A water cycle in a bag is a simple school project that lets you watch water move through evaporation, condensation, and precipitation. By taping a clear zip-top bag to a sunny window, you create a tiny model of how water changes form and moves around Earth. The blue water at the bottom represents oceans, lakes, or puddles, while the droplets on the bag represent water in the air.
This project matters because the water cycle helps explain weather, clouds, rain, and the freshwater we use every day.
Sunlight warms the water in the bag, giving some water molecules enough energy to evaporate into water vapor. When that vapor touches the cooler inside surface of the bag, it condenses into tiny liquid droplets. As droplets grow heavier, they slide down the bag like rain, showing precipitation in a small, visible system.
Students can use the bag to observe changes over time and connect a classroom model to real outdoor weather.
Key Facts
- Evaporation is when liquid water changes into water vapor because it gains energy.
- Condensation is when water vapor cools and changes back into liquid droplets.
- Precipitation happens when water droplets become heavy enough to fall as rain, snow, sleet, or hail.
- The Sun is the main energy source that powers the water cycle.
- In a closed bag model, water changes location and form, but most of the water stays inside the bag.
- Temperature change = final temperature - starting temperature
Vocabulary
- Evaporation
- Evaporation is the process in which liquid water changes into water vapor.
- Condensation
- Condensation is the process in which water vapor cools and changes into liquid water droplets.
- Precipitation
- Precipitation is water that falls from clouds to Earth as rain, snow, sleet, or hail.
- Water vapor
- Water vapor is the invisible gas form of water in the air.
- Model
- A model is a simplified version of a real system that helps people study and explain how it works.
Common Mistakes to Avoid
- Filling the bag too full is a mistake because there will not be enough air space for water vapor and condensation to form clearly.
- Taping the bag in a shady place is a mistake because the water may not warm enough for noticeable evaporation.
- Thinking the blue water disappears forever is a mistake because it changes into water vapor and then back into liquid droplets.
- Opening the bag often is a mistake because water vapor can escape and the model no longer acts like a mostly closed water cycle system.
Practice Questions
- 1 A student adds 60 mL of blue water to the bag. After several hours, 45 mL remains at the bottom and the rest is on the sides as droplets. How many milliliters of water moved away from the bottom?
- 2 The water in a bag starts at 20°C and warms to 31°C in the sunlight. What is the temperature change?
- 3 Two water cycle bags are made the same way, but one is placed on a sunny window and one is placed on a shaded wall. Explain which bag should show more condensation and why.