Properties of Matter — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. A student burns a piece of paper in an open container and finds that the ash weighs less than the original paper. Does this violate conservation of mass?
A) Yes, because mass was clearly lost when the paper burned.
B) No, because the missing mass escaped as gases and smoke into the air.
C) Yes, because chemical changes always reduce the total amount of mass.
D) No, because ash is a different substance so it should weigh less.
Mass was conserved — the 'missing' mass became gases (CO₂, water vapor) that escaped into the open air.
2. Before a reaction, the total mass of the reactants is 50 grams. After the reaction in a sealed container, what is the total mass of the products?
A) Less than 50 grams because some mass is converted into energy.
B) Exactly 50 grams because mass is conserved in chemical reactions.
C) More than 50 grams because new substances are created during the reaction.
D) It depends on whether the reaction is physical or chemical.
Conservation of mass: total mass = 50 g before = 50 g after. This applies to ALL changes in a closed system.
3. Lemonade is made by mixing water, lemon juice, and sugar. Which classification of matter best describes lemonade?
A) An element because it is a single liquid substance.
B) A compound because the ingredients are chemically combined.
C) A heterogeneous mixture because the sugar sinks to the bottom.
D) A homogeneous mixture because the ingredients are evenly mixed throughout.
Well-mixed lemonade is a homogeneous (uniform) mixture — ingredients blend evenly and are not chemically combined.
4. A student adds 10 grams of baking soda to 20 grams of vinegar in a sealed bag. After the reaction stops and gas fills the bag, what is the total mass?
A) Less than 30 grams because the gas weighs less than the original liquids.
B) Exactly 30 grams because all the matter is still inside the sealed bag.
C) More than 30 grams because the gas takes up more space so it has more mass.
D) It cannot be determined without knowing what products the reaction made.
The sealed bag keeps all matter inside. The CO₂ gas produced is still in the bag — total mass stays 30 g.
Part B: Fill in the Blank
Write the correct answer on each line.
1. In a closed system, the total mass stays the same before and after any change.
Conservation of mass holds in any closed system. Neither physical nor chemical changes create or destroy mass.
2. Air is classified as a mixture because it contains several different gases mixed together.
Air contains nitrogen (~78%), oxygen (~21%), and other gases — all physically mixed, not chemically bonded.
3. H₂O is the chemical formula for water, showing it contains hydrogen and oxygen.
A chemical formula uses element symbols and subscripts to show the composition of a compound.
4. When matter seems to disappear during a reaction, it has usually changed into a gas.
Gases are invisible and can escape into the air, making it appear that mass was lost. It's actually conserved.
5. An element is a pure substance that cannot be broken down into simpler substances.
Elements are the fundamental building blocks of matter. They appear on the periodic table and cannot be broken down by chemical means.