Conservation of mass | Exit Quiz | Oak National Academy

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

• ingredients

• products

• reactants

Answer explanation

Reactants are the starting chemicals in a reaction.

Order the steps in a chemical reaction.

• Formation of products

• Presence of reactants

• Rearrangement of atoms

Answer explanation

Start with the reactants, the atoms rearrange during the reaction to form the products.

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

• atoms are destroyed

• atoms change into different elements

• atoms are not lost or gained, but rearranged

Answer explanation

Atoms are not lost or gained but rearranged in a chemical reaction, so the mass of the reactants will equal the mass of the products.

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.

• What could have happened to the product?

• open

• What other reactant was present?

• 12 g - 10 g = 2 g

• What type of system was this?

• Some could have escaped into the atmosphere.

• How much of this reactant reacted?

• oxygen

Answer explanation

Oxygen was the other reactant, reacting with magnesium to form magnesium oxide. 2 grams of oxygen reacted, inferred from the mass increase of the magnesium oxide product. This was an open system. Some of the magnesium oxide could have escaped.

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.

• Na(s) + Cl₂(g) → NaCl(s)

• Fe(s) + S(s) → FeS(s)

• 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

• 2Al(s) + 3Br₂(l) → 2AlBr₃(s)

• Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)

Answer explanation

Na(s) + Cl₂(g) → NaCl(s) and 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s) Chlorine and oxygen are used as reactants, so mass may appear to increase. Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g) hydrogen is a product and would escape so mass would appear to decrease.

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

• X = 5 (g), Y = 3 (g) and Z = 4 (l)

• X = 4 (g), Y = 2 (g) and Z = 2 (g)

• X = 3 (g), Y = 3 (g) and Z = 3 (g)

• X = 5 (g), Y = 3 (g) and Z = 4 (g)

• X = 2 (g), Y = 2 (g) and Z = 4 (g)

Answer explanation

The equation is C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g). The hydrocarbon is propane which is a gas. Water would also be in the gas state as combustion involves the release of much heat energy.