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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. * students know just how to determine solids and liquids hosted together by valve der Waals forces or hydrogen bonding and relate these forces to volatility and also boiling/ melting point temperatures.


A steel needle or file clip has greater density the that of water. So when it is dropped into water, it falls down into the bottom the water. However, if we put it horizontally ~ above the water through a lot of caution, it can float nearly forever because of the surface stress of water.

Surfaces, or an ext generally interfaces, show some physics properties which room not found in the bulk of the exact same material. The finest known example of such property is surface ar tension. Intermolecular forces produce the surface ar tension. The water molecules at the surface space pulled in by the cohesive force between themselves and molecules within the bulk and thus have higher potential energy than those inside. The water keeps that is round surface shape due to the fact that there room no water molecules exterior the surface to balance this inward pull. This cohesive force acts prefer a elastic membrane i m sorry resists being torn.

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

The magnitude of press exerted by an object depends top top its weight and also contact area. Once we drop a needle vertically, also though it has actually the very same weight, the has small contact area, which leader to huge pressure on the surface ar of water resulting in 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 of water, it has actually larger call area and also less press exerting top top the water surface.

When soap is included to the water, the needle will certainly be moved to sink by the disruption that the water"s surface ar tension. Make certain students observe what happens to the needle. The ax surfactant is a mix of "surface acting agent". Surfactants space usually organic 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 organic solvents and water. Surfactants alleviate the surface anxiety of water by adsorbing in ~ the liquid-gas interface. They also reduce the interfacial tension in between oil and also water through adsorbing at the liquid-liquid interface.

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Questioning Script

Prior understanding & experience: metal iron has greater density than that that water, so an object made of iron sinks into the water.

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

Target response: Students know the impact of surface ar tension and fudamentally the toughness of intermolecular bonding.

Common Misconceptions:

Students commonly think heavy metal always sink into water there is no considering the impact of shape even though the form of an item determines the buoyance and exerting pressure onto water. Young student don"t think the there space intermolecular forces within a liquid. Students believe that just the load of an item affects the push exerted through the object.

Question because that students

Why execute we must put the needle onto the item of record to make it float? Why do we need to dry the needle up prior to we use it? What makes the needle float? how the detergent makes the floating needle sink? carry out you think the needle deserve to float ~ above the oil v the exact same method?

Procedure

to fill a maker or glass tumbler fifty percent with water. drop a steel needle or file clip right into water in stimulate to present the reality that the metal has greater density 보다 that the water. acquire the needle the end of water and also dry the needle out to reuse it. Cut paper towel to obtain a item of paper. Put the needle onto the piece of paper. Gain a glass that water. Then placed the piece of file with the needle onto the water. As soon as the file is soaked, press the paper into the water to remove it. Stir the water carefully to show the needle is floating. Prepare a tiny amount soap water with kitchen soap. Include a autumn of soap water right into the glass, climate watch the needle sinking. because that the comparison of surface stress of assorted liquids, usage ethanol or vegetables oil.

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Surface anxiety is crucial for the transfer of energy from wind come water to create waves. Tide are important for fast oxygen diffusion in lakes and also seas.
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Because of the high surface tension in between water and air, water striders room able come literally walk top top water.
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Surface tension reasons these water droplets on pipeline to bead up developing the smallest surface ar possible.

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