The 2–bottle systems — all three of them — look different today than they did when the class first set them up. Students have evidence of this because they made drawings of the systems on the first day, and have observations from today.

Purpose of the discussion

The purpose of this discussion is for students to jointly construct explanations for the changes they observed in the two-bottle system, explanations that are consistent with their observations and evidence.

Engage students in the focus question

What changes in the 2-bottle system have we observed and how do we explain these changes?

Students should refer to data in their Science Notebooks as they make claims, and provide evidence or reasoning. Possible responses include:

• Water is appearing as drops on the inside surfaces of the lower and upper bottles.
• The drops are not evenly distributed. For example, there are no drops on the surface closest to the lamp.

Where do you think the drops in the upper bottle came from? What makes you think that?

• The only water in the system is in the lower bottle; water can't get into the bottles from the outside. (Reasoning)

Did anyone see water move from the puddle in the lower bottle to the drops in both bottles?

• No.

How do you think the water got from the puddle in the lower bottle to the drops in other places?

• Tiny bits of water too small to see moved from the puddle into the air, the air had no place to go other than inside the bottles, and the water particles in the air (water vapor) formed water drops that collected on the inside surfaces of the bottles. (Reasoning)

What's the difference between parts of the bottle where there are no droplets and parts where there are lots of droplets?

• There are no droplets on the warmest part of the lower bottle.

Summarize the discussion and recap the investigation

Using the same language students have used, summarize their main ideas. Include the following key ideas:

• The change that has occurred in the two-bottle systems is that water drops have appeared on previously dry surfaces inside the bottles.
• Some claim that the drops of water come from the puddles in the lower bottles. They reason that the 2-bottle systems are closed and so there is no other source of water.
• No one saw the drops move from the puddles to the sides of the bottles. Some suggest that the water particles that moved were too small to see.

Other observations include:

• The water drops are not evenly distributed; some places remain dry.
• The sand, salt, and blue dye have not appeared anywhere else.

Introduce the term "condensation" and explain condensation is the reverse of evaporation: condensation involves the formation of water drops while evaporation involves the disappearance of water drops.

As you recap the investigation, be sure there is understanding of these points:

• A system is a set of components that interact within a boundary.
• The components of the 2-bottle systems include water, air, plastic bottles, and the salt, dye, or Earth materials.
• The 2-bottle systems are closed systems because matter cannot get into or out of those systems.
• The change that occurred in all three of the 2-bottle systems is that drops of water have appeared on surfaces that were originally dry.
• We reason that the drops came from puddles in the lower bottles, because the 2-bottle systems are closed systems.

The class will continue to observe and develop explanations for changes in the system.