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

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Earth and Environmental Science

Earth's Systems

ESS.EES.3.1

Cohesion

- [Istruttore] La coesione
è la tendenza che hanno

le molecole dello stesso
tipo a rimanere unite.

L'acqua è un esempio classico.

Molecole diverse si raggruppano in gocce

in quanto formano un tutt'uno
tramite il legame a idrogeno.

Mentre le gocce si accumulano,

alcune molecole in superficie
sono esposte all'aria.

Data la mancanza di sostanze adatte

a legarsi a questa connessione

l'acqua sarà maggiormente
attratta verso se stessa

fino a creare una spinta verso l'interno.

Tale attrazione forma molti legami

fino a creare una superficie
in grado di resistere

alle rotture in situazioni di stress,

un fenomeno noto come
tensione superficiale.

Di conseguenza piccoli oggetti
possono camminare sull'acqua

fintanto che non spezzano le
forze di coesione in gioco.

Yang, Eunjin, Jae Hak Son, Sang-im Lee, Piotr G. Jablonski e Ho-Young Kim. "I camminatori d'acqua regolano la velocità di movimento delle gambe per ottimizzare la velocità di decollo per la loro morfologia". Nature Communications 7 (7 dicembre 2016). [<a href="https://doi.org/10.1038/ncomms13698">Fonte</a>]

coesione

La coesione è l'attrazione tra molecole dello stesso tipo - come le molecole d'acqua. L'ossigeno negativo parzialmente caricato di una molecola d'acqua si lega all'idrogeno positivo parzialmente caricato di una seconda molecola d'acqua tramite il legame di idrogeno. Ogni molecola d'acqua può formare fino a quattro legami di idrogeno con altre molecole d'acqua.

Su una superficie, quando più molecole d'acqua si uniscono attraverso la coesione, si forma una goccia. L'acqua in genere non si sviluppa su tutta la superficie. Questo perché le molecole d'acqua sono più attratte l'una dall'altra che dalle molecole che compongono la superficie o l'aria circostante.

La tensione superficiale deriva dalla coesione. I "pattinatori di stagno", noti anche come sciatori d'acqua, sono insetti che usano questo fenomeno per camminare sull'acqua. La tensione superficiale si verifica all'interfaccia tra acqua ed aria (ad esempio, la superficie dell'acqua). Ancora una volta, le molecole d'acqua sono più attratte l'una all'altra rispetto che alle molecole nell'aria. Così, le molecole d'acqua in superficie formano legami con le molecole d'acqua vicine, sopra e sotto di loro. Poiché le molecole di acqua in superficie non possono formare legami con altre molecole d'acqua in corrispondenza di un lato (il lato a contatto con l'aria), formano legami più forti con le loro molecole d'acqua vicine. Le molecole fortemente legate hanno una superficie compressa, creando goccioline sferiche di molecole d'acqua. L'elevata tensione superficiale dell'acqua e la spinta idrostatica delle gambe del "pattinatore dello stagno" consentono all'insetto di rimanere sulla superficie dell'acqua.

Il mercurio è un'altra molecola coesa che può essere facilmente vista. In un contenitore di vetro, il mercurio non si estende nè bagna il vetro, perché le forze coese tra le molecole di mercurio sono più forti delle forze adesive tra il mercurio e il vetro. Anche se l'acqua è altamente coesa, ha anche un'affinità per la silice. Così, l'acqua si disperde uniformemente sul fondo di un contenitore di vetro.

Coesione

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.

Water Molecules and Surface Tension

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hydrogen bonds

Hydrogen Bonds in Water

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van der waals interactions

Intermolecular Forces in Polar Solutions

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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

Water Climbing by Adhesion

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states of water

Water Phase Changes and Ice Density

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solvents

Water as a Biological Solvent

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carbon skeletons

Carbon Backbones in Biomolecules

Carbon backbones give biological molecules their structure and flexibility. Living things are built from macromolecules that contain carbon atoms, and organic compounds are defined by a carbon backbone. Carbon is tetravalent, so each carbon atom can bond with four other atoms. Its valence electrons are stable, so carbon rarely becomes an ion. As carbon chains get longer, they can form ring structures, double bonds, and branching side chains. Carbon-carbon bonds create the carbon skeleton.

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.

Water Molecules and Surface Tension | JoVE K12

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Water Molecules and Surface Tension

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