Atoms, elements and compounds · Quantitative
Conservation of mass
A candle burns down to nothing. A nail rusts and gets heavier. One of those looks like mass being destroyed and one like mass being made. Neither is.
Start here
Sixty grams of candle, and by morning there is nothing on the plate.
Light a candle in the evening and leave it. In the morning the wax is gone, the plate is clean, and the balance reads what an empty plate reads. Nothing was taken away and nobody touched it.
So where did sixty grams of wax go?
Into the air, as two invisible gases, and it weighs more than the wax did — because oxygen from the room joined it. Every atom that was in the wax is still somewhere in the room. Catching them is a matter of choosing the right container.
A reaction rearranges atoms. It does not make any and it does not destroy any. So the mass of everything you started with must equal the mass of everything you end with — as long as you can weigh all of it.
At the bench · two reactions, two vessels
The balance reports. It does not explain.
0 of 3 runs done
One reaction gives off a gas and one takes a gas in. Run each of them open to the air and then sealed, and read what the balance says.
Commit first. Marble chips react with acid in an open flask, fizzing hard. What does the balance do?
Reaction
Vessel
The balance is zeroed with everything on it, flask included.
Mass before
152.00 g
Mass after
—
Where it went
not measured — you work it out
Nothing has happened yet. The display shows the mass of the whole apparatus before the reaction starts.
The rule
total mass of everything before = total mass of everything after
everything means the gases too
mass is measured in grams (g)
Worked example · one step at a time
2.40 g of magnesium burns and leaves 4.00 g of white powder.
Step 0 of 4
Formula
total mass before = total mass after
Nothing is created and nothing is destroyed. Everything has to be counted, including the gas.
Insert
2.40 g + mass of oxygen = 4.00 g
The magnesium was weighed; the oxygen came from the air and was not.
Fine-tune
mass of oxygen = 4.00 − 2.40
Cover the part you want on the bar. Rearranged so the unknown is on its own, with both masses in grams.
Answer
1.60 g of oxygen joined from the air
The powder is heavier than the metal because it contains something that was not on the balance to start with.
Your turn · the same four steps
Marble and acid, open flask: 152.00 g before, 149.80 g after.
Work out the mass of gas that left the flask. Commit to each line, then open the worked version.
Step 1 · The rule
Step 2 · Insert
Steps 3 and 4 · Work it out, then answer
The open flask, done four ways
Formula
total mass before = total mass after
The gas counts, even though it has left the flask.
Insert
152.00 = 149.80 + mass of gas
Everything that started on the balance is on the left; everything that ended up anywhere is on the right.
Fine-tune
mass of gas = 152.00 − 149.80
Cover the gas on the bar. Rearranged so the unknown is alone, and both masses are already in grams.
Answer
mass of gas = 2.20 g
That is carbon dioxide, now in the room. Seal the flask and those 2.20 g stay on the balance.
Key fact
Atoms are rearranged in a reaction, never created or destroyed. If the mass appears to change, a gas has come in from the air or escaped into it.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“When something burns, the mass is destroyed — it turns into heat and light.”
This is the same wrong idea you met when a puddle dried up, wearing different clothes. Commit before you read on.
Heat and light are not made of atoms, so nothing that leaves as heat or light can account for a gram of anything. Burn 60 g of candle wax in a sealed box and weigh the box afterwards: it reads exactly what it read before, warm or not.
What actually happens is that the wax joins with oxygen from the air and leaves as carbon dioxide and water vapour — which together weigh more than the wax did. The reason it looks like destruction is that the products are invisible and they float away. Put a lid on and the illusion goes with them.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
18 g of a metal reacts completely with 32 g of sulfur in a sealed tube. What mass of metal sulfide is made?
Rung 2 · The one that catches people
An iron nail is weighed, left to rust for a month, and weighed again. It is heavier. Why?
Rung 3 · Explain
A candle is weighed, burnt for an hour, and weighed again. It has lost 12 g. Explain where the 12 g went, and describe an experiment that would prove you right.
Rung 4 · Take it somewhere new
A sealed bag of steel wool is weighed, shaken and left until the wool has rusted, then weighed again. Predict the reading, then say what would be different if the bag had a small hole in it, and explain both.
Key note
Total mass before a reaction equals total mass after it, because atoms are only rearranged. In an open vessel a gas can leave or join, so the balance changes; seal the vessel and it never does.
Going further
For two hundred years chemists explained burning by saying that a substance called phlogiston escaped from things as they burnt. It fitted: burning wood loses mass, and a candle goes out in a sealed jar because the air is full of phlogiston and cannot take any more. Then someone weighed a burning metal carefully, found it got heavier, and the theory needed phlogiston to have negative mass. That is the point at which a model stops being repairable — and the weighing had been possible the whole time.
Before this lesson
Connects to
At GCSE this becomes
- Balancing equations by mass, reacting masses, and calculating what a reaction can produce.
Where to next
- Next: Pure or mixture?
Mixtures and separation
- Previous: Formulae
Ask Mr Badmus AI
Not sure why a sealed flask behaves differently?
Burning magnesium is dangerously bright. Never look straight at it, and never seal a flask that is being heated.
Lesson content © MrBadmusAI.