The Mystery of the Sweating Glass
If you take a glass of cold water containing ice cubes and leave it on a table, you will soon notice tiny water droplets forming on the outer surface of the glass.
Where do these droplets come from?
- Did the water seep through the glass?
- Did it spill over?
- Or did it come from the air?
Investigating the Source (Activity 8.4)
To prove where the water comes from, we can perform a weighing experiment.
Steps:
- Take a glass tumbler with ice water.
- Wipe the outside completely dry.
- Place it on a digital weighing scale.
- Observe the weight over 30 minutes.
Observation:
- Water droplets appear on the outside.
- The mass (weight) increases slightly as water accumulates on the outside.
Conclusion: The water did not leak from inside (if it leaked, the total mass wouldn’t increase, it would just move from inside to outside). The extra mass comes from the air surrounding the glass.
Tip
Definition: The process of conversion of water vapour into its liquid state is called Condensation.
How it works: The air around us contains invisible water vapour. When this warm vapour touches the cold surface of the glass, it cools down and turns back into liquid water droplets.
Other Examples of Condensation
- Dew Drops: Water droplets found on leaves and grass in the early morning.
- Breath in Winter: Seeing your breath on a cold day.
- Lids on Hot Food: Water drops on the underside of a plate covering hot food.