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

- [Formateur] La cohésion est la tendancedes molécules de même natureà se coller les unes aux autres. 
L'eau est un exemple classique. 
De multiples molécules forment des gouttelettes,en s'attirant par cohésion grâce à la liaison hydrogène. 
Lors de l'accumulation des gouttes,les molécules en surface sont exposées à l'air. 
Étant donné l'absence de substance capablede se lier à cette interface,l'eau va davantage s'attirer à elle-même,provoquant une tension vers l'intérieur. 
Ce pouvoir d'attirance crée de nombreux liens,générant ainsi une surface capable de résister à la ruptureen situation de stress,phénomène appelé tension superficielle. 
Par conséquent, des corps de petite taillepeuvent marcher sur l'eau,mais sans rompre les forces de cohésion en présence.

Yang, Eunjin, Jae Hak Son, Sang-im Lee, Piotr G. Jablonski et Ho-Young Kim. « Les striders de l’eau ajustent la vitesse de mouvement des jambes pour optimiser la vitesse de décollage en fonction de leur morphologie. » Nature Communications 7 (7 décembre 2016). [<a href="https://doi.org/10.1038/ncomms13698">Source</a>]

cohésion

La cohésion est l’attraction entre les molécules du même type, comme les molécules d’eau. L’oxygène partiellement chargé négativement d’une molécule d’eau se lie à l’hydrogène partiellement chargé positivement d’une deuxième molécule d’eau grâce à des liaisons hydrogène. Chaque molécule d’eau peut former jusqu’à quatre liaisons hydrogène avec d’autres molécules d’eau. 
 
 Sur une surface, lorsque plusieurs molécules d’eau s'assemblent par le biais de la cohésion, une gouttelette se forme. L’eau ne s’étale généralement pas le long de la surface. C’est parce que les molécules d’eau sont plus attirées entre elles que par les molécules qui composent la surface ou l’air environnant.
 
 La tension superficielle résulte de la cohésion. Les araignées d’eau, également connues sous le nom de patineurs d’eau, sont des insectes qui utilisent ce phénomène pour marcher sur l’eau. La tension superficielle se produit à l’interface entre l’eau et l’air (c.-à-d. la surface de l’eau). Là encore, les molécules d’eau sont plus attirées les unes aux autres qu’elles ne le sont par les molécules dans l’air. Ainsi, les molécules d’eau à la surface forment des liaisons avec les molécules d’eau voisines à côté et en dessous d’elles. Comme les molécules d’eau à la surface ne peuvent pas former de liaisons avec d’autres molécules d’eau d’un côté (le côté le plus proche de l’air), elles forment des liaisons plus fortes avec les molécules d’eau voisines. Les molécules fortement liées ont une surface comprimée, créant des gouttelettes sphériques de molécules d’eau. La tension superficielle élevée de l’eau et la flottabilité des pattes de l'araignée d’eau permettent à l’insecte de rester à la surface de l’eau. 
 
 Le mercure est une autre molécule cohésive qui peut être facilement observée. Dans un récipient en verre, le mercure ne s’étale pas et ne mouille pas le verre parce que les forces cohésives entre les molécules de mercure sont plus fortes que les forces adhésives entre le mercure et le verre. Bien que l’eau soit très cohésive, elle a également une affinité pour la silice. Ainsi, l’eau se disperse uniformément dans le fond d’un récipient en verre.

Cohésion

La cohésion est l’attraction entre des molécules du même type, telles que les molécules d’eau. Les molécules d’eau ont une charge globalement neutre mais sont polaires ...

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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.

Water Molecules and Surface Tension

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cohesion

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...

adhesion

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key terms and concepts

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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3.3.9-12.K

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Water Molecules and Surface Tension

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