Predict State Changes - Middle School Physical Science

1. A puddle of liquid water is left outside on a sunny day. The water absorbs thermal energy even though it never reaches $100^\circ\text{C}$. Over time, the puddle gets smaller. Which process best describes what is happening?
   Answer: Evaporating (liquid to gas) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For evaporating: When liquid water absorbs thermal energy below 100°C, such as from sunlight, some surface particles gain enough speed to overcome attractions and escape into the air as gas (water vapor)—this is evaporation, a slower liquid→gas transition that doesn't require reaching the boiling point or forming bubbles throughout. Choice C is correct because it correctly names the process: evaporating for liquid→gas below 100°C when thermal energy is added. Choice A reverses the direction, incorrectly predicting freezing when thermal energy is added, when adding heat actually causes melting or boiling, not freezing. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
2. Which statement correctly describes what happens to water particles when thermal energy is removed from water vapor and it turns into liquid droplets on a cold surface?
   Answer: Particles slow down and move closer together, forming a liquid This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For condensing: When water vapor (gas) comes into contact with a cold surface like a cold glass or mirror, thermal energy transfers from the fast-moving gas particles to the cold surface, slowing the water vapor particles down—as they slow, the attractions between water molecules become strong enough to pull the particles together from widely separated (gas) to touching (liquid), forming water droplets on the surface in the process called condensation (gas → liquid when thermal energy is removed). Choice B is correct because it properly explains that thermal energy change affects particle motion which causes state change, specifically that removing heat slows particles and allows attractions to pull them closer into liquid. Choice A reverses the direction, incorrectly stating particles speed up and spread apart forming gas when thermal energy is removed, when removing heat actually slows them down. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
3. Ice (solid water) at $0^\circ\text{C}$ is heated on a warm plate. While it is changing state, the temperature stays at $0^\circ\text{C}$ even though thermal energy is still being added. What process is happening?
   Answer: Melting This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For melting: When thermal energy is added to ice (solid water at or below 0°C), the particles gain energy and vibrate more and more vigorously until at 0°C they have enough energy to break free from their fixed positions in the crystal pattern—at this point, melting occurs and ice transitions to liquid water as particles begin sliding past each other instead of vibrating in place. Choice C is correct because it correctly names the process: melting for solid→liquid, where temperature stays constant during the state change as energy goes into breaking attractions. Choice A reverses the direction, incorrectly predicting freezing when thermal energy is added, when adding heat actually causes melting or boiling, not freezing. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
4. A cup of liquid water is heated from $20^\circ\text{C}$ to $90^\circ\text{C}$ at normal air pressure. Based on water's boiling point of $100^\circ\text{C}$, what is the water's state at $90^\circ\text{C}$?
   Answer: Liquid (water), because it is below $100^\circ\text{C}$ This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For boiling: When liquid water is heated to 100°C (boiling point at normal pressure), continued addition of thermal energy causes particles to move so rapidly that they completely overcome the attractions to neighboring particles and escape into the air as gas (steam or water vapor)—this is called boiling, the liquid → gas transition that occurs at the boiling point temperature when thermal energy input gives particles enough kinetic energy to break free from the liquid. Choice C is correct because it identifies correct final state based on thermal energy direction, as heating to 90°C adds energy but does not reach the boiling point of 100°C, so the water remains liquid since particles have more motion but not enough to overcome attractions and become gas. Choice B predicts the wrong final state: claims water heated to 90°C becomes gas (steam), when it actually stays liquid below the boiling point. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
5. A student heats a beaker of liquid water from $20^\circ\text{C}$ to $120^\circ\text{C}$ (at normal air pressure). What is the state of the water at $120^\circ\text{C}$?
   Answer: Gas (water vapor/steam) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. When liquid water is heated to 100°C (boiling point at normal pressure), continued addition of thermal energy causes particles to move so rapidly that they completely overcome the attractions to neighboring particles and escape into the air as gas (steam or water vapor)—since the water is heated to 120°C, which is above the boiling point of 100°C, all the liquid water will have changed to gas (water vapor/steam). Choice C is correct because it accurately identifies that at 120°C (above water's boiling point of 100°C), water exists as gas (water vapor/steam), as the thermal energy is sufficient to keep particles moving fast enough to remain in the gas state. Choice A incorrectly predicts solid ice at 120°C, which is impossible as this temperature is far above both melting point (0°C) and boiling point (100°C). Choice B incorrectly predicts liquid water at 120°C, but at normal pressure water cannot remain liquid above 100°C—it must be gas. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions. At temperatures above the boiling point, water molecules have so much kinetic energy that they cannot be held together in the liquid state—they must exist as widely separated gas particles moving rapidly through space.
6. Warm, humid air contains water vapor (a gas). When this air touches the outside of a glass filled with ice water, thermal energy is removed from the water vapor near the cold surface. What state change occurs on the outside of the glass?
   Answer: Condensation (water vapor turns into liquid water droplets) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. When water vapor (gas) comes into contact with a cold surface like a cold glass or mirror, thermal energy transfers from the fast-moving gas particles to the cold surface, slowing the water vapor particles down—as they slow, the attractions between water molecules become strong enough to pull the particles together from widely separated (gas) to touching (liquid), forming water droplets on the surface in the process called condensation (gas → liquid when thermal energy is removed). Choice B is correct because it accurately predicts the state change direction: removing heat from water vapor causes condensation (gas→liquid), correctly names the process as condensation, and properly explains that water vapor turns into liquid water droplets when cooled by the cold glass surface. Choice A reverses the direction, incorrectly predicting evaporation (liquid→gas) when thermal energy is removed from a gas—evaporation requires adding heat to a liquid, not removing heat from a gas. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them. When warm water vapor touches a cold surface, it loses thermal energy and condenses into liquid water droplets, which is why you see water form on the outside of cold glasses.
7. Liquid water at $20^\circ\text{C}$ is heated on a stove until it reaches $100^\circ\text{C}$, and the burner stays on. What will the water do at $100^\circ\text{C}$ as more thermal energy is added?​
   Answer: It will boil and change from liquid water to water vapor (gas) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. When liquid water is heated to 100°C (boiling point at normal pressure), continued addition of thermal energy causes particles to move so rapidly that they completely overcome the attractions to neighboring particles and escape into the air as gas (steam or water vapor)—this is called boiling, the liquid → gas transition that occurs at the boiling point temperature when thermal energy input gives particles enough kinetic energy to break free from the liquid. Choice A is correct because it accurately predicts the state change direction: adding heat at 100°C moves liquid water toward gas state, correctly names the process as boiling for liquid→gas at 100°C, and properly explains that thermal energy change affects particle motion which causes state change. Choice B reverses the direction, incorrectly predicting freezing when thermal energy is added, when adding heat actually causes boiling at 100°C, not freezing. Choice C predicts the wrong initial state: condensing is gas→liquid, but we start with liquid water, not gas. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them. Specific state changes have names and temperatures: melting (solid→liquid at melting point, 0°C for ice), freezing (liquid→solid at freezing point, also 0°C for water), boiling (liquid→gas at boiling point, 100°C for water with rapid bubbles throughout), evaporating (liquid→gas below boiling point, slower from surface only), condensing (gas→liquid when cooled), and the particle model explains all of these: particles speed up with heat and slow down with cooling.
8. A beaker contains liquid water at $90^\circ\text{C}$. Heat is added, but the water is not yet at its boiling point. Which change is expected as thermal energy continues to be added until $100^\circ\text{C}$ is reached?
   Answer: The water will begin boiling (liquid → gas) at $100^\circ\text{C}$ This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For boiling: When liquid water is heated to 100°C (boiling point at normal pressure), continued addition of thermal energy causes particles to move so rapidly that they completely overcome the attractions to neighboring particles and escape into the air as gas (steam or water vapor)—this is called boiling, the liquid → gas transition that occurs at the boiling point temperature when thermal energy input gives particles enough kinetic energy to break free from the liquid. Choice B is correct because it accurately predicts the state change direction: adding heat moves toward gas, specifically boiling for liquid→gas at 100°C. Choice A reverses the direction, incorrectly predicting the water will freeze into ice when thermal energy is added, when adding heat actually causes melting or boiling, not freezing. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
9. A sealed container holds liquid water at $20^\circ\text{C}$. Thermal energy is added until the water reaches $100^\circ\text{C}$. What must happen for the water to change from liquid to gas at this point?
   Answer: More thermal energy must be added at $100^\circ\text{C}$ so particles can overcome attractions and separate This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For boiling: When liquid water is heated to 100°C (boiling point at normal pressure), continued addition of thermal energy causes particles to move so rapidly that they completely overcome the attractions to neighboring particles and escape into the air as gas (steam or water vapor)—this is called boiling, the liquid → gas transition that occurs at the boiling point temperature when thermal energy input gives particles enough kinetic energy to break free from the liquid. Choice B is correct because it properly explains that thermal energy change affects particle motion which causes state change: more heat at 100°C allows particles to overcome attractions for liquid→gas. Choice A reverses the direction, incorrectly predicting that removing heat is needed for boiling, when removing heat causes freezing or condensing, not boiling. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
10. Warm, humid air contains water vapor (a gas). When this air touches the outside of a metal can filled with ice water, droplets form on the can. What state change is happening to the water as thermal energy is removed?
    Answer: Condensation (gas to liquid) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. When water vapor (gas) comes into contact with a cold surface like a cold glass or mirror, thermal energy transfers from the fast-moving gas particles to the cold surface, slowing the water vapor particles down—as they slow, the attractions between water molecules become strong enough to pull the particles together from widely separated (gas) to touching (liquid), forming water droplets on the surface in the process called condensation (gas → liquid when thermal energy is removed). Choice C is correct because it accurately predicts the state change direction: removing heat moves water vapor toward liquid state, correctly names the process as condensation for gas→liquid, and properly explains that thermal energy removal affects particle motion which causes state change. Choice A predicts evaporation (liquid→gas), which is the opposite direction and requires adding thermal energy, not removing it. Choice B names melting (solid→liquid), but we start with gas, not solid, and melting requires adding heat, not removing it. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them. The cold can surface acts as a thermal energy sink, pulling heat from the water vapor and causing the gas particles to slow down enough that intermolecular attractions can pull them together into liquid droplets—this is why you see water droplets on cold glasses on humid days.
11. A student removes thermal energy from liquid water, cooling it from $5^\circ\text{C}$ to $-2^\circ\text{C}$. What will the water most likely become?
    Answer: Ice (solid water) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For freezing: Placing liquid water in a freezer removes thermal energy, causing the water molecules to slow down—when temperature reaches 0°C, the particles are moving slowly enough that the attractions between molecules can pull them into fixed positions forming a crystal pattern (ice structure), and the water freezes solid as particles transition from sliding past each other to vibrating in locked positions. Choice C is correct because it identifies correct final state based on thermal energy direction, as removing heat from liquid below 0°C leads to solid ice. Choice A predicts the wrong final state: claims water vapor (gas), but cooling moves toward solid, not gas. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
12. Which statement best explains why ice melts when thermal energy is added and it reaches $0^\circ\text{C}$?
    Answer: Particles gain energy, vibrate faster, and can slide past each other This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For melting: When thermal energy is added to ice (solid water at or below 0°C), the particles gain energy and vibrate more and more vigorously until at 0°C they have enough energy to break free from their fixed positions in the crystal pattern—at this point, melting occurs and ice transitions to liquid water as particles begin sliding past each other instead of vibrating in place. Choice B is correct because it properly explains that thermal energy change affects particle motion which causes state change, with particles gaining energy to slide past each other in liquid. Choice C confuses the thermal energy direction, stating particles lose energy and spread farther apart into a gas, when losing energy causes condensing or freezing, not spreading into gas. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
13. A student says, "When you add thermal energy to ice at $0^\circ\text{C}$, the particles move faster and can slide past each other." Which state change is the student describing?
    Answer: Melting (solid to liquid) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For melting: When thermal energy is added to ice (solid water at or below 0°C), the particles gain energy and vibrate more and more vigorously until at 0°C they have enough energy to break free from their fixed positions in the crystal pattern—at this point, melting occurs and ice transitions to liquid water as particles begin sliding past each other instead of vibrating in place. Choice B is correct because it accurately predicts the state change direction: adding heat moves toward gas, correctly names the process as melting for solid→liquid, and properly explains that thermal energy change affects particle motion which causes state change. Choice A reverses the direction, incorrectly predicting freezing when thermal energy is added, when adding heat actually causes melting or boiling, not freezing. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
14. A tray of ice (solid water) at $-5^\circ\text{C}$ is left on a kitchen counter at about $20^\circ\text{C}$. Thermal energy is added to the ice until it reaches $0^\circ\text{C}$ and continues to absorb heat. What state change will occur?
    Answer: Melting (solid → liquid) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For melting: When thermal energy is added to ice (solid water at or below 0°C), the particles gain energy and vibrate more and more vigorously until at 0°C they have enough energy to break free from their fixed positions in the crystal pattern—at this point, melting occurs and ice transitions to liquid water as particles begin sliding past each other instead of vibrating in place. Choice C is correct because it accurately predicts the state change direction: adding heat moves toward gas, specifically melting for solid→liquid, and properly explains that thermal energy change affects particle motion which causes state change. Choice B reverses the direction, incorrectly predicting freezing when thermal energy is added, when adding heat actually causes melting or boiling, not freezing. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
15. Which statement correctly describes what happens to particle motion when thermal energy is removed from liquid water until it freezes at $0^\circ\text{C}$?
    Answer: Particles slow down and lock into a fixed pattern, forming a solid This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For freezing: Placing liquid water in a freezer removes thermal energy, causing the water molecules to slow down—when temperature reaches 0°C, the particles are moving slowly enough that the attractions between molecules can pull them into fixed positions forming a crystal pattern (ice structure), and the water freezes solid as particles transition from sliding past each other to vibrating in locked positions. Choice B is correct because it properly explains that thermal energy change affects particle motion which causes state change: removing heat slows particles to form solid. Choice D confuses the thermal energy direction, stating particles slow down but separate into gas, when slowing leads to solid, not gas. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
16. Warm, humid air contains water vapor (a gas). When this air touches the outside of a cold glass of ice water, tiny liquid droplets form on the glass. Which state change is happening because thermal energy is removed from the water vapor?
    Answer: Condensing (gas to liquid) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For condensing: When water vapor (gas) comes into contact with a cold surface like a cold glass or mirror, thermal energy transfers from the fast-moving gas particles to the cold surface, slowing the water vapor particles down—as they slow, the attractions between water molecules become strong enough to pull the particles together from widely separated (gas) to touching (liquid), forming water droplets on the surface in the process called condensation (gas → liquid when thermal energy is removed). Choice C is correct because it correctly names the process: condensing for gas→liquid, and properly explains that thermal energy change affects particle motion which causes state change. Choice A predicts the wrong final state: claims liquid→gas when thermal energy is removed, when removing heat causes freezing or condensing, not evaporating. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
17. Liquid water at $20^\circ\text{C}$ is placed in a freezer. Thermal energy is removed until the water cools to $0^\circ\text{C}$ and continues to lose heat. What state change will occur?
    Answer: Freezing (liquid → solid) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For freezing: Placing liquid water in a freezer removes thermal energy, causing the water molecules to slow down—when temperature reaches 0°C, the particles are moving slowly enough that the attractions between molecules can pull them into fixed positions forming a crystal pattern (ice structure), and the water freezes solid as particles transition from sliding past each other to vibrating in locked positions. Choice A is correct because it accurately predicts the state change direction: removing heat moves toward solid, specifically freezing for liquid→solid. Choice B reverses the direction, incorrectly predicting evaporating when thermal energy is removed, when removing heat causes freezing or condensing, not evaporating. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
18. A puddle of liquid water is left outside on a hot day. Thermal energy from sunlight is added, and some water molecules escape from the surface into the air even though the water is not at $100^\circ\text{C}$. What is this state change called?
    Answer: Evaporation (liquid → gas) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For boiling: When liquid water is heated to 100°C (boiling point at normal pressure), continued addition of thermal energy causes particles to move so rapidly that they completely overcome the attractions to neighboring particles and escape into the air as gas (steam or water vapor)—this is called boiling, the liquid → gas transition that occurs at the boiling point temperature when thermal energy input gives particles enough kinetic energy to break free from the liquid. Choice A is correct because it correctly names the process: evaporating for liquid→gas below boiling point, where thermal energy addition allows surface particles to escape. Choice C reverses the direction, incorrectly predicting freezing when thermal energy is added, when adding heat actually causes melting or boiling, not freezing. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
19. Warm, humid air contains water vapor (a gas). When this air touches the outside of a glass filled with ice water, thermal energy is removed from the water vapor near the glass. What state change occurs on the outside of the glass?​
    Answer: Condensing: gas changes to liquid droplets This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For condensing: When water vapor (gas) comes into contact with a cold surface like a cold glass or mirror, thermal energy transfers from the fast-moving gas particles to the cold surface, slowing the water vapor particles down—as they slow, the attractions between water molecules become strong enough to pull the particles together from widely separated (gas) to touching (liquid), forming water droplets on the surface in the process called condensation (gas → liquid when thermal energy is removed). Choice A is correct because it accurately predicts the state change direction: removing heat moves toward solid, correctly names the process as condensing for gas→liquid, and properly explains that thermal energy change affects particle motion which causes state change. Choice B reverses the direction, incorrectly predicting boiling when thermal energy is removed, when removing heat causes freezing or condensing, not boiling. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.
20. Liquid water in a pot starts at $20^\circ\text{C}$ and is heated on a stove until it reaches $100^\circ\text{C}$, and then heating continues. What will happen to the water at $100^\circ\text{C}$ as thermal energy continues to be added?
    Answer: It will boil, changing from liquid water to water vapor (gas) This question tests understanding of how changes in thermal energy cause substances to change state by affecting particle motion. State changes are driven by thermal energy changes: adding thermal energy (heating) makes particles move faster, which can cause transitions toward gas state—solid + heat → liquid (melting, like ice → water) because particles vibrate so vigorously they break from fixed positions and start sliding, and liquid + heat → gas (boiling/evaporating, like water → steam) because particles move so fast they overcome attractions and separate into space. Removing thermal energy (cooling) makes particles slow down, causing transitions toward solid state—gas - heat → liquid (condensing, like steam → water) because slowing particles are pulled closer by attractions, and liquid - heat → solid (freezing, like water → ice) because particles move slowly enough to lock into fixed positions. For boiling: When liquid water is heated to 100°C (boiling point at normal pressure), continued addition of thermal energy causes particles to move so rapidly that they completely overcome the attractions to neighboring particles and escape into the air as gas (steam or water vapor)—this is called boiling, the liquid → gas transition that occurs at the boiling point temperature when thermal energy input gives particles enough kinetic energy to break free from the liquid. Choice A is correct because it accurately predicts the state change direction: adding heat moves toward gas, specifically boiling for liquid→gas at 100°C. Choice B reverses the direction, incorrectly predicting freezing when thermal energy is added, when adding heat actually causes melting or boiling, not freezing. Remember the pattern for thermal energy and states: adding thermal energy always moves substances toward gas state (solid → liquid → gas) because particles speed up and overcome attractions, while removing thermal energy always moves toward solid state (gas → liquid → solid) because particles slow down and attractions become more effective at organizing them—you can never freeze something by heating it or boil something by cooling it, the direction is set by energy flow.

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