Bryson DeChambeau illustrating the property of water molecules called surface tension using dish soap in a simple science experiment for kids

Does It Sink or Float? Depends on the Soap!

How does soap break down surface tension?

Key Concepts

  • Chemistry icon
    Chemistry
  • Surface tension icon
    Surface tension
  • Surfactant icon
    Surfactant
  • Molecules icon
    Molecules
  • Chemical bonds icon
    Chemical bonds

  • Introduction

    If you’ve ever washed dishes, you know the right dish soap can make a dirty job a lot easier. Have you ever wondered how dish soap is able to clean dishes so much more effectively than water alone? Like many household cleaners, dish soap is a surfactant—it helps break up leftover food on plates by making it easier for food particles to dissolve in water. This soap also breaks up the water molecules themselves, which leads to some pretty interesting kitchen science! In this activity you’ll be observing some surprising properties of dish soap in water!

  • Background

    Giant ships, people and rubber ducks can all float on water’s surface, thanks to one very important trait of water molecules: hydrogen bonds! Water molecules cling strongly to one another by forming these bonds from one molecule to another. They allow water molecules on the surface of water to behave like a membrane, which can even support the weight of small objects, such as water strider insects. This property of water is known as surface tension. You have probably observed this phenomenon many times in your life. Have you ever noticed a single drop of water sitting on your car windshield? Instead of flattening out or splashing, these raindrops are able to hold a spherical shape because the water molecules comprising the raindrop are more attracted to one another than they are to the windshield of your car. As a result, those molecules hold tightly to one another, forming the raindrop’s spherical shape.

    Surfactants such as dish soap break up water’s surface tension. As a result, objects floating in water will sink or change shape as the surface tension changes. In this activity you’ll explore how surface tension affects the behavior of objects in water—and why it’s so important!

  • Preparation

    Fill your bowl at least halfway with water.
  • Procedure

    1. Place your rubber band on a table or other flat surface. Notice its shape when it sits on the table.
    2. Place your rubber band into your bowl of water. Notice the shape of the rubber band when it is in the water. Does it look different than it did when it was on the table? If so, what is different about it?
    3. Place your pin in the water in the center of the rubber band. Does the pin sink or float?
    4. Drop a few drops of dishwashing liquid into the water surrounded by the rubber band. Notice the rubber band’s shape. Did anything about it change when you added the liquid soap?
    5. Observe the pin. If it is still floating, add a few more drops of liquid dishwashing soap. Does the pin continue to float or does it sink? Why do you think this is the case?
  • Observations and Results

    In this activity you used dishwashing soap to examine how surface tension affects the behavior of objects floating in water.

    In the beginning of the activity you should have noticed the rubber band and the pin floated on the water’s surface. They float because water molecules hold on to one another in a way that creates surface tension. This property allows many things to float on water, including your rubber band and pin. 

    You should also have noticed the rubber band had a loose, or irregular, shape. It did not hold a perfectly round circle but instead floated in a shape similar to what it looked like before you put it in the water.
    When you added the dishwashing soap, however, you should have observed a change in the behavior of the rubber band and pin: The band should have suddenly popped out into a circle. In addition you should have noticed that adding the dish soap caused the pin to sink to the bottom of the bowl instead of floating.

    Both of these results happen for the same reason—and you probably already guessed it has something to do with dishwashing soap, which is a surfactant and designed to break down water’s surface tension. This helps make it an excellent cleaner. After you added the dishwashing liquid to the center of the rubber band the surface tension on the inside of the band broke down. The water on the outside of the band, however, hadn’t come in contact with the dishwashing soap. As a result, the water on the outside of the band still had strong surface tension, which caused it to pull the rubber band outward in all directions. This made the rubber band pop suddenly into a circle shape.

    Similarly, after you added the soap to the water you should have seen the pin sink to the bottom of the bowl. Because the soap broke down the surface tension the water could no longer support the weight of the pin.

  • 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.
  • 2-PS1-3. A great variety of objects can be built up from a small set of pieces.

Grades 3-5

  • 5-PS1-1. Matter of any type can be subdivided into particles that are too small to see, but even then, the matter still exists and can be detected by other means.
  • 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-PS1-2, 3. Each pure substance has characteristic physical and chemical properties that can be used to identify it.
  • 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-PS1-2. The periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states.
  • HS-PS1-3. The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms.
  • HS-PS1-4. Stable forms of matter are those in which the electric and magnetic field energy is minimized. A stable molecule has less energy than the same set of atoms separated; one must provide at least this energy in order to take the molecule apart.

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-PS1-4,5. Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules and the rearrangement of atoms into new molecules with consequent changes in the sum of all bond energies in the set of molecules that are matched b the changes in kinetic energy.
  • HS-PS1-6. In many situations, a dynamic and condition-dependent balance between a reaction and the reverse reaction determines the numbers of all types of molecules present.
  • 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. 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.
  • 3-PS2-3. Electric and magnetic forces between a pair of objects do not require that the objects be in contact.
  • 3-PS2-4. The sizes of the forces in each situation depend on the properties of the objects and their distances apart and, for forces between two magnets, on their orientation relative to 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-PS2-3. Electric and magnetic (electromagnetic) forces can be attractive or repulsive, and their sizes depend on the magnitudes of the charges, currents, or magnetic strengths involved and on the distances between the interacting objects.
  • 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.