Name: water molecules and surface tension
Uploaded: 2019-03-11T09:21:00.000Z
Duration: 1 min 7 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Earth and Environmental Science

Earth's Systems

ESS.EES.3.1

Cohesion

- [Verteller] cohesie is de neiging voor hetzelfde soort moleculen om aan elkaar te kleven. Water is een klassiek voorbeeld. Meerdere moleculen vormen druppeltjes, omdat ze samenhangend worden aangetrokken via waterstofbinding.

Terwijl de druppels zich verzamelen, worden sommige moleculen aan het oppervlak blootgesteld aan lucht. Gezien het gebrek aan geschikte stoffen om zich te binden op dit grensvlak, zal water zich meer aangetrokken voelen tot zichzelf en een trek naar binnen creëren. Zo'n aantrekkingskracht bouwt veel bindingen op die een oppervlak genereren dat in staat is om in stresssituaties scheuren te weerstaan, een fenomeen dat oppervlaktespanning wordt genoemd.

Zodoende kunnen kleine objecten op water lopen zolang ze de samenhangende krachten niet breken.

cohesie

Cohesie is de aantrekkingskracht tussen moleculen van hetzelfde type, zoals watermoleculen. Watermoleculen hebben een algehele neutrale lading, maar zijn polaire moleculen. Een zuurstofatoom in één watermolecuul heeft een gedeeltelijke negatieve lading die zich kan binden aan een waterstofatoom met een gedeeltelijke positieve lading in een ander watermolecuul, waardoor een waterstofbrug wordt gevormd. Elk watermolecuul kan maximaal vier waterstofbruggen vormen met andere watermoleculen. Waterstofbruggen zijn verantwoordelijk voor de cohesieve natuur van water.

Op een oppervlak, wanneer meerdere watermoleculen samenkomen door cohesie, wordt een druppel gevormd. Water verspreidt zich doorgaans niet over het oppervlak omdat de watermoleculen meer tot elkaar aangetrokken worden dan tot de moleculen die het oppervlak of de omringende lucht vormen. De aantrekkingskracht tussen de watermoleculen aan het oppervlak en het bulkwater wordt oppervlaktespanning genoemd.

Oppervlaktespanning is het resultaat van cohesie. schaatsenrijder, ook wel poelschaatsenrijder genoemd, zijn insecten die dit fenomeen gebruiken om op water te lopen. Oppervlaktespanning treedt op op het grensvlak tussen water en lucht (dat wil zeggen het wateroppervlak). Watermoleculen aan het oppervlak interacteren met aangrenzende watermoleculen naast en onder hen. Aangezien de oppervlaktewatermoleculen geen waterstofbruggen kunnen vormen met andere watermoleculen aan de ene kant (de kant naast de lucht), vormen ze sterkere interacties met hun aangrenzende watermoleculen. De sterk gebonden moleculen hebben een gecomprimeerd oppervlak, waardoor er bolvormige druppels van watermoleculen ontstaan. De hoge oppervlaktespanning van het water en de drijfkracht van de poten van de pschaatsenrijder stellen het insect in staat om op het wateroppervlak te blijven.

Kwik is een andere cohesieve vloeistof. In een glazen container blijft kwik in bolvormige druppels omdat de cohesieve krachten tussen kwikmoleculen sterker zijn dan de adhesieve krachten tussen kwik en het glas. Daarentegen heeft water, hoewel het zeer cohesief is, ook een affiniteit voor silica (een belangrijk bestanddeel van glas). Daarom verspreidt water zich gelijkmatig over de bodem van een glazen container.

Deze tekst is aangepast van <a href="http://openstax.org/books/chemistry-2e/pages/10-2-properties-of-liquids">Openstax/Chemistry-2e/section 10.2; Properties of liquids</a>

Cohesie

Cohesion is the attraction between molecules of the same type, such as water molecules. Water molecules have an overall neutral charge but are polar ...

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water molecules and surface tension

Water molecules stick to each other through cohesion. Water is neutral overall, but it is a polar molecule. The oxygen atom in one water molecule has a partial negative charge, and it can bind with a hydrogen atom that has a partial positive charge in another water molecule. This forms a hydrogen bond. Each water molecule can form up to four hydrogen bonds with other water molecules, and these bonds give water its cohesive nature.

Cohesion helps water form droplets on a surface. Water does not usually spread out because the water molecules are more attracted to each other than to the surface or the air around them. The attraction between water molecules at the surface and the bulk water is called surface tension, and it comes from cohesion. At the water-air boundary, surface molecules can only make hydrogen bonds beside and below them. That limited bonding creates a compressed surface area and spherical droplets.

Surface tension is strong enough to support pond skaters, also called water striders. These insects use the surface of water to walk across it. Their buoyant legs and the high surface tension of water help them stay on top of the water.

Mercury is another cohesive liquid. In a glass container, mercury stays in spherical droplets because the cohesive forces between mercury molecules are stronger than the adhesive forces between mercury and glass. Water is also highly cohesive, but it has an affinity for silica, which is a main component of glass. Because of that, water spreads evenly at the bottom of a glass container.

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Water molecules stick to each other through cohesion. Water is neutral overall, but it is a polar molecule. The oxygen atom in one water molecule has a partial negative charge, and it can bind with a hydrogen atom that has a partial positive charge in another water molecule. This forms a hydrogen bond. Each water molecule can form up to four hydrogen bonds with other water molecules, and these bonds give water its cohesive nature. Cohesion helps water form droplets on a surface. Water does not...

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Learn how water molecules bond through cohesion, create surface tension, form droplets, and help pond skaters walk on water in this high school science topic.

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Water Molecules and Surface Tension

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