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

Eine Lösung ist ein homogenes Gemisch aus zwei Hauptsubstanzen: dem gelösten Stoff und dem Lösungsmittel, wobei das Lösungsmittel in größeren Mengen vorhanden ist als der gelöste Stoff.
In einer verdünnten Lösung ist der Anteil des gelösten Stoffes relativ zum Lösungsmittel gering, während er bei einer konzentrierten Lösung groß ist.
Bei der Bildung einer Lösung reagieren der gelöste Stoff und das Lösungsmittel nicht chemisch. Stattdessen verteilen sich die Moleküle neu, wobei einzelne Lösungsmittelmoleküle die gelösten Moleküle umgeben und durch Anziehungskräfte interagieren. Ein solcher Auflösungsprozess wird als Solvatation bezeichnet.
Um die Solvatation zu begünstigen, sollten die Wechselwirkungen zwischen gelöstem Stoff und Lösungsmittel stärker sein als die Wechselwirkungen zwischen Lösungsmittel und gelöstem Stoff.
In einer Lösung können der gelöste Stoff und das Lösungsmittel einen beliebigen Aggregatzustand haben. Die Lösung selbst ist eine einzige "Phase".
Das Gegenteil der Solvatation ist die Ausfällung, die durch starke Wechselwirkungen zwischen gelösten Stoffen und gelösten Stoffen ausgelöst wird. Wenn die Solvatationsgeschwindigkeit der Niederschlagsrate entspricht, wird ein Löslichkeitsgleichgewicht hergestellt.
Die Löslichkeit eines gelösten Stoffes ist seine maximal mögliche Konzentration im Löslichkeitsgleichgewicht in einer gegebenen Menge Lösungsmittel. Die Löslichkeit wird durch die Temperatur und andere physikalische Bedingungen wie die molekulare Polarität beeinflusst.
Unpolare gelöste Stoffe lösen sich in unpolaren Lösungsmitteln durch intermolekulare Londoner Dispersionskräfte, während polare oder ionische gelöste Stoffe sich in polaren Lösungsmitteln durch Ionen-Dipol- oder Dipol-Dipol-Wechselwirkungen auflösen.
Wasser ist ein polares Lösungsmittel. Hydrophile oder wasserliebende Verbindungen sind aufgrund der ionischen Ladungen oder polaren Gruppen, die ausreichend starke Wechselwirkungen zwischen Wasser und gelösten Stoffen ermöglichen, wasserlöslich. Sie sind in unpolaren Lösungsmitteln weniger löslich, da die Wechselwirkungen zwischen gelösten und gelösten Stoffen viel stärker sind als jede mögliche Wechselwirkung zwischen unpolaren Lösungsmitteln und polaren gelösten Molekülen.
Wasserunlösliche, fettlösliche unpolare Verbindungen wie Öle sind hydrophob oder wasserfürchtend, da sie keine Wasserstoffbrückenbindungen bilden können, um mit Lösungsmittel-Lösungsmittel-Wechselwirkungen zu konkurrieren.
Amphiphile oder amphipathische Verbindungen haben sowohl polare als auch unpolare Gruppen. Seifen sind zum Beispiel Metallsalze von Fettsäuren. In Wasser aggregieren die Seifenmoleküle in Kugeln, die Mizellen genannt werden. Ihre unpolaren hydrophoben Fettsäureschwänze zeigen nach innen, während ihre polaren hydrophilen Köpfe nach außen zeigen.
Bei der Entfernung von Ölflecken mit Seife umschließen die Schwänze das unpolare Öl und bilden wasserlösliche Mizellen, wodurch das Öl weggewaschen werden kann.

löslichkeit

Lösung, Löslichkeit und Löslichkeitsgleichgewicht

Eine Lösung ist eine homogene Mischung, die aus einem Lösungsmittel, der Hauptkomponente, und einem gelösten Stoff, der Nebenkomponente, besteht. Der physikalische Zustand einer Lösung – fest, flüssig oder gasförmig – ist typischerweise derselbe wie der des Lösungsmittels. Konzentrationen gelöster Stoffe werden häufig mit qualitativen Begriffen wie „verdünnt“ (mit relativ geringer Konzentration) und „Konzentrat“ (mit relativ hoher Konzentration) beschrieben.

In einer Lösung sind die gelösten Teilchen (Moleküle, Atome und/oder Ionen) eng von Lösungsmittelspezies umgeben und interagieren durch Anziehungskräfte. Dieser Auflösungsprozess wird Solvatation genannt. Wenn Wasser das Lösungsmittel ist, wird der Vorgang als Hydratation bezeichnet. Bei der Solvatation sollten die Wechselwirkungen zwischen gelöster Substanz und Lösungsmittel stärker sein als die Wechselwirkungen zwischen gelöster Substanz und gelöster Substanz sowie zwischen Lösungsmittel und Lösungsmittel. Die Präzipitation ist das Gegenteil der Solvatation und erfolgt aufgrund starker Wechselwirkungen zwischen den gelösten Substanzen.

Die Löslichkeit ist das Maß für die maximale Menge an gelöster Substanz, die in einer bestimmten Menge Lösungsmittel gelöst werden kann. Temperatur, Druck und molekulare Polarität sind einige der wichtigen Faktoren, die die Löslichkeit beeinflussen. Ein Löslichkeitsgleichgewicht stellt sich ein, wenn die Auflösung und Präzipitation einer gelösten Spezies mit gleichen Geschwindigkeiten erfolgt.

Gleiches löst Gleiches

Um vorherzusagen, ob eine gelöste Substanz in einem bestimmten Lösungsmittel löslich sein wird, gilt die Faustregel „Gleiches löst Gleiches“. Polare oder ionische gelöste Stoffe lösen sich in polaren Lösungsmitteln aufgrund der resultierenden Ion-Dipol- oder Dipol-Dipol-Wechselwirkungen mit den Lösungsmittelmolekülen. Eine solche Wechselwirkung ist mit einem unpolaren Lösungsmittel nicht möglich. Unpolare gelöste Substanzen lösen sich in unpolaren Lösungsmitteln durch intermolekulare Dispersionskräfte.

Hydrophile, hydrophobe und amphiphile Verbindungen

Wasser ist ein polares Lösungsmittel. Gelöste Stoffe, die in Wasser löslich sind, werden als "hydrophil" oder "wasserliebend" bezeichnet. Wenn beispielsweise festes KCl zu Wasser gegeben wird, werden die positiven (Wasserstoff-)Enden der polaren Wassermoleküle von den negativen Chloridionen und die negativen (Sauerstoff-)Enden des Wassers von den positiven Kaliumionen angezogen. Die Wassermoleküle umgeben einzelne K+ und Cl-Ionen, reduzieren die starken Kräfte, die die Ionen aneinander binden, und lassen sie als solvatisierte Ionen in die Lösung übergehen.

Eine gelöste Substanz, die in Wasser unlöslich ist, wird als „hydrophob“ oder „wasserscheu“ bezeichnet. Solche gelösten Substanzen wie Öl sind aufgrund der stärkeren Wechselwirkungen zwischen gelösten Stoffen nicht in der Lage, Wasserstoffbrückenbindungen mit den umgebenden Wassermolekülen zu bilden. Dadurch verklumpen die gelösten Teilchen und bleiben ungelöst.

Verbindungen, die sowohl polare als auch unpolare Gruppen haben, werden als „amphipathisch“ oder „amphiphil“ bezeichnet. Zum Beispiel Seifen, bei denen es sich um Salze von Fettsäuren handelt. Sie haben einen hydrophoben Schwanz aus unpolaren Kohlenwasserstoffen und einen polaren hydrophilen Kopf. Die Reinigungswirkung von Seifen und Reinigungsmitteln lässt sich durch die Struktur der beteiligten Moleküle erklären. Das Kohlenwasserstoffende (unpolar) eines Seifen- oder Waschmittelmoleküls löst sich in unpolaren Substanzen wie Öl, Fett oder Schmutzpartikeln oder wird von ihnen angezogen. Das ionische Ende wird von Wasser (polar) angezogen. Dadurch orientieren sich die Seifen- oder Waschmittelmoleküle an der Grenzfläche zwischen den Schmutzpartikeln und dem Wasser und fungieren so als eine Art Brücke zwischen zwei unterschiedlichen Substanzarten, unpolaren und polaren. Dadurch werden Schmutzpartikel als kolloidale Partikel suspendiert und leicht abgewaschen.

Dieser Text wurde angepasst von<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>.

Löslichkeit

Lösung, Löslichkeit und Löslichkeitsgleichgewicht
Eine Lösung ist ein homogenes Gemisch, das aus einem Lösungsmittel, der Hauptkomponente, und einem gelösten Stoff, dem ...

Education

Lehre

JoVE Core

Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Covalent Bonding and Structure

Kovalente Bindung und Struktur

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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.

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Arizona Science Standards

High School

Physical Science

HS+C.P1

HS+C.P1U1.4

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9C.3.2.1.2

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Why Some Substances Dissolve in Water

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