In the previous session students investigated the transformation of salt from a solid granular material into something they could no longer see. They could infer from weight measurements that the salt was still present.

Review with students some of the important ideas from the last investigation when salt was dissolved.

• When salt dissolves in water it separates into particles too small to see.
• We can use weight as evidence that salt is still there, even if we can't see it.
• The salt particles take up space, even though they are too small to see.
• Each particle became smaller, but since the weight remained the same, the number of particles must have increased.
• Salt is matter before it is dissolved and it is still matter after it dissolves.

Connect today's investigation to the previous one. Today, the class will investigate another kind of transformation with some similarities to salt dissolving in water. Elicit students' ideas on evaporation with the following question:

• The puddle evaporates because the sun dried it up and it is now simply gone.
• The puddle has turned in to clouds.
• The puddle is in the air.

Explain that the class will continue to work with water, and that over the next few investigations they'll be taking a closer look at what happens when water disappears.

Introduce the investigation question:

What happens to the water?

Demonstrate the following for students:

• Soak one paper towel and squeeze it gently to remove any water not fully absorbed by the paper, so no drops will drip from the wet towel.
• Weigh the wet paper towel.
• Announce the weight to students.
• Drape the wet towel over something where it can remain until later in the class.