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

Una soluzione è una miscela omogenea di due sostanze principali: il soluto e il solvente. Il solvente è presente in quantità maggiore del soluto.
In una soluzione diluita, la proporzione di soluto rispetto al solvente è piccola, mentre per una soluzione concentrata, la proporzione è grande.
Quando si forma una soluzione, il soluto e il solvente non reagiscono chimicamente. Invece, le molecole si ridistribuiscono, con le singole molecole di solvente che circondano le molecole di soluto e interagiscono attraverso le forze attrattive. Tale processo di dissoluzione è chiamato solvatazione.
Per favorire la solvatazione, le interazioni soluto-solvente dovrebbero essere più forti delle interazioni solvente-solvente e soluto-soluto.
In una soluzione, il soluto e il solvente possono essere qualsiasi stato della materia. La soluzione stessa è un'unica "fase".
L'opposto della solvatazione è la precipitazione, innescata da forti interazioni soluto-soluto. Quando la velocità di solvatazione è uguale alla velocità di precipitazione, si stabilisce un equilibrio di solubilità.
La solubilità di un soluto è la sua massima concentrazione possibile all'equilibrio di solubilità in una data quantità di solvente. La solubilità è influenzata dalla temperatura e da altre condizioni fisiche come la polarità molecolare.
I soluti non polari si dissolvono in solventi non polari attraverso le forze di dispersione intermolecolari di London, mentre i soluti polari o ionici si dissolvono in solventi polari attraverso interazioni ione-dipolo o dipolo-dipolo.
L'acqua è un solvente polare. I composti idrofili, o amanti dell'acqua, sono solubili in acqua a causa delle cariche ioniche o dei gruppi polari che consentono interazioni acqua-soluto sufficientemente forti. Sono meno solubili nei solventi non polari perché le interazioni soluto-soluto sono molto più forti di qualsiasi possibile interazione tra solvente non polare e molecole di soluto polare.
I composti non polari insolubili in acqua e liposolubili come gli oli sono idrofobici o temono l'acqua, in quanto non possono formare legami idrogeno per competere con le interazioni solvente-solvente.
I composti anfifilici o anfipatici hanno sia gruppi polari che non polari. Ad esempio, i saponi sono sali metallici di acidi grassi. Nell'acqua, le molecole di sapone si aggregano in sfere chiamate micelle. Le loro code di acidi grassi idrofobici non polari puntano verso l'interno, mentre le loro teste idrofile polari sono all'esterno.
Quando si rimuovono le macchie di olio con il sapone, le code circondano l'olio non polare per formare micelle idrosolubili, consentendo all'olio di essere lavato via.

solubilità

Soluzione, solubilità e costante di solubilità

Una soluzione è una miscela omogenea composta da un solvente, il componente maggiore, e un soluto, il componente minore. Lo stato fisico di una soluzione (solido, liquido o gassoso) è tipicamente lo stesso del solvente. Le concentrazioni di soluto sono spesso descritte con termini qualitativi come diluito (di concentrazione relativamente bassa) e concentrato (di concentrazione relativamente alta).

In una soluzione, le particelle del soluto (molecole, atomi e/o ioni) sono strettamente circondate da specie solventi e interagiscono attraverso forze attrattive. Questo processo di dissoluzione è chiamato solvatazione. Quando l'acqua è il solvente, il processo è noto come idratazione. Per la solvatazione, le interazioni soluto-solvente dovrebbero essere più forti delle interazioni soluto-soluto e solvente-solvente. La precipitazione è l'opposto della solvatazione e si verifica a causa di forti interazioni soluto-soluto.

La solubilità è la misura della quantità massima di soluto che può essere disciolta in una data quantità di solvente. La temperatura, la pressione e la polarità molecolare sono alcuni dei fattori importanti che influenzano la solubilità. L'equilibrio di solubilità viene stabilito quando la dissoluzione e la precipitazione di un tipo di soluto avvengono a velocità uguali.

Il simile scioglie il simile

Per prevedere se un soluto sarà solubile in un dato solvente, la regola pratica è “il simile scioglie il simile”. I soluti polari o ionici si dissolvono in solventi polari a causa delle risultanti interazioni ione-dipolo o dipolo-dipolo con le molecole del solvente. Tale interazione non sarà possibile con un solvente non polare. I soluti non polari si dissolvono in solventi non polari attraverso le forze di dispersione intermolecolari.

Composti idrofili, idrofobi e anfifilici

L'acqua è un solvente polare. I soluti solubili in acqua sono chiamati “idrofili” o “amanti dell’acqua”. Ad esempio, quando all'acqua viene aggiunto KCl solido, l'estremità positiva (idrogeno) delle molecole polari di acqua viene attratta dagli ioni cloruro negativi e le estremità negative (ossigeno) dell'acqua vengono attratte dagli ioni positivi di potassio. Le molecole d’acqua circondano i singoli ioni K+ e Cl−, riducendo le forze forti che legano insieme gli ioni e permettendo loro di spostarsi in soluzione come ioni solvatati.

Un soluto insolubile in acqua è definito “idrofobo” o “che ha paura dell’acqua”. Tali soluti, come l’olio, non sono in grado di formare legami a idrogeno con le molecole d’acqua circostanti a causa delle interazioni soluto-soluto più forti. Di conseguenza, le particelle di soluto si raggruppano e rimangono non disciolte.

I composti che hanno sia gruppi polari che non polari sono chiamati “anfipatici” o “anfifilici”. Ad esempio, i saponi, che sono i sali degli acidi grassi. Hanno una coda idrofobica di idrocarburi non polari e una testa idrofila polare. L'azione pulente di saponi e detergenti può essere spiegata in termini di strutture delle molecole coinvolte. L'estremità idrocarburica (non polare) di una molecola di sapone o detergente si dissolve o è attratta da sostanze non polari, come olio, grasso o particelle di sporco. L'estremità ionica è attratta dall'acqua (polare). Di conseguenza, le molecole di sapone o detersivo si orientano nell'interfaccia tra le particelle di sporco e l'acqua, fungendo quindi come una sorta di ponte tra due diversi tipi di materia, non polare e polare. Di conseguenza, le particelle di sporco vengono sospese come particelle colloidali e vengono facilmente lavate via.

Questo testo è adattato da <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>, e <a href="https://openstax.org/books/chemistry-2e/pages/11-5-colloids">Section 11.5: Colloids</a>.

Solubilità

Soluzione, solubilità ed equilibrio di solubilità
Una soluzione è una miscela omogenea composta da un solvente, il componente principale, e da un soluto, il ...

Education

Didattica

JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Covalent Bonding and Structure

Legame covalente e struttura

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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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chemical bonds

Chemical Bonds: Types and Strength

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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.

Why Some Substances Dissolve in Water | JoVE K12

organic chemistry

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Arizona Science Standards

High School

Physical Science

HS+C.P1

HS+C.P1U1.4

Molecular Orbital (MO) Theory

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
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Electron Behavior

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Concentration and Rate Law

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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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Matter and Its Interactions: Develop a model based on evidence to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy. (P: 2, CC: 5, CI: PS1)

9C.3.1.1.2

Solution Formation

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Energetics of Solution Formation

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General Properties of Solutions

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Solutions and Concentrations

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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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