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Sideways Pour

This demonstration uses simple materials to introduce the adhesive and cohesive properties of water.

The two forces at play in this 'trick'. One is adhesion (the attraction between the water and the surface of the string), and the other is cohesion (the attraction of water to itself). Together, they are stronger than gravity, which pulls the water toward the ground.

Adhesion and cohesion attractive forces occur because water molecules are polar. Each molecule of water has two atoms of hydrogen and one atom of oxygen (H2O). Because the (slightly positive) hydrogen atoms from one molecule will attract the (slightly negative) oxygen atoms from another, the molecules tend to stick together and to other surfaces/substances.

The properties of adhesion and cohesion can be easily observed by watching raindrops on a windowpane. Adhesion holds the drops to the glass. Even if the window is tilted forward, some drops will cling to the underside of the pane. Cohesion can be seen if you trace the path of drops down the pane. Drops close to one another will be drawn together by cohesion, forming larger drops. Drops are more likely to join together than split apart. Splitting a water drop requires some energy or change to loosen the bonds that hold the molecules together.

Objectives

  • Describe the adhesive and cohesive properties of water.

Materials

  • Per Student:
    two cups
    tape
    water
    cotton string (or twine)
    food colouring (optional)

Key Questions

  • Why are there no puddles of water?
  • How does this activity 'defy gravity'?
  • What other forces are at play?

What To Do

  1. Fill one cup about two thirds full
  2. Ask the class if it is possible to pour the water from one cup to the other, without holding one directly above the other.
  3. Submerge the string so it is thoroughly wet
  4. Tape the wet string to the bottom of the empty cup firmly.
  5. (optional) Colour the water in the first cup
  6. Pick up the loose end of the string, submerge it in the water of the other cup.
  7. Pinch the string against the side of the cup with your index finger and thumb, so you can both lift the cup and ensure the string is taut and submerged while pouring.
  8. Use your other hand to keep the empty cup on the table.
  9. Lift the full cup so it is above but aside from the first cup. Remeber the string must be taut.
  10. Tilt the full cup so that the water pours down the string.

You will see the water cling tightly to the string as it moves down toward the glass. Not a drop will spill on the table!

Extensions

  • How low can you bring the pitcher before the water stops flowing into the cup?
  • How far apart can you put the pitcher and cup and still pour water (successfully!) from one to the other?