Chemical reactions · Quantitative
Mass in a reaction
One reaction on a balance loses two grams and another gains one and a half. Both obey the same rule — so what is the balance actually telling you?
Start here
A candle burns away to almost nothing. Steel wool burns and gets heavier.
Both are burning. Both are on a balance. One reading falls and the other rises, and neither of them is doing anything strange — the same rule is behind both.
What is the rule?
Nothing is created and nothing is destroyed — but the balance only weighs what is on the pan. The candle's products float away, so the reading falls. The steel wool takes oxygen out of the air and keeps it, so the reading rises. Count everything, including the gases, and the total never changes at all.
Your turn · the balance bench
Two reactions, two flasks. Predict what the balance does.
The reaction
The flask
Before you run it: what will the balance reading do?
Balance before
152.00 g
Flask, contents and everything on the pan.
Balance after
149.80 g
The same pan, once the reaction has finished.
Mass of gas
not measured — you work it out
The mass of carbon dioxide that left the flask.
The reading fell by 2.20 g. Nothing was destroyed: carbon dioxide bubbled out of the open neck and walked off the pan. The 2.20 g is now in the room.
Balance before
152.00 g
Flask, contents and everything on the pan.
Balance after
152.00 g
The same pan, once the reaction has finished.
Mass of gas
0.00 g
Nothing left the flask.
Not a hundredth of a gram. The same reaction happened — the chips still fizzed away — and with the gas trapped on the pan the balance has nothing to report.
Balance before
84.60 g
Flask, contents and everything on the pan.
Balance after
86.20 g
The same pan, once the reaction has finished.
Mass of gas
not measured — you work it out
The mass of oxygen that joined the magnesium.
The reading rose by 1.60 g. The extra mass came out of the air: oxygen atoms are now part of the white powder, and they are being weighed for the first time.
Balance before
250.00 g
Flask, contents and everything on the pan.
Balance after
250.00 g
The same pan, once the reaction has finished.
Mass of gas
0.00 g
Nothing entered or left the flask.
Sealed, with the air already inside and weighed. The magnesium still burns and still takes oxygen, but the oxygen was on the pan before the reaction and is on the pan after it. No change.
All four runs. Open and fizzing: the reading falls. Open and burning: the reading rises. Sealed, either reaction: the reading does not move by so much as a hundredth of a gram. The reaction never changes the mass. The lid decides whether the balance can see it.
The rule
total mass of reactants = total mass of products
Every calculation in this lesson is that line with one of its numbers missing.
The bar
Cover the one you want
Total mass of everything before = total mass of everything after
The cells are sized so you can read them. Drawn to scale, the gas would be about one part in seventy of the whole bar.
everything before = left in the flask + the gas
An addition. Use this when you know what stayed and what left, and want the mass you started with — 149.80 + 2.20 = 152.00 g.
left in the flask = everything before − the gas
A subtraction. Use this when you know the starting mass and how much gas escaped, and want the mass remaining.
the gas = everything before − left in the flask
A subtraction, and the one an open flask always asks for: the gas is the quantity nobody measured, so it is the difference between the two readings.
Two parts side by side make the whole. Cover the part you want and take the other one away from the whole.
Watched first · FIFA
How much carbon dioxide left the open flask?
Marble chips and acid in an open flask. The balance read 152.00 g at the start and 149.80 g when the fizzing stopped. Four steps, one at a time.
Formula
total mass of reactants = total mass of products
Write the rule before you touch a number. It is the same rule for every question in this lesson.
Insert
152.00 = 149.80 + mass of gas
The flask started at 152.00 g. Afterwards, 149.80 g is still on the balance and the rest of the products left as gas.
Fine-tune
mass of gas = 152.00 − 149.80
The quantity asked for is not on the left, so rearrange until it is on its own. Covering "the gas" on the bar gives you this line.
Answer
mass of carbon dioxide = 2.20 g
Two decimal places, because that is what the balance gave you, and grams — the unit belongs to the answer.
Now you · same four steps
2.40 g of magnesium burns and leaves 4.00 g of magnesium oxide. What mass of oxygen joined in?
Do each step yourself, then open it to compare. The steps are the same four; only the numbers and the missing quantity have moved.
Formula
Write the rule down.
total mass of reactants = total mass of products
Unchanged. It does not matter that this reaction gains mass rather than losing it.
Insert
Put in what you know. The reactants are the magnesium and the oxygen; the product is the magnesium oxide.
2.40 + mass of oxygen = 4.00
Both reactants go on the left of the equals sign, because both of them were there at the start — even though one came out of the air.
Fine-tune
Rearrange so the quantity you want is on its own.
mass of oxygen = 4.00 − 2.40
A part, again — so a subtraction, again. The bar with "the gas" covered is the same picture, with the gas joining instead of leaving.
Answer
Work it out, and give the unit.
mass of oxygen = 1.60 g
1.60 g of oxygen came out of the air and is now part of the powder. Weigh the flask before and after in a sealed vessel and you would see no change at all.
Notice what changed between the two questions and what did not. The formula did not change. The rearranging did — in the first one the missing quantity was a part, in this one it is also a part, but it was on the other side of the arrow. Which side a substance is on decides whether it is added or subtracted; the rule itself never moves.
Key fact
Total mass of reactants = total mass of products. A balance reading that changes is telling you a gas has entered or left the pan — not that mass has been created or destroyed.
Think again
“Gases do not weigh anything, so the missing 2.20 g cannot be the carbon dioxide.”
You cannot feel the air, and nothing you have ever picked up has felt heavier for having gas in it. Commit before you read on.
Gases have mass, and it is easy to measure. The air in an ordinary classroom weighs around 150 kilograms. A cubic metre of carbon dioxide is nearly two kilograms. Weigh a football, pump it up hard, weigh it again: the reading goes up, and the only thing you added was air.
What gases lack is not mass but weight you can notice — they are spread out, and the air around you pushes up on everything, so nothing feels heavier for containing gas. The sealed flask settles it: seal the reaction so the gas cannot leave and the balance does not move at all. The gas was on the pan the whole time. This is the same wrong idea as thinking a dried-up puddle was destroyed, in a chemical costume.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
A reaction is carried out in a sealed flask on a balance. What happens to the reading?
Rung 2 · The one that catches people
2.40 g of magnesium is burned in an open dish and 4.00 g of white powder is left. What has happened?
Rung 3 · Produce a calculation
8.00 g of calcium carbonate is heated in an open crucible. It decomposes to calcium oxide and carbon dioxide, and 4.48 g of calcium oxide is left. Work out the mass of carbon dioxide given off, showing all four steps, and then explain what the balance would have read if the crucible had been sealed.
Rung 4 · Take it somewhere new
A student burns a candle on a balance and records a loss of 4.10 g. They conclude that burning destroys matter. Design the measurement that would prove them wrong, say what result you would expect, and explain why their reading was not evidence of destruction.
Key note
Total mass of reactants = total mass of products. Atoms are rearranged, never created or destroyed, so the total cannot change. A balance reading that falls means a gas has left the pan; one that rises means a gas has joined from the air. Seal the vessel and the reading does not move. The relationship is a sum, so it is drawn as a part-whole bar: cover the quantity you want and what is left is the calculation.
Going further
This rule was the argument that ended a whole theory. For most of the eighteenth century, burning was explained by phlogiston — a substance said to escape from things as they burned, which neatly explained why a candle got lighter. Then metals were burned in sealed vessels and weighed, and they got heavier. Defenders of the theory were reduced to suggesting phlogiston had negative mass. Lavoisier weighed everything, including the air, and showed that the gain in the metal was exactly the loss from the air in the vessel. The theory did not survive a balance.
Two honest footnotes. First, a school balance reads to a hundredth of a gram, so a reaction losing a milligram of gas looks perfectly conserved — the rule is exact and the measurement is not. Second, mass is conserved in every chemical reaction there is. In a nuclear reaction — inside a star or a reactor, where the atoms themselves change rather than rearrange — a tiny amount of mass becomes energy instead, and that is where the Sun's output comes from. That is not chemistry, nothing on any bench in any school behaves that way, and it is never an exception a chemistry answer needs. The rule you have just learned is exact for the whole of chemistry.
Before this lesson
Next in this unit
At GCSE this becomes
- Relative formula mass, moles, and reacting-mass calculations — all of them this rule with the arithmetic done in particles.
Where to next
Ask Mr Badmus AI
Still not sure how burning magnesium can get heavier?
Lesson content © MrBadmusAI.