GCSE · Chemistry · AQA · Spec 8462

Mass changes when a reactant or product is a gas

Burn magnesium and the powder left weighs MORE than the metal. Heat calcium carbonate and the solid weighs LESS. Yet mass is never made or destroyed. So what's going on?

Chemistry · Where did the gas go?

Will the balance reading go up, down or stay the same?

Picture a reaction sitting on a balance. Answer each question in turn and see what the reading does, and why.

Is a gas involved? → Is the gas a reactant or a product? → Is the container open or sealed?

5 possible routes.

On A reaction happens on a balance. 2 branches to choose from.

Every route obeys the same law: no atoms are made or destroyed. The reading only moves when a gas crosses the edge of an open container.

Watch out: Never write that mass was ‘made’ or ‘lost’. Say which gas came in from the air, or which gas escaped into it.

Chemistry · Where does the extra mass come from?

The magnesium oxide weighs more. Why?

A strip of magnesium burns in an open crucible. When it has finished, the white powder left behind, magnesium oxide, weighs more than the magnesium did at the start.

Which of these is closest to what you think right now?
How sure are you?

Chemistry · Reaction balancer

Count the atoms, then count the mass

Tap + or − to change the big numbers in front. Get the same number of magnesium atoms and oxygen atoms on both sides.

Reactants
1
Mg
1
O2
Products
1
MgO
Mg1→1✓
O2→1✗
Conservation check: count atoms on both sides.Not yet.

Once it balances, check the mass. Multiply each relative formula mass (Mg = 24, O₂ = 32, MgO = 40) by the number in front of it, then add up each side. You get 48 + 32 on the left and 80 on the right: the same atoms, so the same mass.

Tap + or − under each molecule to set its coefficient.
Watch out: Don't balance it by writing MgO₂. That would be a different substance. Only the numbers in front can change.

Chemistry · Gut feeling first

How heavy will the powder be?

45 g of magnesium burns completely in air. Before doing any maths, what mass of magnesium oxide do you think it makes?

Your estimate

60g

0g120g

Calculating the mass of gas that reacted

Problem

45 g of magnesium burns completely: 2Mg + O₂ → 2MgO. Relative formula masses: Mg = 24, O₂ = 32, MgO = 40. What mass of magnesium oxide forms, and what mass of oxygen reacted?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • Atoms are never made or destroyed in a reaction, so the total mass of everything involved never changes.
  • A balance only weighs what is in the container. In an open container, a gas can come in from the air or escape into it.
  • Gas coming in makes the reading go up; gas escaping makes it go down; in a sealed container the reading stays the same.
  • You can calculate the mass of a gas that reacted from the balanced equation, using relative formula masses multiplied by the numbers in front.

The big picture

Mass is never made or destroyed in a reaction: atoms are only rearranged. If a balance reading changes, a gas has come in from the air (like oxygen when magnesium burns) or escaped into it (like carbon dioxide when calcium carbonate is heated). Count the gas and the totals match, just as they do in a sealed container.

Key points

1Conservation of mass: the total mass of the reactants equals the total mass of the products, because atoms are rearranged, not created or destroyed.
2Burning magnesium: magnesium oxide is heavier than the magnesium used, and the difference is the mass of oxygen that reacted.
3Heating calcium carbonate: it breaks down into calcium oxide and carbon dioxide; in an open container the carbon dioxide escapes, so the solid left is lighter.
4Particle model: gas particles move into the reacting mixture from the surroundings, or out of it, so the particles being weighed change even though no atoms are made or lost.
5In 2Mg + O₂ → 2MgO the relative masses are 2 × 24 = 48, 32 and 2 × 40 = 80, and 48 + 32 = 80.

Worked example

Problem

A student burns some magnesium in an open crucible. The mass of the solid goes up by 8 g. What mass of magnesium burned? Use 2Mg + O₂ → 2MgO, with Mg = 24 and O₂ = 32.

⚠ Watch out

Adding relative masses without the numbers in front. For 2Mg + O₂ → 2MgO, 24 + 32 = 56 doesn't match 40, which makes it look as if mass has vanished. Multiply first: 48 + 32 = 80.

🧠

Memory hook

Gas in, reading up. Gas out, reading down. Lid on, no change. And the atoms? They never change at all.

✓

Check yourself

A sealed flask of magnesium and air reads 150.0 g. The magnesium burns inside it. What does the balance read now, and why would an open crucible be different?

Flashcards

(13)
What is conservation of mass?
The total mass of the reactants equals the total mass of the products, because atoms are rearranged, never created or destroyed.
Why can a reaction in an open container seem to change mass?
A reactant or product is a gas, and its mass isn't included in the balance reading.
Magnesium burns in an open crucible. What happens to the reading?
It goes up: oxygen from the air joins on to make magnesium oxide.
When magnesium burns, what does the increase in mass of the solid equal?
The mass of oxygen that reacted.
What does calcium carbonate break down into when heated?
Calcium oxide (a solid) and carbon dioxide (a gas).
Why does heated calcium carbonate lose mass in an open container?
The carbon dioxide produced escapes into the air, so it is no longer weighed.
What happens to the reading if the reaction is in a sealed container?
It stays the same, because the gas is trapped inside and still weighed.
Use the particle model: how does the mass of an open reaction mixture change?
Gas particles move in from the surroundings or out into them, so the particles being weighed change. No atoms are made or destroyed.
Can heat from a flame add mass to a substance?
No. Heat is energy, not atoms, so it adds no mass.
Relative masses in 2Mg + O₂ → 2MgO?
Mg: 2 × 24 = 48. O₂: 32. MgO: 2 × 40 = 80. And 48 + 32 = 80.
Before adding relative formula masses from an equation, what must you do?
Multiply each one by the number in front of that substance in the balanced equation.
How do you find the mass of a product from the mass of a reactant?
Use the ratio of their relative masses from the balanced equation: mass known × (product total ÷ reactant total).
Why can't you balance Mg + O₂ → MgO by writing MgO₂?
Changing a formula changes the substance. Only the numbers in front can change.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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