What happened to the water?
3. Introduce the 2-bottle systems
Return to the investigation question:
What happened to the water?
Show students one of the 2-bottle systems, without the lamp.
The 2-bottle system is a closed system. Elements inside the system will remain there, and nothing new will be added to the system. The mini-lakes with the covers off are open systems. The concept of systems will be introduced in Investigation 8.
The class will have three different closed systems to observe. The lower bottle of each system represents a mini-lake or a real lake. It will hold water plus one other material.
The upper bottle in each system represents the air in the room, or the air that makes up Earth's atmosphere. Each upper bottle will start off holding just air.
Share with students that evaporation happens slowly when the air above the water is very humid. The lamp will be used to add heat and cause evaporation to happen more quickly in the 2-bottle system, even after the air inside the bottles becomes very humid. Show students a lamp and how it is added to the 2-bottle system.
Remind students that no extra heat was added to the paper towels or the plastic cups (in the previous investigation) while the drops of water were evaporating from them. Water can easily evaporate at cooler temperatures when it is not sealed in a closed system.
Add materials to the 2-bottle systems
Note: When adding materials to the 2-bottle systems and when moving them once they hold the materials, work carefully to keep the materials from splashing around inside the bottles. Drops of sandy, salty, or blue-colored water that are splashed around will leave deposits that may later lead students to conclude that sand, salt, or food coloring has evaporated.
As you work, describe the materials in each of the 2-bottle systems.
- Add the sand-gravel mix to a 1-liter bottle using a paper funnel. Add the 30g of water, close the system, and set it on its stand. These are the same materials used in the mini-lakes.
- Mix 5g of salt with 30g of water, add it to a 1-liter bottle, close the system, and set it on its stand.
- Dissolve 5 drops of blue food coloring in 30g of water and add it to a 1-liter bottle. Close the system and set it on its stand.
Note: Do not add the lamps to the systems until after students have made their initial observations and predictions in Part 4. Evaporation and condensation will start shortly after the lamps have been turned on.
Note: You may choose to add materials to the 2-bottle systems before class. In this case, describe what you put in each one of the lower bottles.
