5.10 Conservation of Mass

Question 1

Conservation of mass

Answer 1

• During a chemical reaction, even though the way atoms are arranged in compounds changes, no atoms are destroyed and no atoms are created.
• This means there are the same number and types of atoms on each side of a reaction equation. Because of this, no mass is lost or gained - we say that mass os conserved during a reaction.

Question 2

What are the two reasons why in an experiment you might observe a change of mass of an unsealed reaction during a reaction?

Answer 2

• Explanation 1: If the mass increases, it’s probably because some of the atoms in gaseous reactants end up in solid, liquid or aqueous products. Before the reaction, the gas is floating around the air. It’s there, but it’s not contained in the reaction vessel, so you can’t account for its mass. When the gas reacts to form part of the product, it becomes contained inside the reaction vessel - so the total mass of the stuff inside the reaction vessel increases.
• Explanation 2: If the mass decreases, it’s probably because some (or all) of the atoms in the solid, liquid or aqueous reactants end up in gaseous products. Before the reaction, all the reactants are contained in the reaction vessel. If the vessel isn’t enclosed, then the gas can escape from the reaction vessel as it’s formed. It’s no longer contained in the reaction vessel, so you can’t account for its mass - the total mass of the stuff inside the reaction vessel decreases.

Question 3

How do you avoid losing or gaining mass during a reaction?

Answer 3

Complete your experiments in a sealed container

Question 4

What does the law of conservation of mass state>

Answer 4

No atoms are lost or made during a chemical reaction

Question 5

What is a chemical change?

Answer 5

A rearrangement of the atoms in the reactants, to form the products

Question 6

Why is it more difficult to measure the mass of a gas than the mass of a liquid or solid?

Answer 6

A gas can easily escape into the air

Question 7

In a chemical reaction, is there a change in the number of atoms?

Answer 7

No

Question 8

A chemical reaction is carried out in a beaker on a balance. Why may the balance reading for the products be more than the reading for the reactants?

Answer 8

Gas from the air reacted with the reactants