Floating Needle

Author(s): Miha Lee
Discrepant Event - Teacher's Guide
SED 695B
Principles illustrated
Standard
  • Surface Tension
  • Pressure
  • Hydrogen Bonding
  • Detergent (Surfactant)

Chemistry 9-12: Chemical bonding

2h. * Students know how to identify solids and liquids held together by van der Waals forces or hydrogen bonding and relate these forces to volatility and boiling/ melting point temperatures.

Detailed Explanation of Discrepant Event

A metal needle or paper clip has higher density that that of water. So when it is dropped into water, it falls down into the bottom of water. However, if we put it horizontally on the water with a lot of caution, it can float almost forever due to the surface tension of water.

Surfaces, or more generally interfaces, show some physical properties which are not found in the bulk of the same material. The best known example of such property is surface tension. Intermolecular forces create the surface tension. The water molecules at the surface are pulled in by the cohesive force between themselves and molecules inside the bulk and thus have higher potential energy than those inside. The water keeps its round surface shape because there are no water molecules outside the surface to balance this inward pull. This cohesive force acts like a elastic membrane which resists being torn.

The photos above show the structure of water molecules. Between water molecules there are hygrogen bonding, which are attractive forces. The yellow lines represent hydrogen bonding. Inside molecules have attractive forces in all direction, so these forces are canceled out. Therefore, they don't feel any net force, so they don't have any potential energy. In contrast, the surface molecules have only side and downward attration forces. So horizontally they attract each other and act like a membrane. In a vertical direction to the center, they have potential energy. To reduce this energy, they try to make their surface area as samll as possible.

The magnitude of pressure exerted by an object depends on its weight and contact area. When we drop a needle vertically, even though it has the same weight, it has small contact area, which leads to large pressure on the surface of water resulting in tearing the surface and falling down. In fact, that's why needles have such sharp tips. In contrast, if is put horizontally to the surface of water, it has larger contact area and less pressure exerting on the water surface.

When soap is added to the water, the needle will be driven to sink by the disruption of the water's surface tension. Make sure students observe what happens to the needle. The term surfactant is a blend of "surface acting agent". Surfactants are usually organic compounds that are amphipathic, meaning they contain both hydrophobic groups (their "tails") and hydrophilic groups (their "heads"). Therefore, they are soluble in both organic solvents and water. Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. They also reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface.

Questioning Script

Prior knowledge & experience: Metal iron has higher density than that of water, so an object made of iron sinks into the water.

Root question: What makes the metal needle float on water?

Target response: Students understand the effect of surface tension and fudamentally the strength of intermolecular bonding.

Common Misconceptions:

  1. Students usually think heavy metal always sink into water without considering the effect of shape even though the shape of an object determines the buoyance and exerting pressure onto water.
  2. Young students don't think that there are intermolecular forces within a liquid.
  3. Students believe that only the weight of an object affects the pressure exerted by the object.

Question for students

  1. Why do we need to put the needle onto the piece of paper to make it float?
  2. Why do we need to dry the needle up before we use it?
  3. What makes the needle float?
  4. How the detergent makes the floating needle sink?
  5. Do you think the needle can float on the oil with the same method?

Procedure

  1. Fill a beaker or glass tumbler half with water.
  2. Drop a metal needle or paper clip into water in order to show the fact that the metal has higher density than that of water.
  3. Get the needle out of water and dry the needle out to reuse it.
  4. Cut paper towel to get a piece of paper.
  5. Put the needle onto the piece of paper.
  6. Get a glass of water. Then put the piece of paper with the needle onto the water.
  7. When the paper is soaked, push the paper into the water to remove it.
  8. Stir the water carefully to show the needle is floating.
  9. Prepare a little amount soap water with kitchen soap.
  10. Add a drop of soap water into the glass, then watch the needle sinking.
  11. For the comparison of surface tension of various liquids, use ethanol or vegetable oil.

Application in Our Life

Surface tension is essential for the transfer of energy from wind to water to create waves. Waves are necessary for rapid oxygen diffusion in lakes and seas.
Because of the high surface tension between water and air, water striders are able to literally walk on water.

Surface tension causes these water droplets on leaves to bead up forming the smallest surface possible. The water molecules at the surface are pulled in by the cohesive force between themselves and molecules inside the droplet. The water keeps its droplet shape because there are no water molecules outside the surface to balance this inward pull.

References & Links:

[TOP]     [BACK to 695B]     [HOME]