Did you know? In a sealed container with liquid still present, evaporation does not simply switch off. Over time, condensation can catch up until evaporation and condensation occur at equal rates. The amount of liquid and vapour then appears steady.
This is called dynamic equilibrium: particles still move between liquid and gas, but there is no net change in the amounts at constant conditions. The everyday phrase “evaporation stopped” describes the stable appearance, not the particle activity.
Try this now: draw a sealed container with liquid at the bottom. Add upward arrows for evaporation and downward arrows for condensation. At equilibrium, draw equal numbers of arrows in both directions.
Choose the task you need
1. Track evaporation and condensation together
Evaporation changes liquid particles into vapour at the surface. Condensation changes vapour back into liquid. A closed container allows vapour to accumulate, so the reverse process becomes increasingly important.
| Process | Direction | What makes it visible in the model |
|---|---|---|
| Evaporation | liquid → gas | particles leave the liquid surface |
| Condensation | gas → liquid | vapour particles return to a surface or liquid |
| Net evaporation | more leave than return | liquid amount decreases |
| Dynamic equilibrium | equal rates | amounts remain approximately constant |
USGS describes evaporation and condensation as opposite changes of state. In a sealed system, both directions must be considered.
2. Build the sealed-container sequence
Start with liquid water and relatively little water vapour above it. At a fixed temperature, surface particles evaporate. As vapour accumulates, more vapour particles are available to condense.
| Stage | Evaporation rate | Condensation rate | Visible trend |
|---|---|---|---|
| Just sealed | evaporation present | low | vapour amount rises |
| Approaching equilibrium | evaporation present | rising | net liquid loss slows |
| At equilibrium | equals condensation | equals evaporation | amounts appear steady |
| Temperature changes | new balance develops | new balance develops | amounts may shift |
OpenStax explains that physical phase changes can establish equilibrium in a sealed container. At equilibrium, forward and reverse rates are equal even though both processes continue.
3. Distinguish steady amounts from stopped particles
A still water level is a macroscopic observation. It cannot, by itself, show that no particles are changing state. Equal opposing changes cancel in the overall measurement.
Original teaching illustration: station platform
Imagine ten passengers entering a station each minute while ten leave. The number inside stays constant, but movement has not stopped. Dynamic equilibrium works in the same bookkeeping way: equal rates create no net change.
| Statement | Accurate? | Better wording |
|---|---|---|
| Evaporation has stopped. | usually misleading if liquid and vapour remain | net evaporation has stopped |
| Nothing is moving. | no | particles continue moving |
| The liquid level is constant. | possible at equilibrium | observable amount is steady |
| The container is saturated forever. | too absolute | equilibrium depends on conditions such as temperature |
For Primary explanations, “evaporation and condensation balance” may be sufficient. Later Science or Chemistry can introduce dynamic equilibrium and vapour pressure with greater precision.
4. Plan a safe observation
Use room-temperature water, transparent plastic containers and normal classroom handling. Never heat a sealed container: pressure can rise and create a serious hazard.
- Place equal small amounts of room-temperature water in two identical clear containers.
- Seal one securely and leave the other open in the same safe location.
- Mark the starting level and observe at the same times over several days.
- Record droplets on the lid or walls as well as the liquid level.
- Do not taste the samples, and clean the containers after the observation.
| Keep the same | Change | Observe |
|---|---|---|
| container shape and starting amount | sealed or open | liquid level |
| location and light exposure | sealed or open | droplets on walls or lid |
| observation time | sealed or open | mass only if suitable equipment is available |
This comparison cannot directly count particles, but it can support the idea that vapour escapes from an open container while a sealed container retains matter and allows condensation to return water.
5. Diagnose the explanation and transfer it
A learner may see a stable liquid level and conclude that particles have stopped, or may think every closed container immediately reaches equilibrium. Ask about rates, time and conditions.
| Observed response | Likely unstable point | Repair prompt |
|---|---|---|
| “Evaporation stops when the lid closes” | vapour accumulation is ignored | What happens to surface particles just after sealing? |
| “The water level is steady, so nothing changes” | net change and particle change are merged | Can equal opposite rates hide movement? |
| “Condensation starts only at 0°C” | dew point is confused with freezing | What state change is condensation? |
| “Every sealed container behaves identically” | temperature and liquid amount are ignored | Which conditions set the new balance? |
A useful transfer question compares a sealed bottle, a covered food container and a terrarium. The materials differ, but the reasoning route remains: evaporation, vapour accumulation, condensation and a condition-dependent balance.
When tuition can help
Tuition may help when the learner can name evaporation and condensation separately but cannot reason about both occurring together, distinguish net change from particle activity or design a fair comparison. A tutor can connect observations, particle models, rates and safety.
A useful review starts with one fresh Science task, the learner’s explanation and the exact point where the reasoning changes. eduKateSG’s verified 3-pax format is designed for close observation and precise feedback; it is not a promise of a particular grade.
Sources and scope
- USGS: evaporation and the water cycle
- OpenStax Chemistry 2e: chemical and physical equilibria
- MOE: secondary school experience under Full Subject-Based Banding
- SEAB: Secondary Education Certificate (SEC)
The worked examples and teaching checks on this page are original educational illustrations. They explain the stated concept; they do not reproduce a live examination question or promise a grade outcome.
For secondary planning, PG1, PG2 and PG3 are Posting Groups used for admission, while G1, G2 and G3 are subject levels where those levels are offered. The SEC begins in 2027. Check the learner’s actual subject, level, examination year and candidate type; not every subject is offered at every G level.