be balanced.
Weigh it
24 g of magnesium burns completely in 16 g of oxygen and makes magnesium oxide.
How many grams of magnesium oxide will you get?
- 40 g
- 24 g
- 8 g
Tip the scale
The scale shows the mass of all atoms on each side (in atomic mass units, u). Change the numbers in front of the formulae until the scale is level.
Atoms stay, so mass stays
Mass can neither be created nor destroyed in a chemical reaction. So the number of atoms of each element stays the same before and after, and chemical equations must always be balanced.
Total: 80 u on each side.
- Mg | O
- LHS: 2Mg + O₂ | 2 atoms (48 u) | 2 atoms (32 u)
- RHS: 2MgO | 2 atoms (48 u) | 2 atoms (32 u)
Notes
Mass can neither be created nor destroyed in a chemical reaction, so the number of atoms of each element remains the same before and after it; hence chemical equations must always be balanced.
Check yourself
2 g of hydrogen burns with 16 g of oxygen to form water. How many grams of water are formed?
Answer: 18 g
Mass of reactants = mass of products: 2 + 16 = 18 g of water.
Why must a chemical equation be balanced?
A skeletal equation shows 3 atoms of copper on the left but only 2 on the right. What does that tell us?
- Because balanced equations look neater. It is not about looks. It is about the law of conservation of mass.
- Because the reaction then goes faster. Balancing does not change speed. It shows that atoms and mass are conserved.
- Mass is not created or destroyed, so atoms of each element must be the same on both sides — correct. Yes! Atoms rearrange; none are lost or gained.
- The equation is not balanced yet: atoms of copper cannot disappear — correct. Yes! Atoms are not lost, so the equation needs balancing.
- One copper atom was destroyed in the reaction. Atoms are never destroyed in a chemical reaction.
- Copper changed into another element. Atoms of one element never change into another in a chemical reaction.