Burning 1 kg of natural gas releases a large amount of energy, and the energy leaves as heat and light. The last lesson says the leftover gases must end very slightly less massive. Yet every chemistry course teaches that mass is conserved. Both are right.
From earlier science you know the law of conservation of mass: in a chemical change, the total mass of the products equals the total mass of the reactants. This lesson reconciles the law with the mass-energy ledger.
A chemical change that releases energy really does lower the products' total mass.
The change is real; the question is whether anyone could ever measure it.
Here is the key again: because the conversion factor c² is enormous (about 9 × 10¹⁶ joules per kilogram of mass), a tiny amount of mass is equivalent to a huge amount of energy.
Run the sentence backward: a huge amount of energy is equivalent to only a tiny amount of mass.
A chemical reaction's energy, huge by everyday standards, is therefore equivalent to a vanishingly small mass.
Burning 1 kg of natural gas changes the total mass of the products by less than a millionth of a gram.
No balance can see a millionth of a gram missing from a kilogram, so the law of conservation of mass keeps working perfectly for chemistry.
Burning 1 kg of natural gas releases a large amount of energy as heat and light. Why does a chemistry balance still read the products' mass as unchanged?
The released energy leaves the gases as heat and light.
The mass equivalent of the released energy is the energy divided by the enormous conversion factor.
Dividing a chemistry-sized energy by an enormous factor leaves less than a millionth of a gram.
A balance cannot detect a millionth of a gram missing from a kilogram.
The products' mass really fell, but by less than a millionth of a gram, far below what any balance can read.
Mireille burns a candle stub for ten minutes, and the heat and light escape the room. By what sort of amount does the mass of the candle's products fall short of the candle plus the oxygen it used?
The candle's burning releases far less energy than burning a full kilogram of natural gas.
Even the kilogram's release is equivalent to less than a millionth of a gram.
So the candle's missing mass is smaller still.
The products fall short by far less than a millionth of a gram: an immeasurably tiny amount.
Burning 1 kg of natural gas releases energy as heat and light. By how much does the total mass of the products change?
Lorcan says: 'E = mc² means that mass is destroyed whenever a change releases energy.' Is Lorcan correct?
Delphine says: 'A chemical change really does change the products' total mass, just by far too little to measure.' Is Delphine correct?
A campfire burns all night, releasing a large amount of energy. Why do the ashes, smoke and gases together have almost exactly the mass of the original wood and oxygen?
Caspian burns a little methane in a sealed glass jar, lets it cool back to room temperature, and checks its mass. The reading matches the reading from before the burn. Which statement explains the matching readings?
One chemical reaction releases ten times as much energy as another. How does its mass change compare?