3M science at home: walking water

Walking Water

How does capillary action help move liquid molecules?

Key Concepts

  • physics icon
  • gravity icon
  • molecules icon
  • surface tension
    Surface tension

  • Introduction

    Imagine this challenge: you have two glasses of water – one empty and one full. You want to pour half of the full glass into the empty one. The twist? You aren’t allowed to pick up either glass! Can you get the water to ‘walk’ between the glasses using nothing but a paper towel? Try this activity to find out!

  • Background

    You’ve probably used paper towels to clean up spilled water or other liquids Have you ever wondered exactly how they soak up so much water? Paper towels are made of many small fibers that have gaps in between them. Water gets pulled into these gaps by capillary action—the same phenomenon that allows trees to suck water out of the ground. This action is partially fueled by surface tension, which is caused by cohesion (water molecules being attracted to one another). Surface tension is what allows water to form beads instead of spreading out, and for some small insects to walk on water. It also allows water to get sucked up into narrow tubes or gaps in materials. The absorption process is also aided by adhesion (the attraction between different types of molecules). The paper towel fibers are made of cellulose, which also comprises wood and many plants. These fibers are polar, meaning they have a slight positive charge on one end and a slight negative charge on the other. Water molecules are also polar. Because opposite electric charges are attracted to each other, this results in the water molecules being attracted to the cellulose fibers.

    Although you might not have tried it yourself, maybe you’ve heard of another phenomenon: siphoning. This process uses a tube to suck liquid from one container into another, somehow seeming to defy gravity. First the liquid gets sucked up into the tube before it moves down into the next container—even though there’s no pump powering the motion. Siphoning is based on some of the same physical principles mentioned above. (For example, water can be drawn up into the tube by capillary action.)

    That might sound like a lot of science for a simple, everyday object. Try this activity to find out if you can use paper towels to move water from one cup to another—and understand the science behind it!

  • Preparation

    1. Line up all your glasses in a row.
    2. Starting with glass on one end fill every other glass with water.
    3. Put a few drops of food coloring in each water-filled glass. You can choose which colors to use, but don’t use the same color twice in a row.
    4. Use the spoon to mix the food coloring in each glass. Use a paper towel to wipe off the spoon in between glasses, so you don’t transfer the colors.
  • Procedure

    1. Fold each half-sheet paper towel (except the one you used to clean the spoon) into a narrow strip about one inch wide.
    2. Fold each paper towel in half lengthwise to form a “V” shape. The V should be only slightly taller than your glasses. If necessary, rip or cut a little bit off each end of the V to make it shorter.
    3. Use one paper towel to connect each pair of adjacent glasses. (Flip the V shape upside-down and put one end in each cup.)
    4. Look closely at the ends of the paper towels that are in the glasses with water. What do you notice?
    5. Take a break! This experiment goes very slowly. Come back in 15 or 20 minutes. What do you see now?
    6. Keep checking on your setup over the next couple of hours. What happens?
    7. Let your test sit overnight and check on it the next day. What does it look like now?
    8. Extra: Try the activity with different brands of paper towels. Do some work better or faster than others?
    9. Extra: Try placing your cups in different arrangements instead of a straight line. For example, what happens if you connect three cups, initially filled with water, to an empty fourth, central cup?
    10. Extra: What happens if you use a paper towel to connect two cups that are initially filled to different heights with water?
  • Observation and Result

    As soon as you place the paper towels in the glasses, you should notice they start to absorb some of the water. The water starts getting sucked up the paper towel due to capillary action (described in the background section) and eventually starts going down the other side into the empty glass (just like a siphon). This process happens very slowly though—it’s like watching paint dry or grass grow! You might need to go do something else for a couple hours before you can start seeing water accumulate in the empty glasses. The water will eventually stop flowing when the water level in all the cups is even—but you will probably have to let it sit overnight before this happens.

    When two different colors of water mix, the food-coloring dyes combine to form a third color.

  • Safety First & Adult Supervision

    • Follow the experiment’s instructions carefully.
    • A responsible adult should assist with each experiment.
    • While science experiments at home are exciting ways to learn about science hands-on, please note that some may require participants to take extra safety precautions and/or make a mess.
    • Adults should handle or assist with potentially harmful materials or sharp objects.
    • Adult should review each experiment and determine what the appropriate age is for the student’s participation in each activity before conducting any experiment.

Next Generation Science Standard (NGSS) Supported - Disciplinary Core Ideas

This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.

Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.

Physical Science (PS)1: Matter and Its Interactions

Grades K-2

  • 2-PS1-1. Different kinds of matter exist and many of them can be either solid or liquid depending on temperature. Matter can be described and classified by its observable properties.
  • 2-PS1-2. Different properties are suited to different purposes.

Grades 3-5

  • 5-PS1-3. Measurements of a variety of properties can be used to identify materials.

Grades 6-8

  • MS-PS1-1. Substances are made from different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms.
  • MS-PS1-4. Gases and liquids are made of molecules or inert atoms that are moving about relative to each other. In a liquid, the molecules are constantly in contact with others.

Grades 9-12

  • HS-PS1-1. Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons.
  • HS-PS1-3. The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms.

Grades 3-5

  • 5-PS1-4. When two or more different substances are mixed. A new substance with different properties may be formed.
  • 5-PS1-2. No matter what reaction or change in properties occurs, the total weight of the substances does not change.

Grades 6-8

  • MS-PS1-2. Substances react chemically in characteristic ways.
  • MS-PS1-3. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants.
  • MS-PS1-5. The total number of each type of atom is conserved, and thus the mass does not change.

Grades 9-12

  • HS-PS1-7. The fact that atoms are conserved, together with the knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions.

Physical Science (PS)2: Motion and Stability: Forces and Interactions

Grades K-2

  • K-PS2-1. Pushes and pulls can have different strengths and directions.
  • K-PS2-2. Pushing or pulling on an object can change the speed or direction of its motion and can start or stop it.

Grades 3-5

  • 3-PS2-1. Each force acts on one particular object and has both strength and direction. An object typically at rest has multiple forces acting on it., but they add to give zero net force on the object. Forces that do not sum to zero can cause changes in the object’s speed or direction of motion.
  • 3-PS2-2. The patterns of an object’s motion in various situations can be observed and measured; when that past motion exhibits a regular patter, future motion can be predicted from it.

Grades 6-8

  • MS-PS2-1. For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first, but in the opposite direction (Newton’s third law.)
  • MS-PS2-2. The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion.

Grades 9-12

  • HS-PS2-1. Newton’s second law accurately predicts changes in the motion of macroscopic objects.

Grades K-2

  • K-PS2-1. When objects touch or collide, they push on one another and can change motion.
  • Grades 3-5

    • 3-PS2-1. Objects in contact exert forces on each other.
    • 5-PS2-1. The gravitational force of Earth acting on an object near the Earth’s surface pulls that object toward the planet’s center.

    Grades 6-8

    • MS-PS2-4. Gravitational forces are always attractive. There is a gravitational force between any two masses, but it is very small except when one or both of the objects have a large mass.
    • MS-PS2-5. Forces that act at a distance (electric, magnetic, and gravitational) can be explained by fields that extend through space and can be mapped by their effect on a test object.

    Grades 9-12

    • HS-PS2-4. Newton’s law of universal gravitation and Coulomb’s law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects.
    • HS-PS2-5. Forces at a distance are explained by fields (gravitational, electric, and magnetic) permeating space that can transfer energy through space. Magnets or electric currents cause magnetic fields; electric charges or changing magnetic fields cause electric fields.
    • HS-PS2-6. Attraction and repulsion between electric charges at the atomic scale explain the structure, properties, and transformations of matter, as well as the contact forces between material object.