Conservation of Mass

Question 1

Explain the law of conservation of mass.

Answer 1

• The total mass of a closed system before and after a reaction doesn’t change
• This is because no atoms are created nor destroyed during a chemical reaction

(total mass of reactants is the same as total mass of products)

Question 2

What is a closed system?

Answer 2

A system where nothing can get in or out

Question 3

What causes a precipitation reaction to occur?

Answer 3

Two solutions reacting to form an insoluble solid, called a precipitate, in the solution

Question 4

Explain what could cause the total mass of a system to increase following a reaction.

Answer 4

• Using an unsealed reaction vessel
• At least one of the reactants is a gas found in air (e.g. oxygen) and the products are solids, liquids or aqueous
• When the gas reacts to form part of the product, it becomes contained in the reaction vessel
• Therefore, the total mass of the system increases

Question 5

Explain what could cause the total mass of a system to decrease following a reaction.

Answer 5

• Using an unsealed reaction vessel
• Some, or all, of the reactants are solids, liquids or aqueous and at least one of the products is a gas
• Because the vessel isn’t enclosed, the gas can escape from it when formed
• Therefore, the total mass of the system decreases

Question 6

A student investigates a reaction within a sealed reaction vessel.

How would you expect the total mass of the system to change over the course of the reaction? Explain your answer.

Answer 6

The total mass would stay the same over the course of the reaction because it is happening in a closed system

Question 7

(Exam-style question)

Answer 7