Balancing equations and conservation of mass

Science · Matter & Materials · ages 15-16

Name ______________________ Date ____________

Which of these equations is balanced?
- Fe + Cl₂ → FeCl₃
- H₂ + Cl₂ → HCl
- Na + O₂ → Na₂O
- 2K + Br₂ → 2KBr
Atoms are neither created nor destroyed during a chemical reaction.
Circle one: True False
A magnesium ribbon is burned in air and the ash weighs more than the ribbon did. Why?
- oxygen from the air has joined the magnesium
- burning creates new atoms out of the flame
- the ash has absorbed heat, and heat has mass
- the balance reads high once it has been warmed
In 2CO + O₂ → 2CO₂, how many oxygen atoms are on the left-hand side?
Answer: ______________
Marble chips are dropped into acid in an open flask on a balance. The reading falls steadily. What has happened?
- carbon dioxide has escaped from the flask
- some of the mass has been destroyed
- the acid has evaporated into the marble
- the balance has drifted while the flask fizzed
A number in front of a formula multiplies everything inside that formula.
Circle one: True False
Look at H2 + O2 -> H2O. Which element is not balanced?
- Oxygen: 2 atoms left, 1 right
- Hydrogen: 1 atom left, 2 right
- Magnesium is missing
- Everything balances already
Lithium plus chlorine gives lithium chloride, written 2Li + Cl₂ → LiCl. What number goes in front of LiCl to balance it?
Answer: ______________
Aluminium reacts with chlorine to give aluminium chloride, AlCl₃. Which line is the balanced symbol equation?
- Al + 3Cl₂ → AlCl₃
- 2Al + Cl₃ → 2AlCl₃
- 2Al + 3Cl₂ → 2AlCl₃
- Al + Cl₂ → AlCl₂
Someone writes 4Fe + 3O₂ → Fe₂O₃. Which element is not balanced, and by how much?
- oxygen, 6 on the left against 2 on the right side
- iron and oxygen: 4 against 2, and 6 against 3
- iron, 2 on the left against 4 on the right
- neither, the line is already balanced

Answer key

For grown-ups. Fold this page away before handing over the rest.

Balancing equations and conservation of mass W1-mt_KUaPCEIA5f-s1

2K + Br₂ → 2KBr · 2K + Br₂ → 2KBr has 2 potassium and 2 bromine on each side. Each of the others is short of at least one element.
True · They are only rearranged into different substances. That is why every element has to appear in the same numbers on both sides of a balanced equation.
oxygen from the air has joined the magnesium · The ash is magnesium oxide, and the oxygen in it came from the air. Weigh the oxygen too and nothing has been gained.
4 · Each CO carries 1 oxygen and there are 2 of them, giving 2, and the O₂ adds 2 more. 2 + 2 = 4.
carbon dioxide has escaped from the flask · The reaction makes carbon dioxide, which is a gas and leaves an open flask. The mass is still there, just no longer on the balance.
True · It counts whole units, so 3SO₂ is three sulfur atoms and six oxygen atoms. That is why a front number can fix a count without changing what the substance is.
Oxygen: 2 atoms left, 1 right · Left has O2 but right has one O, so oxygen is unbalanced.
2 · The left has 2 lithium and 2 chlorine. One LiCl gives only 1 of each, so 2 are needed.
2Al + 3Cl₂ → 2AlCl₃ · Chlorine comes in 2s on the left and 3s on the right, so the smallest number both divide into is 6. That needs 3Cl₂ and 2AlCl₃, and the 2 aluminiums on the right force a 2 in front of Al.
iron and oxygen: 4 against 2, and 6 against 3 · The left has 4 iron and 6 oxygen. One Fe₂O₃ gives only 2 iron and 3 oxygen, so both are exactly half of what is needed. Putting a 2 in front of Fe₂O₃ fixes both at once.