You are watching: Why (if you are careful) are you able to float a needle on the surface of water

Chemisattempt 9-12: Chemical bonding

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

A steel needle or paper clip has actually higher thickness that that of water. So when it is dropped right into water, it falls dvery own right into the bottom of water. However, if we put it horizontally on the water through most caution, it deserve to float nearly forever because of the surconfront stress and anxiety of water.

Surencounters, or even more mostly interdeals with, display some physical properties which are not uncovered in the bulk of the exact same product. The finest known example of such building is surchallenge stress. Intermolecular forces produce the surface stress and anxiety. The water molecules at the surconfront are pulled in by the cohesive force between themselves and molecules inside the mass and hence have actually greater potential energy than those inside. The water keeps its round surconfront form because tright here are no water molecules exterior the surconfront to balance this inward pull. This cohesive force acts prefer a elastic membrane which resists being torn.


The photos over show the framework of water molecules. Between water molecules tbelow are hygrogen bonding, which are attrenergetic forces. The yellow lines represent hydrogen bonding. Inside molecules have attractive pressures in all direction, so these forces are canceled out. As such, they don"t feel any type of net force, so they do not have actually any kind of potential energy. In contrast, the surconfront molecules have just side and also downward attration pressures. So horizontally they attract each other and also act like a membrane. In a vertical direction to the facility, they have actually potential power. To minimize this energy, they try to make their surchallenge location as samll as possible.

The magnitude of press exerted by a things depends on its weight and call area. When we drop a needle vertically, even though it has actually the very same weight, it has tiny contact location, which leads to large press on the surconfront of water resulting in tearing the surface and also falling down. In fact, that"s why needles have actually such sharp tips. In contrast, if is put horizontally to the surconfront of water, it has larger contact area and less push exerting on the water surchallenge.

When soap is included to the water, the needle will certainly be moved to sink by the disruption of the water"s surconfront tension. Make sure students observe what happens to the needle. The term surfactant is a blfinish of "surface acting agent". Surfactants are commonly organic compounds that are amphipathic, meaning they contain both hydrophobic teams (their "tails") and hydrophilic groups (their "heads"). As such, they are soluble in both organic solvents and water. Surfactants reduce the surchallenge stress of water by adsorbing at the liquid-gas interconfront. They likewise reduce the interfacial tension between oil and also water by adsorbing at the liquid-liquid interchallenge.


Questioning Script

Prior knowledge & experience: Metal iron has greater density than that of water, so a things made of iron sinks right into the water.

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

Taracquire response: Students understand the effect of surconfront tension and fudamentally the strength of intermolecular bonding.

Usual Misconceptions:

Students commonly think heavy metal always sink into water without considering the result of shape also though the form of an object determines the buoyance and exerting push onto water. Young students do not think that tright here are intermolecular forces within a liquid. Students believe that just the weight of a things affects the pressure exerted by the object.

Concern for students

Why execute we must put the needle onto the piece of paper to make it float? Why execute we should dry the needle up before we use it? What makes the needle float? How the detergent makes the floating needle sink? Do you think the needle have the right to float on the oil through the very same method?


Fill a beaker or glass tumbler half with water. Drop a steel needle or paper clip right into water in order to display the reality that the steel has greater density than that of water. Get the needle out of water and dry the needle out to reusage it. Cut paper towel to gain a item of paper. Put the needle onto the item of paper. Get a glass of water. Then put the piece of paper with the needle onto the water. When the paper is soaked, push the paper right into the water to rerelocate it. Stir the water very closely to display the needle is floating. Prepare a small amount soap water with kitchen soap. Add a drop of soap water into the glass, then watch the needle sinking. For the comparikid of surchallenge stress of assorted liquids, use ethanol or vegetable oil.

Surface anxiety is crucial for the deliver of power from wind to water to create waves. Waves are necessary for quick oxygen diffusion in lakes and also seas.
Since of the high surface anxiety in between water and air, water striders are able to literally walk on water.

Surface tension reasons these water droplets on leaves to bead up developing the smallest surface feasible.

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The water molecules at the surface are pulled in by the cohesive pressure in between themselves and also molecules inside the droplet. The water keeps its droplet form because tright here are no water molecules exterior the surconfront to balance this inward pull.