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

Chemistry 9-12: chemical bonding

2h. * college student know just how to identify solids and liquids hosted together by valve der Waals forces or hydrogen bonding and relate these forces to volatility and boiling/ melting suggest temperatures.

A metal needle or record clip has greater density that that the water. So as soon as it is dropped into water, it drops down into the bottom that water. However, if we put it horizontally top top the water with a lot of caution, it deserve to float nearly forever due to the surface stress and anxiety of water.

Surfaces, or much more generally interfaces, display some physical properties which room not found in the bulk of the same material. The best known instance of such residential property is surface tension. Intermolecular forces create the surface tension. The water molecule at the surface room pulled in by the cohesive force in between themselves and also molecules within the bulk and thus have higher potential power than those inside. The water keeps that is round surface shape due to the fact that there room no water molecules exterior the surface ar to balance this inward pull. This cohesive force acts choose a elastic membrane i m sorry resists being torn.


The photos above show the framework of water molecules. Between water molecules there are hygrogen bonding, which room attractive forces. The yellow lines represent hydrogen bonding. Inside molecules have attractive pressures in every direction, so these pressures are canceled out. Therefore, they don"t feel any type of net force, for this reason they don"t have any type of potential energy. In contrast, the surface ar molecules have actually only side and also downward attration forces. So horizontally they tempt each other and act choose a membrane. In a upright direction come the center, they have actually potential energy. To minimize this energy, they try to make their surface area together samll together possible.

The magnitude of press exerted by things depends top top its weight and also contact area. When we drop a needle vertically, even though it has actually the exact same weight, that has small contact area, which leads to large pressure on the surface of water leading to tearing the surface and also falling down. In fact, that"s why needles have such spicy tips. In contrast, if is placed horizontally come the surface ar of water, it has actually larger contact area and also less press exerting on the water surface.

When soap is added to the water, the needle will certainly be propelled to sink through the disruption of the water"s surface ar tension. Make sure students observe what wake up to the needle. The term surfactant is a mix of "surface acting agent". Surfactants room usually organic compounds that room amphipathic, meaning they save on computer both hydrophobic groups (their "tails") and also hydrophilic groups (their "heads"). Therefore, they are soluble in both necessary solvents and also water. Surfactants alleviate the surface anxiety of water by adsorbing at the liquid-gas interface. They likewise reduce the interfacial tension between oil and also water through adsorbing in ~ the liquid-liquid interface.


Questioning Script

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

Root question: What provides the steel needle rise on water?

Target response: Students know the result of surface ar tension and fudamentally the strength of intermolecular bonding.

Common Misconceptions:

Students usually think hefty metal constantly sink right into water there is no considering the result of shape also though the form of an object determines the buoyance and exerting push onto water. Young college student don"t think the there are intermolecular forces within a liquid. Students believe that only the weight of an object affects the push exerted through the object.

Question for students

Why execute we need to put the needle ~ above the piece of paper to do it float? Why do we should dry the needle up prior to we use it? What provides the needle float? how the detergent makes the floating needle sink? do you think the needle can float on the oil with the very same method?


fill a maker or glass tumbler half with water. drop a steel needle or paper clip into water in order to present the truth that the metal has greater density 보다 that that water. obtain the needle the end of water and also dry the needle the end to reuse it. Cut paper towel to get a piece of paper. Put the needle ~ above the item of paper. Gain a glass of water. Then put the item of paper with the needle top top the water. Once the document is soaked, push the file into the water to remove it. Row the water closely to present the needle is floating. Prepare a little amount soap water v kitchen soap. Include a fall of soap water right into the glass, then watch the needle sinking. because that the comparison of surface stress and anxiety of various liquids, usage ethanol or vegetables oil.

Surface tension is essential for the transport of energy from wind come water to produce waves. Tide are important for fast oxygen diffusion in lakes and also seas.
Because that the high surface ar tension in between water and also air, water striders space able come literally walk top top water.

Surface tension reasons these water droplets on leaves to bead up forming the smallest surface possible.

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The water molecules at the surface room pulled in by the cohesive force in between themselves and molecules inside the droplet. The water keeps the droplet shape since there space no water molecules exterior the surface ar to balance this inward pull.