Properties of Matter — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. A student stirs sugar into cold water until no more dissolves. She then heats the water and adds more sugar, which dissolves. What conclusion is best supported?
A) Heating water creates a chemical reaction that breaks down the sugar molecules.
B) Increasing the temperature of the solvent allows more solute to dissolve.
C) The sugar melted from the heat rather than actually dissolving in the water.
D) Cold water cannot dissolve any sugar, so all the sugar settled on the bottom.
Heating water gives its molecules more energy, which allows them to break apart and surround more sugar particles. That is why the heated water could dissolve additional sugar after the cold water was already saturated.
2. A pot of boiling water is left on the stove for a long time and the water level drops. Where did the water go?
A) The water was destroyed by the heat energy from the stove burner.
B) The water molecules changed into oxygen and hydrogen gas permanently.
C) The water evaporated and entered the air as invisible water vapor.
D) The water was absorbed into the metal of the pot through tiny holes.
Boiling gives water molecules enough energy to escape the liquid as water vapor, an invisible gas. The water was not destroyed; it changed state and entered the surrounding air.
3. A glass of ice water is left on a table and water droplets form on the outside of the glass. What caused this?
A) Water leaked through microscopic cracks in the glass wall.
B) The cold glass caused water vapor in the surrounding air to condense on its surface.
C) The ice inside the glass pushed liquid water through the glass by pressure.
D) Warm air heated the glass and made it sweat like a person exercising.
The cold glass cools the air right next to it, causing invisible water vapor in that air to lose energy and change back into liquid droplets on the glass surface. This process is called condensation.
4. A student wants to dissolve a sugar cube faster without changing the temperature of the water. Which strategy would work best?
A) Use a larger container so the water has more room to react with the sugar.
B) Add more sugar cubes so they help each other dissolve more quickly.
C) Crush the sugar cube into powder to increase the surface area touching the water.
D) Let the sugar sit longer without stirring because time alone speeds dissolving.
Crushing the sugar cube into powder creates many small pieces with much more total surface area exposed to the water. More surface contact means water molecules can dissolve the sugar faster without needing to raise the temperature.
Part B: Fill in the Blank
Write the correct answer on each line.
1. The ability of a substance to dissolve in another substance is called its solubility.
Solubility measures how well one substance can dissolve in another. For example, sugar has high solubility in water, while sand has almost no solubility in water.
2. During freezing, a liquid loses thermal energy and its particles slow down enough to form a solid.
Thermal energy is the energy of moving particles. When a liquid loses enough thermal energy, its particles slow down and lock into fixed positions, forming a solid.
3. Crushing a solute into smaller pieces helps it dissolve faster by increasing its surface area.
Smaller pieces expose more total surface area to the solvent. More contact between solute and solvent means the dissolving process happens faster.
4. Water droplets forming on a cold glass is an example of condensation, where gas turns to liquid.
The cold glass cools nearby water vapor below its dew point, causing the gas molecules to lose energy and form visible liquid droplets on the glass surface through condensation.
5. During boiling, the vapor pressure of the liquid equals the air pressure pushing down on its surface.
Boiling occurs when the vapor pressure inside the liquid matches the atmospheric pressure above it. At that point, bubbles of gas can form throughout the liquid and rise to the surface.