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

A metal needle or file clip has higher density that that that water. So as soon as it is dropped into water, it falls down into the bottom of water. However, if we placed it horizontally on the water v a most caution, it have the right to float practically forever due to the surface stress and anxiety of water.

Surfaces, or much more generally interfaces, show some physics properties which are not uncovered in the mass of the very same material. The best known instance of such residential or commercial property is surface tension. Intermolecular forces produce the surface tension. The water molecules at the surface are pulled in by the cohesive force in between themselves and also molecules within the bulk and thus have greater potential power than those inside. The water keeps the round surface ar shape due to the fact that there space no water molecules external the surface to balance this inward pull. This cohesive pressure acts favor a elastic membrane which resists being torn.


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

The magnitude of press exerted by an object depends top top its weight and also contact area. As soon as we fall a needle vertically, also though it has the same weight, it has tiny contact area, which leads to big pressure top top the surface ar of water bring about tearing the surface and also falling down. In fact, that\"s why needles have actually such sharp tips. In contrast, if is placed horizontally to the surface ar of water, it has larger contact area and less push exerting on the water surface.

When soap is included to the water, the needle will be driven to sink through the disruption the the water\"s surface ar tension. Make certain students watch what wake up to the needle. The hatchet surfactant is a blend of \"surface acting agent\". Surfactants are usually essential compounds that are amphipathic, an interpretation they save both hydrophobic groups (their \"tails\") and also hydrophilic groups (their \"heads\"). Therefore, they space soluble in both necessary solvents and also water. Surfactants minimize the surface stress and anxiety of water by adsorbing at the liquid-gas interface. They likewise reduce the interfacial tension in between oil and also water by adsorbing in ~ the liquid-liquid interface.


Questioning Script

Prior understanding & experience: metal iron has greater density 보다 that of water, so an item made of stole sinks right into the water.

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

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

Common Misconceptions:

Students commonly think heavy metal constantly sink into water there is no considering the impact of shape also though the form of things determines the buoyance and also exerting press onto water. Young college student don\"t think that there space intermolecular pressures within a liquid. Students believe that only the weight of an object affects the pressure exerted by the object.

Question because that students

Why carry out we should put the needle onto the piece of document to do it float? Why perform we must dry the needle up prior to we usage it? What renders the needle float? how the detergent provides the floating needle sink? do you think the needle have the right to float top top the oil v the same method?


fill a manufacturer or glass tumbler half with water. drop a steel needle or document clip into water in stimulate to display the reality that the metal has greater density 보다 that that water. obtain the needle out of water and also dry the needle the end to reuse it. Cut document towel to acquire a piece of paper. Put the needle ~ above the item of paper. Acquire a glass of water. Then placed the piece of document with the needle onto the water. When the record is soaked, push the document into the water to eliminate it. Row the water closely to display the needle is floating. Prepare a small amount soap water v kitchen soap. Include a drop of soap water into the glass, climate watch the needle sinking. because that the to compare of surface stress of various liquids, use ethanol or vegetables oil.

Surface stress and anxiety is important for the move of power from wind to water to develop waves. Waves are necessary for fast oxygen diffusion in lakes and also seas.
Because the the high surface tension between water and air, water striders are able to literally walk top top water.

Surface tension reasons these water droplets on pipeline to bead up creating the smallest surface possible.

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The water molecule at the surface are pulled in through the cohesive force between themselves and also molecules inside the droplet. The water keeps that droplet shape since there space no water molecules outside the surface ar to balance this inward pull.