Watching colored water magically defy gravity, climb up paper towels, travel across open air gaps, and fill empty glasses is a mesmerizing kitchen science experiment for young children. Known as the Walking Water Experiment, this simple, colorful STEM activity uses household items to demonstrate fundamental physics and chemistry principles: capillary action, molecular cohesion, surface adhesion, and primary color mixing. In this step-by-step experiment guide, children create a continuous liquid bridge that “walks” colored water between glasses, offering an engaging introduction to fluid mechanics.
The Core Physics Science: Capillary Action, Cohesion, and Adhesion
How can liquid water travel upward against the downward pull of gravity without a mechanical pump? The answer is capillary action.
Understanding Capillary Action. Capillary action is the physical process that allows liquids to flow through narrow spaces or porous materials without the assistance of external forces. Capillary action is driven by the dynamic interaction between two molecular forces: Adhesion and Cohesion.
1. Adhesion (Attraction Between Different Molecules). Water molecules ($H_2O$) are strongly attracted to the cellulose fibers that make up paper towels. This attraction between water molecules and paper fibers is called adhesion. As paper towel fibers absorb water, adhesion pulls water molecules upward along the microscopic porous pathways of the paper towel.
2. Cohesion (Attraction Between Identical Water Molecules). Water molecules are also strongly attracted to one another due to polar hydrogen bonding—a property called cohesion. As the lead water molecules are pulled upward by adhesion to the paper towel fibers, their cohesive bonds drag neighboring water molecules along behind them in a continuous liquid chain!
Plumbing in Nature: How Trees Drink Water
Capillary action is not just a fun kitchen science trick—it is a vital biological mechanism that sustains plant life across the planet!
How Giant Redwood Trees Drink. Giant California Redwood trees stand over 300 feet tall without mechanical pumps. Tree roots absorb liquid water from soil, and capillary action pulls water upward through microscopic vascular tubes (xylem vessels) all the way to leaves at the top of the forest canopy!
Capillary Action Science Variables Matrix
| Experimental Variable | Experimental Option A | Experimental Option B | Impact on Water Walking Speed & Volume |
|---|---|---|---|
| Paper Towel Thickness | Thin 1-Ply Napkins | Thick 2-Ply Absorbent Paper Towels| Porous 2-Ply towels provide wider capillary channels, accelerating flow | |
| Liquid Viscosity | Pure Water + Food Dye | Water + Sugar / Corn Syrup | Higher viscosity liquids slow down capillary flow rate |
| Capillary Path Length | Short 4-Inch Towel Bridge | Long 10-Inch Towel Bridge | Shorter towel bridges reduce travel distance, completing walk faster |
| Glass Elevation Level | Level Counter Setup | Elevated Source Glasses | Elevating source glasses uses gravity to accelerate siphon flow |
Step-by-Step Guide: Setting Up the Walking Water Experiment
Follow these structured instructions to build a 6-glass primary color walking water circuit.
Materials Required:
– 6 Clear glass cups or jars of identical height
– Water
– Liquid food coloring (Red, Yellow, Blue)
– 6 Sheets of absorbent paper towels
– Scissors
Step 1: Arrange the Glass Circuit. Place 6 clear glass cups in a tight circle or straight line on a level kitchen counter. Label or number the cups 1 through 6.
Step 2: Add Water and Primary Food Dye. Fill Cup 1, Cup 3, and Cup 5 three-quarters full with clean water. Leave Cup 2, Cup 4, and Cup 6 completely empty!
– Cup 1: Add 5 drops of RED food coloring.
– Cup 3: Add 5 drops of YELLOW food coloring.
– Cup 5: Add 5 drops of BLUE food coloring.
Step 3: Prepare Paper Towel Bridges. Fold a sheet of paper towel lengthwise into a 1-inch-wide strip. Trim the length so it forms a neat “U” shape that reaches from the bottom of one cup to the bottom of the adjacent cup. Repeat to create 6 paper towel bridges.
Step 4: Place Bridges and Observe Capillary Action! Insert one end of a paper towel bridge into Cup 1 (Red water) and the other end into empty Cup 2. Place a second bridge from empty Cup 2 into Cup 3 (Yellow water). Continue placing bridges around the circuit. Within 5 minutes, observe colored water climbing upward against gravity along the paper towel bridges!
Step 5: Observe Color Mixing and Equilibrium! Over 2 to 4 hours, water walks along the bridges into empty cups. As red water and yellow water walk into empty Cup 2, they mix to form vibrant ORANGE water! Yellow and blue water walk into Cup 4 to create GREEN water, while blue and red water walk into Cup 6 to create PURPLE water! Eventually, all 6 glasses reach equal liquid levels—achieving hydrostatic equilibrium!
Concluding Recommendation
Run the Walking Water Experiment with your child using primary food colors and absorbent paper towels, observing capillary action transport water upward while mixing secondary colors in empty glasses.
Evaluating Surface Tension Reduction in Fluid Dynamics
Exploring how surfactants alter liquid surface tension provides a deeper understanding of fluid dynamics.
Surfactants and Interfacial Tension. Adding a few drops of dish soap or detergent to water lowers its surface tension from 72 mN/m (milliNewtons per meter) down to roughly 25 mN/m. In the walking water experiment, using pure water ensures optimal hydrogen bonding cohesion and adhesion across paper towel cellulose fibers, allowing water to walk smoothly without surface tension breakdown.
Investigating Evaporation Rates and Environmental Humidity. The speed at which walking water achieves liquid equilibrium depends on ambient room humidity and temperature. In dry, warm environments, minor water evaporation occurs along the paper towel bridge, which can be minimized by covering the glasses with plastic wrap to maintain a closed capillary system.
Evaluating Primary and Secondary Color Mixing Mechanics
In addition to capillary action physics, the walking water experiment provides a memorable demonstration of color theory.
Primary Color Mixing Principles. Red, yellow, and blue are primary colors that cannot be created by mixing other dyes. As capillary action walks red and yellow water into empty Cup 2, they combine to form secondary orange water; yellow and blue mix into green in Cup 4; and blue and red mix into purple in Cup 6.
Summary Guidelines for the Walking Water Experiment
To run a successful walking water capillary action demonstration:
1. Use Absorbent 2-Ply Paper Towels: Fold sturdy paper towels into neat U-shaped bridges to maximize capillary flow.
2. Fill Alternating Source Cups: Fill Cup 1 (Red), Cup 3 (Yellow), and Cup 5 (Blue), leaving Cups 2, 4, and 6 empty.
3. Observe Water Walking and Color Mixing: Watch capillary action transport water upward while creating secondary colors over 2 to 4 hours.
Our pick: Learning Resources Primary Science Lab Set or DIY Capillary Action Chemistry Kit Stack
