Conservation of mass means that the combined mass of the starting reactants equals the combined mass of the products formed.

A closed system is one in which matter cannot enter or leave the observed environment, allowing only energy transfer between the system and its surrounding environment.

Reactants are the elements or compounds before a reaction takes place.

Products are the elements or compounds that are formed after a reaction has taken place.

Atoms are the building blocks of all matter.

Conservation of mass

Combined science

Three types of chemical reaction

Balancing equations

Uncertainties in measurements: chemistry

Mass in a chemical reaction

In the Making salts unit, pupils will learn about the production of pure salts, concentration and strength of acids, and what makes acids acidic and how bases can neutralise them.

In the Understanding Chemical Reactions unit, pupils learned about how atoms rearrange during chemical reactions and the conservation of mass.

This unit covers writing chemical formulae, balancing equations, and using the conservation of mass. It includes calculating moles, empirical formulae, relative formula masses, and theoretical yields. Emphasis is on stoichiometry, concentration, gas volumes, and using standard form.

Calculations involving masses

Physical activity and equipment requiring safe use

What is the name of the starting chemicals in a reaction?

atoms are not lost or gained, but rearranged

Which of the following statements explains why mass is conserved in a chemical reaction?

Some could have escaped into the atmosphere.

10 g of magnesium is heated in a crucible and 12g of magnesium oxide is obtained at the end of the reaction. Match the questions to the answers below about this reaction.

Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)

Some open system reactions where a gas is used as a reactant or formed as a product, may appear as if there has been a gain or loss in mass. Choose the reactions below where this could be the case.

Balance the following equation showing the combustion of a hydrocarbon by replacing X, Y and Z and add state symbols to show the expected state of the reactants and products.
C₃H₈ + XO₂ → YCO₂ + ZH₂O

What are the building blocks of all matter?

Match the formulae of the substances to the name of the substance. Use your periodic table to help.

nitrogen (2), hydrogen (8), sulfur (1) and oxygen (4)

nitrogen (2), hydrogen (8) and oxygen (4)

Name the elements and number of atoms of each type in (NH₄)₂SO₄

of oxygen (2) and hydrogen (4) in the reactants and products.

hydrogen + oxygen → water (dihydrogenoxide)

Hydrogen reacts with oxygen to form water as steam. Match up the following statements to make complete sentences about this reaction.

carbon (1), hydrogen (4) oxygen (4)-> carbon (1), hydrogen (4) oxygen (4) No

carbon (1), hydrogen (4) oxygen (2)-> carbon (1), hydrogen (4) oxygen (3) Yes

carbon (4), hydrogen (4) oxygen (2)-> carbon (2), hydrogen (4) oxygen (3) Yes

carbon (1), hydrogen (4) oxygen (4)-> carbon (1), hydrogen (4) oxygen (4) Yes

Methane (CH₄) burns to produce carbon dioxide (CO₂) and water (H₂O). Count how many atoms of each element are on the left and right hand side of the equation. Are they equal? CH₄ + 2O₂ → CO₂ + 2H₂O

Conservation of mass

Lesson outcome

Content guidance