Ask students to check their paper towels.

Purpose of the discussion

The purpose of this discussion is for students to construct explanations of what happened to the drops of water, explanations that are consistent with their observations and evidence.

Engage students in the focus question

What happened to the drops of water you put on the paper towels, or that I sprayed on the plastic cups? Does water still exist after it disappears, or has it been destroyed?

Provide time for students to consider their position and the reasoning to back it up. Students may offer the following:

• The water has moved into the air. (Students may suggest that the existence of clouds supports this claim.)
• The water has moved from the surface to inside the plastic or inside the paper. (What evidence can students point to to support this claim?)
• The water is gone; it no longer exists.

If the evaporated water still exists, and where do you think it is now?

If the evaporated water still exists, why can't we see it?

Who thinks water is destroyed by evaporation and why do you think so?

Remind students to support their position with evidence or reasoning. They should refer to their annotated drawings and notebook entries as they make their claims and provide their reasoning.

Weigh the paper towel: If some students think the water has moved inside the plastic or paper, weigh the paper towel saturated at the start of the investigation. Very likely it is now dry or almost dry. The weight measurement, which should be 3g to 5g less than the wet weight, provides evidence that evaporated water does not move from the surface of a material to inside of it.

How might we test the idea that the water still exists after it evaporates, or that evaporation destroys water?

• Make evaporation happen in a closed system, and look for evidence that the water still exists.

Summarize the discussion and recap the investigation

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

• The only evidence we have is that evaporated water does not just move from the surface to the inside of a material. The weight of the saturated paper towel became less as water evaporated.
• There is no clear answer to the question of whether or not evaporated water has been destroyed, but the class will continue to explore that question in the science classes that follow.

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

• Today you watched as drops of water on a plastic cup became smaller; some disappeared.
• You also put drops of water on a paper towel, which also disappeared.
• This disappearance of water in the classroom matches other experiences you have had: wet clothing dries; rain puddles disappear.
• We can claim that evaporated water does not move from the surface to inside a material. The evidence for this is that the weight of the paper towel became less as water evaporated from it.