A Journey through States of Water · Lesson 2 of 8
Where Does Disappearing Water Go?
“Investigate puddles and wet surfaces to discover how liquid water enters the air.”
• Investigate whether water on a steel plate disappears by seeping through it. • Explain evaporation as liquid water changing into water vapour. • Recognise that evaporation can happen at room temperature. • Distinguish evaporation from absorption or seepage in everyday situations.
A puddle that shrinks
A puddle after rain may be smaller by evening. Where did its water go? Some water may enter the ground, and some may move into the air. To separate these possibilities, consider water left on a washed steel utensil: the metal surface is different from soil, yet the wetness eventually disappears. We can test the idea that the water soaked through the metal.
Investigate water on a plate
Place a tablespoon of water on a dry steel plate. Look beneath it immediately and at regular intervals as the water on top decreases. In the chapter’s investigation, water does not appear below the plate, yet the water on top eventually disappears. This rules against seepage through that steel plate in this situation. It directs us to a different explanation: liquid water becomes water vapour and mixes with the air.
The process by which liquid water changes into water vapour. It can happen from an exposed surface even when the water is not boiling.
Water vapour is still water. Evaporation explains drying clothes, a mopped floor, drops on washed utensils, and sweat disappearing from skin. It is continuous at ordinary room temperature, although it can happen more quickly in some conditions. A little water sprinkled on a hot pan changes rapidly; the visible mist above it contains tiny liquid droplets, while water vapour itself is invisible.
Choose the explanation that fits the surface
Soil can absorb or allow water to seep into it. A steel plate does not behave like soil. A puddle on a playground can shrink through both movement into the ground and evaporation into the air. Evidence from one surface should not be applied without thought to every other surface.
| Observation | Possible process | Reason to investigate |
|---|---|---|
| Water on a steel plate disappears | Evaporation | Check whether the underside becomes wet |
| A puddle on soil shrinks | Evaporation and seepage | Both air and ground can receive water |
| Wet clothes become dry | Evaporation | Water can leave the fabric for the air |
Problem
One puddle is on a paved surface and another is on soil. Both shrink. Must both shrink for exactly the same reasons?
- 1.Water can evaporate from the surface of either puddle.
- 2.Some water in the soil puddle may also seep into the ground.
- 3.Observe the surfaces and avoid claiming that only one process is possible in both places.
Draw the steel plate and water at the start, then a later view with less liquid. Label the dry underside and use arrows towards the air for the water that has evaporated. The arrows represent a process; the vapour is not drawn as a visible cloud.
Hand sanitiser also seems to disappear when rubbed on hands because its volatile liquid ingredients evaporate. This is a useful comparison, but the liquid is not necessarily pure water. Later we will explore why evaporation can cool the skin.
Quiz
What is evaporation?
What observation most directly challenges the claim that water passes through a steel plate?
Why might a playground puddle shrink?
When can evaporation take place?
What makes a whitish mist near hot water visible?
Practice Problems
- Describe a fair observation using a steel plate to test whether water seeps through it.
- Explain where water goes when a mopped floor dries without being heated to boiling.
- Give two possible reasons a puddle on soil becomes smaller and explain how they differ.
- Draw and label the change when water left on a plate evaporates.
- Why should the disappearance of sanitiser not be described as pure water evaporating?
- Predict whether drying utensils in an ordinary room involves evaporation and justify your answer.
Key Takeaways
• Evaporation changes liquid water into invisible water vapour, even at room temperature. • A shrinking puddle may lose water to both the air and the ground. • An investigation should test a proposed explanation instead of assuming that all surfaces behave alike.