A Journey through States of Water · Lesson 3 of 8
Why Do Droplets Form on a Cold Glass?
“Test where droplets on a chilled glass come from and learn how vapour returns to liquid water.”
• Explain condensation as water vapour changing into liquid water on a cool surface. • Use observations and measurements to compare competing explanations for droplets outside a glass. • Distinguish what a mass reading alone shows from what a marked water level also shows. • Connect condensation with dew, droplets on a lid, humidity, and collecting water from air.
A cold glass becomes wet outside
Fill a glass with cold water and add ice. Leave it undisturbed for five minutes. Small drops appear on the outer surface and may join into larger drops. Record what you observe separately from what you wonder about it: “drops are outside” is an observation; “did water pass through the glass?” is a question. You can try a cold metal container as well.
| I observe | I wonder |
|---|---|
| Drops appear outside a cold tumbler. | Where did the drops come from? |
| Several tiny drops join into a bigger drop. | Would this happen on a tumbler at room temperature? |
Possible explanations include water seeping through the glass, melted ice escaping, and water coming from the air. Each is a proposal to test, not a fact to accept. You could compare a room-temperature glass or use a tall narrow container to look carefully for a falling internal water level. One observation alone may not settle every possibility.
Water vapour can become liquid
Air around us contains invisible water vapour. When that air touches a sufficiently cool surface, some vapour becomes tiny liquid drops. Those drops are the water seen outside the cold glass. A similar change can produce dew on plants in the morning, or drops on the cooler underside of a plate covering hot water.
The change of water vapour from the gaseous state into liquid water, often when vapour meets a cooler surface.
A measurement and a better test
Cover a half-filled glass containing ice and water with a small steel plate, then weigh the whole arrangement. Record the reading every five minutes for 30 minutes. If drops collect on the outside, the reading may increase as water from the surrounding air joins the weighed arrangement. A gain is evidence consistent with condensation, but the mass reading alone does not prove that none of the water also seeped through the glass.
| Time | Mass reading | What to look for |
|---|---|---|
| Start | Record the starting mass | Check that the outside is initially dry |
| 5, 10, 15, 20, 25 minutes | Record each reading | Observe any new outside drops |
| 30 minutes | Compare with the starting mass | Explain the overall change |
Improve the test by marking the initial water level inside the glass. If that level does not go down while extra water collects outside, the added drops cannot be explained by water leaving the inside through the glass. The level mark and the mass reading answer different parts of the question. Keep the cover in place so water loss from the opening does not confuse the result.
Problem
The outside of a covered chilled glass becomes wet and its balance reading increases. A student says, “The balance proves the glass never leaked.” Is that conclusion justified?
- 1.The increase supports the idea that water from outside the glass has joined the weighed arrangement.
- 2.A mass reading alone cannot rule out simultaneous small changes inside the glass.
- 3.Mark the internal level and look for any decrease; the combined evidence gives a stronger conclusion.
The ice cools the glass. It need not pass through the glass to create outside droplets; water vapour already present in the surrounding air can condense there.
The amount of water vapour in air is called humidity. Daily humidity records can be compared across seasons to find patterns. A machine that collects water from humid air works on the same principle: it cools air so some vapour condenses and can be collected. Water intended for drinking still needs appropriate treatment and testing.
Quiz
What is condensation?
Why do droplets appear outside a chilled glass?
Which pair gives stronger evidence against the seepage explanation?
Why does a balance reading alone not conclusively rule out seepage?
What does humidity describe?
Practice Problems
- Set up the cold-glass observation and write two observations and two questions in separate columns.
- Explain why morning dew and droplets under a lid are examples of condensation.
- Describe how you would use both a balance and a marked water level to investigate drops outside a chilled glass.
- A balance reading increases while the inner water level is unchanged. Explain where the extra outside water came from.
- Find humidity readings for several days and suggest one question that the records might help you explore.
- Explain why cooling humid air can help collect liquid water.
Key Takeaways
• Condensation changes water vapour in air into liquid droplets on a cool surface. • A mass reading and a marked water level test different parts of the cold-glass explanation. • Dew, covered-pot droplets, and water collected from humid air all involve condensation.