Conservation of Mass: 17 MCQs with answers

B · Matter is neither created nor destroyed in a closed system.

The law states that in a closed system, matter is neither created nor destroyed; total mass before equals total mass after.

B · Equal to the total mass of the products

Because atoms are rearranged and not lost or gained, the mass of reactants equals the mass of products.

B · Rearranged to form new products

Atoms are simply rearranged during a reaction; none are created or destroyed.

C · Ice melting into water with the same mass

Melting is a physical change; the water molecules are simply rearranged, so the mass of ice equals the mass of melted water.

B · Antoine Lavoisier

Lavoisier's experiments in the late 18th century provided early strong evidence for the law.

A · True

Antoine Lavoisier is credited with establishing the law of conservation of mass in the late 18th century.

D · The mass stays the same because all products remain inside

In a sealed container, gases such as carbon dioxide and water vapour remain trapped, so total mass is conserved.

A · Carbon dioxide gas escaped from the flask

In an open system, escaping carbon dioxide carries mass away, causing an apparent mass loss.

B · To show that the number of each type of atom is equal on both sides

Balancing reflects the conservation of atoms, and therefore mass, in a chemical reaction.

D · It is mass that is conserved; weight depends on gravity

Mass is the amount of matter (kg); weight is a force (N) that depends on gravity. Conservation applies to mass.

C · 2

The left side has $O_2$ (2 oxygen atoms); the right side has $2H_2O$ (2 oxygen atoms). Both sides have 2.

A · The system must be closed so no matter escapes or enters

A closed system prevents matter from entering or leaving, allowing accurate mass comparison.

B · A small amount of mass converts to energy, so mass is not exactly conserved

In nuclear reactions, mass-energy equivalence ($E=mc^2$) means a tiny amount of mass converts to energy; for chemistry, mass conservation is an excellent approximation.

C · All products, including carbon dioxide gas, remain inside the closed flask

In a closed system, gases cannot escape, so the total mass of reactants equals the total mass of products.

B · 1 C, 4 H, 4 O

Counting: left has 1 C, 4 H, 4 O; right has 1 C (in $CO_2$), 4 H (in $2H_2O$), and 4 O (2 in $CO_2$ + 2 in $2H_2O$).

A · It allows calculation of reactant and product quantities because no mass is lost

Industries use the law to calculate yields and control processes, such as in the Haber process for ammonia.

A · The reaction produced a gas that escaped the sealed container.

The correct option is that the reaction produced a gas that escaped the sealed container. In a closed system, mass should be conserved, but if gases escape, it can lead to a decrease in measured mass, demonstrating the importance of maintaining a closed system during reactions.