Water changing state and cycling
How can water leave a liquid surface and later become droplets again?
Use particle motion and energy transfer to connect evaporation, condensation, precipitation, and collection.
Learning goals
- Use a particle model to distinguish liquid water from water vapour.
- Connect evaporation and condensation with energy transfer.
- Predict the direction of a state change from a temperature change.
Before you begin
- Know that matter can be modelled as tiny particles.
- Read temperatures in degrees Celsius and simple arrow diagrams.
Interactive science experiment
- 1Evaporation
- 2Condensation
- 3Precipitation
- 4Collection
Preparing the interactive model…
Qualitative teaching model: values and diagrams compare trends; they are not precision measurements.
Observe, then explain
Work through four steps. Each button moves the interactive model to the matching observation state.
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As water gains energy, particles move faster on average. Some surface particles overcome attractions and become invisible water vapour; evaporation also occurs below boiling.
Your task: Raise the temperature and observe how the number of particles leaving the surface changes.
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After water vapour transfers energy to cooler surroundings, particles slow and gather into droplets. Visible cloud droplets are liquid water, not vapour itself.
Your task: Increase humidity or lower temperature and observe where droplets form.
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Droplets collide, combine, and grow. When gravity can make them fall, precipitation occurs. Falling is a change of position, not another change of state.
Your task: Watch small droplets combine and identify what changes before they begin to fall.
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Fallen liquid water collects at lower places. Matter has not vanished; when water gains energy again, surface evaporation can restart the cycle.
Your task: Follow the arrows back to the liquid surface and identify which processes in the cycle changed the state of water.
Build an explanation
Evaporation is not boiling
Evaporation needs only some surface particles to have enough energy, so it occurs at many temperatures. During boiling, bubbles also form within the liquid at the boiling point for that pressure.
Conservation and cycling
A change of state does not make a new substance. Arrangement and motion change, but H₂O remains H₂O; a closed model keeps the same number of particles.
Check your understanding
Printable worksheet
Complete the worksheet first. Answers and explanations are on a separate page.
Chemistry
Water changing state and cycling
How can water leave a liquid surface and later become droplets again?
- A cup is not boiling, yet its water level slowly falls. What best explains this?
- Droplets appear on the outside of a cold cup. What most likely happened?
- During condensation, how does water exchange energy with its surroundings?
Answer page
Water changing state and cycling
- Some surface particles have enough energy to enter the gas
Evaporation can occur below boiling; the escaping particles are still H₂O.
- Water vapour in the air condensed
Nearby water vapour transfers energy to the cold cup and gathers into droplets.
- Water transfers energy to the surroundings
When gas becomes liquid, water transfers energy to cooler surroundings.