Name: why some substances dissolve in water
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 3 min
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

3.2.1 9C.3.1.1.3

Solubility

Une solution est un mélange homogène de deux substances principales : le soluté et le solvant. Le solvant est présent en plus grande quantité que le soluté.
Dans une solution diluée, la proportion de soluté par rapport au solvant est faible, tandis que pour une solution concentrée, la proportion est importante.
Lors de la formation d’une solution, le soluté et le solvant ne réagissent pas chimiquement. Au lieu de cela, les molécules se redistribuent, avec des molécules de solvant individuelles entourant les molécules de soluté et interagissant par des forces d’attraction. Un tel processus de dissolution est appelé solvatation.
Pour favoriser la solvatation, les interactions soluté-solvant doivent être plus fortes que les interactions solvant-solvant et soluté-soluté.
Dans une solution, le soluté et le solvant peuvent être n’importe quel état de la matière. La solution elle-même est une « phase » unique.
L’opposé de la solvatation est la précipitation, déclenchée par de fortes interactions soluté-soluté. Lorsque le taux de solvatation est égal au taux de précipitation, un équilibre de solubilité est établi.
La solubilité d’un soluté est sa concentration maximale possible à l’équilibre de solubilité dans une quantité donnée de solvant. La solubilité est affectée par la température et d’autres conditions physiques telles que la polarité moléculaire.
Les solutés non polaires se dissolvent dans les solvants non polaires par les forces de dispersion intermoléculaires de Londres, tandis que les solutés polaires ou ioniques se dissolvent dans les solvants polaires par des interactions ion-dipôle ou dipôle-dipôle.
L’eau est un solvant polaire. Les composés hydrophiles, ou hydrophiles, sont solubles dans l’eau en raison des charges ioniques ou des groupes polaires qui permettent des interactions eau-soluté suffisamment fortes. Ils sont moins solubles dans les solvants non polaires car les interactions soluté-soluté sont beaucoup plus fortes que toute interaction possible entre les molécules de solvant non polaire et de soluté polaire.
Les composés non polaires liposolubles et insolubles dans l’eau, comme les huiles, sont hydrophobes ou craignent l’eau, car ils ne peuvent pas former de liaisons hydrogène pour rivaliser avec les interactions solvant-solvant.
Les composés amphiphiles ou amphipathiques ont à la fois des groupes polaires et non polaires. Par exemple, les savons sont des sels métalliques d’acides gras. Dans l’eau, les molécules de savon s’agrègent en sphères appelées micelles. Leur queue d’acide gras hydrophobe non polaire pointe vers l’intérieur tandis que leur tête hydrophile polaire est à l’extérieur.
Lors de l’élimination des taches d’huile à l’aide de savon, les queues entourent l’huile non polaire pour former des micelles solubles dans l’eau, ce qui permet à l’huile d’être emportée.

solubilité

<h4>Solution, solubilité et équilibre de solubilité</h4>

Une solution est un mélange homogène composé d'un solvant, le composant majeur, et d'un soluté, le composant mineur. L'état physique d'une solution (solide, liquide ou gazeux) est généralement le même que celui du solvant. Les concentrations de solutés sont souvent décrites avec des termes qualitatifs tels que dilué (de concentration relativement faible) et concentré (de concentration relativement élevée).

Dans une solution, les particules de soluté (molécules, atomes et/ou ions) sont étroitement entourées d'espèces de solvant et interagissent par l'intermédiaire de forces d'attraction. Ce processus de dissolution est appelé solvatation. Lorsque l’eau est le solvant, le processus est appelé hydratation. Pour la solvatation, les interactions soluté-solvant doivent être plus fortes que les interactions soluté-soluté et solvant-solvant. La précipitation est l’opposé de la solvatation et se produit en raison de fortes interactions soluté-soluté.

La solubilité est la mesure de la quantité maximale de soluté pouvant être dissoute dans une quantité donnée de solvant. La température, la pression et la polarité moléculaire sont quelques-uns des facteurs importants qui affectent la solubilité. L'équilibre de solubilité est établi lorsque la dissolution et la précipitation d'une espèce solutée se produisent à des taux égaux.

<h4>Dissout comme tels</h4>

Pour prédire si un soluté sera soluble dans un solvant donné, la règle générale est « le semblable dissout le semblable ». Les solutés polaires ou ioniques se dissolvent dans les solvants polaires en raison des interactions ion-dipôle ou dipôle-dipôle qui en résultent avec les molécules de solvant. Une telle interaction ne sera pas possible avec un solvant non polaire. Les solutés non polaires se dissolvent dans les solvants non polaires grâce aux forces de dispersion intermoléculaires.

<h4>Composés hydrophiles, hydrophobes et amphiphiles</h4>

L'eau est un solvant polaire. Les solutés solubles dans l’eau sont appelés « hydrophiles » ou « qui aiment l’eau ». Par exemple, lorsque du KCl solide est ajouté à l’eau, l’extrémité positive (hydrogène) des molécules d’eau polaires est attirée par les ions chlorure négatifs et les extrémités négatives (oxygène) de l’eau sont attirées par les ions potassium positifs. Les molécules d’eau entourent les ions K+ et Cl− individuels, réduisant ainsi les fortes forces qui lient les ions entre eux et les laissant passer en solution sous forme d’ions solvatés.

Un soluté insoluble dans l’eau est qualifié de « hydrophobe » ou de « craignant l’eau ». De tels solutés, comme le pétrole, sont incapables de former des liaisons hydrogène avec les molécules d’eau environnantes en raison des interactions soluté-soluté plus fortes. En conséquence, les particules de soluté se regroupent et restent non dissoutes.

Les composés qui ont à la fois des groupes polaires et non polaires sont appelés « amphipathiques » ou « amphiphiles ». Par exemple, les savons, qui sont des sels d'acides gras. Ils ont une queue hydrophobe d'hydrocarbures non polaires et une tête hydrophile polaire. L’action nettoyante des savons et détergents peut s’expliquer par la structure des molécules impliquées. L'extrémité hydrocarbure (non polaire) d'une molécule de savon ou de détergent se dissout dans des substances non polaires, telles que l'huile, la graisse ou les particules de saleté, ou est attirée par ces éléments. L'extrémité ionique est attirée par l'eau (polaire). En conséquence, les molécules de savon ou de détergent s'orientent à l'interface entre les particules de saleté et l'eau, elles agissent donc comme une sorte de pont entre deux types de matière différents, apolaire et polaire. En conséquence, les particules de saleté se mettent en suspension sous forme de particules colloïdales et sont facilement éliminées.

Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/11-1-the-dissolution-process">OpenStax Chemistry 2e, Section 11.1: The Dissolution Process</a>, <a href="https://openstax.org/books/chemistry-2e/pages/11-3-solubility">Section 11.3: Solubility</a>, and <a href="https://openstax.org/books/chemistry-2e/pages/11-5-colloids">Section 11.5: Colloids</a>.

Solubilité

Équilibre, équilibre, solubilité et solubilité
Une solution est un mélange homogène composé d’un solvant, le composant principal, et d’un soluté, le ...

Education

Enseignement

JoVE Core

Organic Chemistry

Chimie organique

Unit 1

Unité 1

Covalent Bonding and Structure

Liaison covalente et structure

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

why some substances dissolve in water

Solution formation depends on how solutes and solvents interact. A solution is a homogeneous mixture made of a solvent, which is the larger part, and a solute, which is the smaller part. The physical state of a solution is usually the same as the solvent. Concentration is often described as dilute for a low amount of solute or concentrated for a high amount of solute.

When a solute dissolves, its particles are surrounded by solvent species. This process is called solvation. If water is the solvent, it is called hydration. For solvation to happen, solute-solvent attractions must be stronger than the attractions within the solute and within the solvent. The opposite process is precipitation, which happens when solute particles are pulled together strongly enough to leave solution.

Solubility is the maximum amount of solute that can dissolve in a given amount of solvent. Temperature, pressure, and molecular polarity can all affect solubility. Solubility equilibrium occurs when dissolution and precipitation happen at the same rate. At that point, the amount dissolved stays constant.

A useful rule for predicting solubility is “like dissolves like.” Polar or ionic solutes usually dissolve in polar solvents because of ion-dipole or dipole-dipole attractions. These interactions do not work well with nonpolar solvents. Nonpolar solutes usually dissolve in nonpolar solvents through dispersion forces.

Water is a polar solvent, so substances that dissolve well in water are called hydrophilic, or water-loving. For example, when KCl is added to water, the hydrogen end of water is attracted to chloride ions, and the oxygen end is attracted to potassium ions. Water molecules surround the ions, weaken the forces holding them together, and allow them to move into solution as solvated ions. Substances that do not dissolve in water are called hydrophobic, or water-fearing. Oil is one example. It cannot form hydrogen bonds with water, so the particles cluster together and stay undissolved.

Some compounds contain both polar and nonpolar parts. These are called amphipathic, or amphiphilic. Soaps are an example because they are salts of fatty acids. They have a nonpolar hydrocarbon tail and a polar hydrophilic head. This structure helps soaps and detergents clean because the nonpolar end interacts with oil, grease, or dirt, while the ionic end interacts with water. The molecules line up at the boundary between the dirt and the water. They act like a bridge between polar and nonpolar matter, which helps suspend dirt as colloidal particles so it can be washed away.

Why Some Substances Dissolve in Water

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Learn how solution formation, solvation, solubility, and the rule “like dissolves like” explain why some substances dissolve in water and others do not.

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Arizona Science Standards

High School

Physical Science

HS+C.P1

HS+C.P1U1.4

Molecular Orbital (MO) Theory

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Measuring Reaction Rates

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Temperature Dependence

Chemical Kinetics 
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Reaction Rate

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Concentration Dependence

Chemical Kinetics and the Reaction Rate Law
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9C.3.1.1.2

Solution Formation

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9C.3.2.1.2

3 Developing possible explanations of phenomena or designing solutions to engineering problems
 3.2 Constructing explanations and designing solutions 
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Why Some Substances Dissolve in Water

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