Why do the water drops form?
4. Make meaning
The 2-bottle systems look very different than they did when the class first set them up. Drops of water continue to collect on the inside surfaces of both bottles, and students have agreed that the systems are closed, so the water is not coming from outside of the 2-bottle systems.
Purpose of the discussion
The purpose of this discussion is for students to jointly construct explanations for why drops of water form on the surfaces inside the 2-bottle systems.
They are encouraged to think about particles of water that are too small to see and to consider the role that temperature differences play in evaporation and condensation.
Engage students in the focus question
How can we explain the formation of water drops on the inside surfaces of the 2-bottle system?
Note: If students use the terms "evaporation" and "condensation", ask for details. What is the story behind the word evaporation, or condensation? What do those terms mean? Explaining what is going on is challenging as it requires students to move beyond what they can observe and to explain what can't be seen. They must move back and forth between visible evidence and inference. This discussion is an opportunity for students collectively to put pieces of the explanation into a coherent whole as if they were solving a puzzle together.
Listen for explanations that include the idea that water particles too small to see leave the puddle, move into the air inside the bottles, and are now a gas called water vapor. When enough particles of water vapor cluster together on the cooler surfaces of the bottle, they form visible drops of water.
We started with a puddle of water in the lower bottle and no water in the upper bottle. Now we see water drops on the inside surfaces of both bottles. But I didn't see any water moving from one bottle to the other. How would you explain that?
- Particles of water too small to see break free from the surface of the puddle in the lower bottle. Those particles, that are too small and too spread apart to see, form a gas called water vapor. The water vapor travels throughout the two bottles as part of the circulating air.
We seem to agree on how water particles behave during evaporation. But what about water vapor turning back to liquid water drops - the process we call condensation - how do these water particles behave?
- The water particles in the air are spread far apart. When lots of those particles cluster together they become visible again, as water drops. This doesn't happen everywhere in the 2-bottle system.
Water drops don't form in every part of the 2-bottle system. Can we figure out what's different about parts of the bottle where there are no drops and parts where are lots of them?
- There are no drops on the surface close to the lamp where it's warmest so maybe it needs to be cool for water vapor to become liquid water drops (a process called condensation).
Have you noticed other places where water drops form?
- Water drops form on the sides of a can or glass that holds a cold liquid.
Do you think it is accurate to say that evaporation and condensation are reverse processes? Explain.
Summarize the discussion and recap the investigation
Using the same language students have used, summarize their main ideas.
In your summary, use the three Dot Sheets to reinforce the concept that particle spacing as well as particle size has a significant influence on our ability to see the particles.
Include the following key ideas:
- Particles of water, which are too small to see individually, break free from the surface of the puddle of water and move into the air.
- These invisible particles form a gas called water vapor, which becomes part of the air and circulates inside the 2-bottle system.
- When the invisible water particles bunch together, they form visible drops on the inside surfaces of the 2-bottle system. This process is called condensation.
- Condensation is not happening on the warmest surface of any one of the three 2-bottle systems.
As you recap the investigation, be sure there is understanding of these points:
- You made new observations of the three 2-bottle systems.
- You discovered that drops of water cover most but not all of the inside surfaces of the systems.
- The warmest part of each 2-bottle system is dry.
- There is now evidence that water isn't destroyed as it evaporates. In the closed 2-bottle system, the only source of the condensation is the water vapor that originally evaporated from puddle in the lower bottle.
Letter from the Engineer
The Two–Bottle System
The two-bottle system you have been using is an example of a design created by two engineers. They followed a design process that is highlighted below:
- Define the problem
The problem they faced was:
“What can we build that will help answer the question?”, “What happens to water when it evaporates?”
- Identify the Criteria
The important features of a design, the features that allow us to know if the design is a success, are called criteria. The criteria for the two-bottle system included:
- It must be a closed system (if water still exists after it evaporates, the instrument must capture it).
- It must have two separate places that can hold water, so we can tell if water moved from its original location to a new place.
- People must be able to see what is happening.
- Identify the Constraints
There were also some restrictions, or what engineers call constraints:
- It must be safe for use in a 5th grade science classroom.
- It must be made from simple materials, so it is not very expensive.
- It must be sturdy, not break easily.
- It must be easy to set up correctly and use.
- Whatever is going to happen needs to happen quickly, so people don’t get bored waiting forever to see what might happen.
- Pose solutions
The engineers made lots of different sketches that they thought might work, and talked about the possible designs.
- Test and Evaluate Solutions
Then over the course of several weeks, they built and tested five different versions of the two-bottle system, improving it each time to better match the criteria and the constraints, before they built the version you have in your classroom.
You decide:
- Did the two-bottle system allow you to answer the question, “What happens to water when it evaporates?”
- Does it meet the criteria?
- Does it comply with the constraints?
What changes would you make to improve the two-bottle system so that it does a better job?
Continuing to observe the 2-bottle system
Given enough time, all of the water in the lower bottle will evaporate, leaving it dry, and will condense in the upper bottle. We recommend you leave the light on overnight and across weekends but not during school vacation periods. If you leave the lamp on continuously this change typically takes about a week. It will take significantly longer if the lamp is on only during school hours.
If the water in the lower bottle has not evaporated and condensed in the upper bottle by the time your class completes this unit, leave the 2-bottle systems in place and encourage your students to check them once or twice a week until the process is complete.
Students will find additional line drawings for the 2-bottle systems and spaces for notes and observations in the Science Notebooks that they can use to record any further observations.
