Hands On: Water Cycle in a Bag

Hands On: Water Cycle in a Bag


Big Idea:

Water never disappears, it moves endlessly between the ocean, atmosphere, and land through evaporation, condensation, precipitation, and collection. The same water has been cycling for billions of years.

Materials:

  • A ziplock bag (quart or gallon size)
  • Water
  • Blue food coloring (optional)
  • A permanent marker
  • Tape to attach to a sunny window


What to Do:


Step 1: Draw the Scene

Use the marker to draw a sun in the top corner of the bag, clouds near the top, and waves at the bottom. "We are making a tiny model of Earth's water cycle in this bag."


Step 2: Add the Ocean

Fill the bag with about 2 inches of water. Add a drop of blue food coloring. Seal it tightly. "This blue water is our ocean."


Step 3: Add Sun Power

Tape the bag to a sunny window with the water at the bottom. "Now the Sun does its work."


Step 4: Observe and Record

Check the bag every 30 to 60 minutes. Watch for condensation forming on the upper inside of the bag. Record what you see on the lab sheet at each check.


Step 5: The Key Observation

Look closely at the droplets on the sides of the bag. Are they blue or clear? "The color stayed in the ocean. When water evaporates, it leaves behind anything dissolved in it, including salt. That is why ocean water is salty but rain is fresh."


Step 6: Discuss

  • Where is the energy coming from that drives this whole cycle? (The Sun.)
  • What would happen to the water cycle if the Sun disappeared?
  • How long has the same water been cycling on Earth?

What's Really Happening (Caregiver Explanation):

The Sun provides energy that converts liquid water into water vapor through evaporation. When that vapor rises and contacts the cooler plastic near the top of the bag, it condenses back into liquid droplets. When droplets grow heavy enough they slide back down as precipitation, returning to the ocean at the bottom. This closed loop is exactly what happens on a planetary scale. The water you drink today may have been inside a dinosaur, or in an ancient glacier, or in clouds above an ocean that no longer exists. Earth's water has been cycling for over 4 billion years.

Digging Deeper:

The PhET 'States of Matter' simulator at phet.colorado.edu lets learners adjust temperature and observe how water molecules move differently as a solid, liquid, and gas. After exploring it, ask: at the molecular level, what is actually happening when water evaporates? Why does salt stay behind? This connects to desalination, the process of turning ocean water into drinking water. Research one method of desalination and explain how it uses the same principle as this activity.