Law of Conservation of Matter
The law of conservation of matter (also called the law of conservation of mass) states that matter is neither created nor destroyed in a chemical reaction or physical change, so the total mass of the products equals the total mass of the reactants in a closed system. In brief: if 24 g of magnesium burns with 16 g of oxygen, exactly 40 g of magnesium oxide forms.
The law explains why a burning candle seems to vanish although its atoms are still present in gases, and why equations must be balanced. This article states the law, shows how to test it with experiments, explains apparent exceptions, includes calculations, describes its history from Lavoisier to modern physics and lists common mistakes.
What does the law say?
In a closed system, where no matter enters or leaves, the total mass stays constant during any physical or chemical change. The mass of the reactants before the reaction equals the mass of the products after it: total mass of reactants = total mass of products.
The law applies to each element too. If 12 g of carbon atoms are present in the reactants, 12 g of carbon atoms are present in the products, although they may be in different compounds.
Why is matter conserved?
According to atomic theory, a chemical reaction rearranges atoms. Bonds between atoms break and new bonds form, but the atoms themselves are not created, destroyed or changed into atoms of other elements. Since the same atoms are present before and after, the total mass is the same.
This idea is the reason for balancing chemical equations. In 2H2 + O2 → 2H2O there are 4 hydrogen atoms and 2 oxygen atoms on each side.
How can you test the law?
- Place two solutions in a flask, with a small test tube of the second solution standing inside the flask, so that they do not mix at first. Use, for example, sodium carbonate in the flask and calcium chloride in the tube, or any pair that gives a precipitate.
- Seal the flask with a stopper and weigh the whole apparatus. Record the mass.
- Tip the flask so the solutions mix. A white precipitate forms.
- Weigh the apparatus again. The mass is the same as before, within the accuracy of the balance.
The sealed flask is essential. If the experiment makes a gas and the container is open, gas escapes and the mass appears to fall.
Why does mass sometimes seem to change?
| Observation | Why it seems to break the law | Explanation |
|---|---|---|
| Burning wood leaves little ash | Mass decreases | Carbon dioxide and water vapor escape into the air |
| Iron wool gains mass when it rusts | Mass increases | Iron combines with oxygen from the air |
| Antacid tablet fizzing in an open cup | Mass decreases | Carbon dioxide gas escapes |
| Heated magnesium ribbon gains mass | Mass increases | Magnesium combines with oxygen |
| Salt dissolving in water | Salt seems to disappear | Mass of solution equals mass of salt plus water |
Worked examples
- Burning magnesium. 12.0 g of magnesium reacts with oxygen to form 20.0 g of magnesium oxide. The mass of oxygen that reacted is 20.0 − 12.0 = 8.0 g.
- Decomposition of calcium carbonate. Heating 50.0 g of calcium carbonate gives 28.0 g of calcium oxide. The mass of carbon dioxide released is 50.0 − 28.0 = 22.0 g, which agrees with the formula masses (100 g/mol CaCO3 gives 56 g/mol CaO and 44 g/mol CO2).
- Reaction in an open beaker. A beaker with an antacid tablet and water has a total mass of 150.00 g. After the fizzing stops, the reading is 149.35 g. The mass of gas lost is 150.00 − 149.35 = 0.65 g.
- Finding a missing mass. 10.0 g of hydrogen peroxide solution decomposes and releases 0.47 g of oxygen gas in a closed system. The mass of the rest is 10.0 − 0.47 = 9.53 g.
How was the law discovered?
Early alchemists and chemists did not weigh gases, so burning seemed to destroy matter and metals seemed to change mass in puzzling ways. In the 1770s and 1780s Antoine Lavoisier carried out careful experiments in sealed vessels and weighed all the substances, including gases. He showed that the mass remains constant and that burning is a combination with oxygen. Earlier, Mikhail Lomonosov in Russia had expressed a similar idea in the 1750s. Lavoisier’s work is usually seen as the foundation of modern chemistry.
Is mass always conserved?
In ordinary chemical reactions the law holds to a very high precision. In nuclear reactions, a small part of the mass is converted into energy according to Einstein’s equation E = mc2, and so mass alone is not conserved. Physicists therefore state a broader law of conservation of mass-energy. Even chemical reactions involve such small energy changes that the change in mass is far too small to measure with any balance.
Which mistakes are common?
Mistake 1: Thinking that burned things lose mass
The substances change into gases, which escape. If all products are collected, the total mass is unchanged.
Mistake 2: Applying the law to open systems without accounting for gases
Gases can enter or leave. Include them in the mass balance.
Mistake 3: Believing that atoms are created in reactions
Atoms are rearranged only. The number of each type of atom stays constant.
Mistake 4: Assuming that volume is also conserved
Mass is conserved, but volume is not. Mixing 50 mL of ethanol and 50 mL of water gives less than 100 mL.