Conservation of mass

No atoms 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 products. This means that the total mass of the products in a chemical reaction will be the same as the total mass of the reactants.

Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide.No atoms are created or destroyed when copper reacts with oxygen to form copper oxide

The total mass stays the same during a chemical reaction. This is the law of conservation of mass.

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:

Calcium carbonate is made up of 28 grams of calcium oxide and 22 grams of carbon dioxide.

Calcium carbonate is made up of 28 grams of calcium oxide and 22 grams of carbon dioxide

Reactions in closed systems

No substances can enter or leave a closed system. A simple closed system could be a sealed container such as a stoppered flask.

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 non-enclosed system. These systems are often open flasks or crucibles that let gases enter or leave. If a gas escapes, the total mass will look as if it has decreased. If a gas is gained, the total mass will look as if it has increased. However, the total mass stays the same if the mass of the gas is included.

Example

Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas.

Zn(s) + 2HCl(aq) \(\rightarrow\) ZnCl2(aq)+ H2(g)

Use the particle model to explain why the reaction between zinc and hydrochloric acid appears to lose mass.

Weight of Zn + HCIThe mass goes down during the experiment

During a reaction no atoms can be created or destroyed. The hydrogen atoms from the hydrochloric acid in solution become hydrogen gas molecules in the products. The reaction takes place in an open container. According to the particle model, particles in a gas are free to move position and are far apart. The hydrogen gas can therefore escape into the atmosphere. This causes the mass measured to decrease.