States of Matter
What state of matter has particles that are very close together and vibrate mostly in place?
Solid
What is the name of the phase change when a liquid becomes a gas at the surface?
Evaporation (or vaporization at boiling).
True or false: Molecules in a liquid are constantly in contact with other molecules.
True.
In everyday language, "heat" can mean two things. In science, what does "heat" specifically refer to?
In science, "heat" is the transfer of thermal energy between objects due to a temperature difference.
Give one everyday example of evaporation (liquid → gas).
Examples: puddles drying, wet clothes drying, water in a cup slowly evaporating.
Which state of matter has particles that are widely spaced and move freely past each other?
gas
What is the phase change called when a gas becomes a liquid?
Condensation
Draw or describe a simple particle model for a gas in a closed container (qualitative: spacing and motion).
Model: many points spread far apart, moving quickly and randomly, bouncing off container walls.
How is temperature related to the motion of atoms or molecules? Give a short definition
Temperature measures the average kinetic energy of the particles — higher temperature means particles move faster on average.
Why does sweating cool your skin? Use particle or energy ideas in your answer.
Sweat evaporates from your skin, taking thermal energy away from your skin and cooling it; evaporation requires energy, which is taken from the skin as heat.
Describe how particle motion in a liquid compares to particle motion in a solid.
In a liquid, particles are close and can move/slide past each other; in a solid, particles are closely packed and mainly vibrate in place.
Name the phase change when a solid turns directly into a gas (skipping liquid).
Sublimation
How does the average kinetic energy of molecules change when thermal energy is added to a sample?
The average kinetic energy increases.
If thermal energy is removed from a gas, what two particle-level changes might you predict (relating to motion and state)?
Particles slow down (average kinetic energy decreases) and the gas may condense into a liquid.
Carbon dioxide can be a gas at room temperature but can become a solid called dry ice. Name the process when CO22 changes from gas directly to solid.
Deposition (if gas to solid directly is sometimes called deposition; sublimation is solid to gas). For gas → solid: deposition.
Give one example of a pure substance and name its state of matter at room temperature
Example: Water — liquid at room temperature. (Other correct examples: helium — gas; carbon dioxide — gas at room temp.)
During melting, is thermal energy being added or removed? How does this affect particle motion?
Thermal energy is being added; particle motion increases until particles can move enough to separate into a liquid.
Describe what happens to molecules during freezing.
During freezing, liquid molecules slow down, come closer together, and attractive forces bring them into a solid state; kinetic energy decreases
A sealed container of helium is heated. Explain how the average kinetic energy and the total thermal energy are affected.
Average kinetic energy of helium atoms increases (temperature rises); total thermal energy also increases because both temperature and the energy per particle rise.
A beaker of water at 25∘C25∘C is heated until it boils. Which happens first: the average kinetic energy of water molecules increases, or the water changes to gas? Explain the sequence.
First the average kinetic energy of water molecules increases as thermal energy is added (temperature rises), and when it reaches the boiling point, the water changes to gas (vaporization). During boiling the temperature stays around the boiling point while energy goes into changing state.
Explain qualitatively how particle spacing and motion change when a solid becomes a gas.
Particles move farther apart and their motion becomes faster; bonds/interactions are overcome and the substance becomes a gas.
Explain why temperature can remain constant during a phase change even while thermal energy is being added or removed.
During a phase change energy goes into changing particle interactions (breaking or forming bonds) rather than increasing temperature; the added/removed thermal energy changes potential energy, not temperature, so temperature remains constant.
Describe at the molecular level what happens to molecules during condensation.
During condensation, gas molecules slow down, come closer together, and attractive forces bring them into a liquid state; kinetic energy decreases and potential energy changes as bonds form.
Explain why two objects at the same temperature can still have different amounts of total thermal energy
Because total thermal energy depends on temperature, number of particles, and state; two objects at the same temperature can have different masses (different numbers of particles) so different total thermal energy.
Design a simple model (brief description or quick sketch) that predicts what happens to particle motion and spacing when thermal energy is removed from liquid water until it freezes.
As thermal energy is removed, particles move more slowly (kinetic energy decreases), they come closer together, and ordered structure forms; at the freezing point the liquid becomes a solid with particles in fixed positions (crystalline structure). (Accept sketch showing widely spaced moving particles becoming closer and vibrating in place.)