Law of conservation of mass
No The smallest part of an element that can exist. are created or destroyed in a chemical reaction. Instead, they just join together in a different way than they were before the reaction, and form A substance formed in a chemical reaction.. This means that the total The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of the products in a chemical reaction will be the same as the total mass of the A substance that reacts together with another substance to form products during a chemical reaction..
Calculations using the law
The mass of one substance in a reaction can be calculated if the masses of the other substances are known. For example:
Image caption, 1. Calcium carbonate is made up of 28 grams of calcium oxide and 22 grams of carbon dioxide
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Reactions in closed systems
No substances can enter or leave a In chemistry, a system in which no substances can enter or leave during a reaction.. A simple closed system could be a sealed container such as a stoppered flask. Sometimes reactions that happen in open beakers are closed systems. These include:
- acid-alkali The reaction between an acid and a base to form a salt plus water. reactions, which produce A compound formed by neutralisation of an acid by a base, eg a metal oxide, as the result of hydrogen ions in the acid being replaced by metal ions or other positive ions. Sodium chloride, common salt, is one such compound. solutions
- A reaction in which an insoluble solid is formed when certain solutions are mixed., which produce an Unable to dissolve in a particular solvent. For example, sand is insoluble in water.A suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction. from two reacting Mixture formed by a solute and a solvent.
In both examples, the reactants and products stay in the beaker. The total mass of the beaker and the substances it contains stays the same during the reaction.
Reactions in non-enclosed systems
Substances can enter or leave a In chemistry, a system in which gases or other substances can enter or leave during a reaction.. These systems are often open flasks or crucibles that let gases enter or leave. The mass may seem to decrease or increase in these systems. The table shows two examples.
| Reaction | What happens | Apparent change in mass |
| Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) | Hydrogen gas escapes | Mass seems to decrease |
| 2Mg(s) + O2(g) → 2MgO(s) | Oxygen is gained from air | Mass seems to increase |
| Reaction | Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) |
|---|---|
| What happens | Hydrogen gas escapes |
| Apparent change in mass | Mass seems to decrease |
| Reaction | 2Mg(s) + O2(g) → 2MgO(s) |
|---|---|
| What happens | Oxygen is gained from air |
| Apparent change in mass | Mass seems to increase |
The mass seems to change during the reaction, but if the mass of the gas is included, it stays the same.
