10. Concepts in Chemistry: Conservation of Mass

Question 1

What happens in a Chemical Reaction?

Answer 1

• Mass is always conserved
• No atoms destroyed or created
• Same number and types of atoms on each side of reaction equation

Question 2

What is a good way to show that total mass before and after a reaction doesn’t change?

Answer 2

• Do reaction in Closed System
• Example: Precipitation reaction
• Copper Sulphate solution reacts with Sodium Hydroxide -> insoluble Copper Hydroxide (precipitate) and soluble Sodium Sulphate

CuSO4 + 2NaOH -> Cu(OH)2 + Na2SO4

Question 3

What is involved if the Mass seems to change?

Answer 3

• If in unsealed reaction vessel
• Gas involved
• Mass Increases: At least 1 reactant is a gas found in air e.g. oxygen and products are solids, liquids or aqueous

Before: Gas floating in air - can measure mass -> gas reacts to form part of product -> becomes contained in reaction vessel i.e. increases mass e.g. metal + oxygen = metal oxide

• Mass Decreases: Some or all of reactants are solids, liquids, aqueous and at least on of products is a gas

Before: Reactants contained in vessel -> Gas forms and leaves vessel so can’t measure mass e.g. Metal carbonate -> metal oxide + carbon dioxide