Ziplock Bag Water Cycle
Steven Carson

"Cloud" made up of tiny droplets forms on the inside surface of the bag. Some clear paths occur where "raindrops" formed and accumulated small droplets as the large drops fell and grew.

Carefully place a plastic cup containing some water into the comer of a Ziplock bag, seal it and suspend it by the opposite corner in a window. Water from the cup evaporates saturating the air in the bag. After a day or so, small droplets will condense on the inside of the bag like a cloud. Some larger drops will begin to run down and grow by accumulating small drops. This is analogous to one way in which rain can form. (The other way involves ice that melts before reaching the ground.)

Make up a few bags and hang them in different places - sunny vs. shady, light vs. dark, warm vs. cool ~ and observe the differences.

Make up a bag with food coloring in the water in the cup. The condensation and the water that accumulates in the bottom corner will be colorless (unless some water from the cup is spilled) showing that when water evaporates the food coloring stays behind. This can also be shown by completely evaporating some colored water.

Make up a bag with salt in the water in the cup. The condensation and the water that accumulates in the bottom corner will be pure. This can be tested by removing the cup and giving students two toothpicks to dip in the water in the cup and the water in the bag and taste. This can show how water that evaporates from the ocean leaves the salt behind and can lead to relatively pure rain. Salt water can also be completely evaporated to show how the salt stays behind.