Can matter just disappear?
A candle gets shorter as it burns, while rusty iron actually gets heavier. Can a chemical reaction make matter vanish, or appear out of nowhere?
In the 18th century, the French chemist Antoine Lavoisier ran reactions inside sealed containers and weighed them carefully before and after. The total mass never changed.
The law of conservation of mass
The total mass of the substances that react equals the total mass of the substances produced.
This is the law of conservation of mass质量守恒定律. Watch for three key ideas:
- That react: anything that doesn’t take part in the reaction (such as leftover excess reactant) doesn’t count.
- Chemical reaction: the law is about chemical changes. You don’t need it to explain physical changes.
- Total: compare the total mass of all the reactants with all the products, including any gases.
Why mass is conserved: the atomic view
A chemical reaction rearranges atoms. Take the electrolysis of water:
Before the reaction there are 4 H atoms and 2 O atoms. Afterward there are still 4 H atoms and 2 O atoms, just joined differently. The kinds, numbers, and masses of atoms don’t change, so the total mass doesn’t either.
Here’s what does and doesn’t change in a reaction:
| What you can see and measure | Particle level | |
|---|---|---|
| Never changes | Total mass, which elements are present, mass of each element | Kinds of atoms, number of atoms, mass of atoms |
| Always changes | Which substances are present | Kinds of molecules |
| May change | Oxidation states of the elements | Number of molecules |
Experiment: why does the balance reading change?
All four reactions below obey the law of conservation of mass, but when you weigh them in an open container, some readings go down, some go up, and one stays the same. Make a guess first, then run each one in an open and in a sealed container.
The gas escapes into the air and isn't weighed, so the reading drops by 0.44 g. The total mass of everything that reacted still equals the total mass of the products.
The gas is trapped in the flask, so the total mass stays the same.
No gas is used or produced, so nothing enters or leaves the container. The reading stays the same even when it's open.
Sealed, it stays the same too. Reactions that don't involve gases show conservation of mass in either kind of container.
from the air reacts and becomes part of the product, so the reading rises by 0.16 g. (In a real experiment, some white smoke drifts away, so the reading may come out lower.)
The that reacts was already inside the flask, so the total mass stays the same.
The and water vapor from the flame escape into the air and aren't weighed, so the reading drops.
The gases are trapped in the jar, so the total mass stays the same.
Conclusion: when a reaction uses or produces a gas, you need a sealed container to show conservation of mass directly.
Example 1: Finding the mass of a product
Heating 15.8 g of potassium permanganate decomposes it completely and leaves 14.2 g of solid. How much oxygen is produced?
- Identify the reactants and products. The only reactant is (15.8 g). The products are the solids and , plus the gas .
- Apply conservation of mass. 15.8 g = mass of leftover solid + mass of oxygen = 14.2 g + mass of oxygen.
- Solve. Mass of oxygen = 15.8 g − 14.2 g = 1.6 g.
Example 2: Working out a formula from conservation of atoms
A substance R burns in oxygen: . What is the formula of R?
- Count atoms on the right. has 2 C and 4 O; has 6 H and 3 O. The right side has 2 C, 6 H, and 7 O in total.
- Count the known atoms on the left. has 6 O.
- Use conservation of atoms. R must contain 2 C, 6 H, and 7 − 6 = 1 O.
- Conclusion. The formula of R is (ethanol, the alcohol in drinks).
Common mistakes
Mistake 1: A burning candle gets lighter, so it breaks the law of conservation of mass. Every chemical reaction obeys the law. The candle gets lighter because the and water vapor it produces escape into the air and aren’t weighed.
Mistake 2: Using the law to explain a physical change. “10 g of ice melts into 10 g of water, which follows the law of conservation of mass” is incorrect. Melting is a physical change, and the law applies to chemical changes.
Mistake 3: Counting substances that didn’t react. When 3 g of carbon reacts with 10 g of oxygen, it doesn’t make 13 g of . The 3 g of carbon can only react with 8 g of oxygen, and the other 2 g of oxygen is left over, so only 11 g of forms.
Quick check
Which of these never changes in a chemical reaction?
A reaction only rearranges atoms, so the kinds, numbers, and masses of atoms stay the same. The kinds of molecules and substances always change, and the number of molecules may change.
In the reaction A + B → C + D, 10 g of A reacts completely with 8 g of B and produces 4 g of D. How much C is produced?
The total mass that reacts is 10 g + 8 g = 18 g, which equals the total mass of the products. So the mass of C = 18 g − 4 g = 14 g.
Summary
- The total mass of the substances that react equals the total mass of the substances produced.
- Why: the kinds, numbers, and masses of atoms don’t change during a reaction.
- When gases are involved, weigh the reaction in a sealed container to see conservation of mass directly.
- The law applies only to chemical changes, and only the mass that actually reacts counts.
On to AP (for later)
AP Chemistry usually talks about conservation of atoms: in a particle diagram, each kind of atom must appear in the same number before and after a reaction. This is the starting point for every stoichiometry calculation in AP Unit 4. Next you’ll learn to read chemical equations, and after that you’ll use conservation of atoms to balance them.