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Purpose
[stextbox id = “info”] To challenge the thinking of students who believe that a liquid absorbs into a surface rather than evaporating.[/stextbox]
Description
[stextbox id = “info”] Pour a small amount of room-temperature water on a dark cookie sheet (or similar type of pan), making a puddle no more than about a couple of inches across. Place a bright desk lamp (ideally with an incandescent bulb) above the pan and turn it on (this will heat the pan and make the water evaporate faster). Have students observe the puddle every 20–30 minutes until it disappears.[/stextbox]
Questions to Ask Students
[stextbox id = “info”]- Before making the puddle:
- Will water soak into the pan?
- After the puddle has disappeared:
- Why did the puddle get smaller and eventually disappear?
- Where did the water go?
Student Thinking
[stextbox id = “info”] Students who initially thought that the water was absorbing into the surface may no longer have an explanation for where the water is going. The next student experience will help them understand the process of evaporation.[/stextbox]
Implementation Tips
[stextbox id = “info”]- Again, the purpose of this activity is to give students evidence that the water is not absorbing into the surface. Don’t worry about explaining evaporation at this point.
- A 2-inch diameter puddle will take several hours to evaporate depending on the darkness of the pan, the brightness of the light, and humidity. Start the activity at the beginning of the day and have students observe the puddle throughout the day.
- Be sure to use a pan that will not rust.
- After all of the water has evaporated, lift the cookie sheet to show that no water has leaked through. Be sure to start the activity with water that is room temperature or warmer. Cold water may cause condensation on the underside of the sheet, which may make students think water has leaked through.
- [/stextbox]