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

Uma solução é uma mistura homogênea de duas substâncias principais: o soluto e o solvente. O solvente está presente em maior quantidade do que o soluto.
Em uma solução diluída, a proporção de soluto em relação ao solvente é pequena, enquanto para uma solução concentrada, a proporção é grande.
Ao formar uma solução, o soluto e o solvente não reagem quimicamente. Em vez disso, as moléculas se redistribuem, com moléculas de solvente individuais ao redor das moléculas de soluto e interagindo por meio de forças atrativas. Esse processo de dissolução é chamado de solvatação.
Para favorecer a solvatação, as interações soluto-solvente devem ser mais fortes do que as interações solvente-solvente e soluto-soluto.
Em uma solução, o soluto e o solvente podem ser qualquer estado da matéria. A solução em si é uma única "fase".
O oposto da solvatação é a precipitação, desencadeada por fortes interações soluto-soluto. Quando a taxa de solvatação é igual à taxa de precipitação, um equilíbrio de solubilidade é estabelecido.
A solubilidade de um soluto é sua concentração máxima possível no equilíbrio de solubilidade em uma determinada quantidade de solvente. A solubilidade é afetada pela temperatura e outras condições físicas, como polaridade molecular.
Os solutos apolares se dissolvem em solventes apolares por meio de forças de dispersão intermoleculares de London, enquanto os solutos polares ou iônicos se dissolvem em solventes polares por meio de interações íon-dipolo ou dipolo-dipolo.
A água é um solvente polar. Os compostos hidrofílicos, ou amantes da água, são solúveis em água devido às cargas iônicas ou grupos polares que permitem interações água-soluto suficientemente fortes. Eles são menos solúveis em solventes apolares porque as interações soluto-soluto são muito mais fortes do que qualquer interação possível entre solvente apolar e moléculas de soluto polar.
Compostos apolares insolúveis em água e solúveis em gordura, como óleos, são hidrofóbicos ou temem a água, pois não podem formar ligações de hidrogênio para competir com as interações solvente-solvente.
Os compostos anfifílicos ou anfipáticos têm grupos polares e apolares. Por exemplo, sabonetes são sais metálicos de ácidos graxos. Na água, as moléculas de sabão se agregam em esferas chamadas micelas. Suas caudas de ácidos graxos hidrofóbicos não polares apontam para dentro, enquanto suas cabeças hidrofílicas polares estão do lado de fora.
Ao remover manchas de óleo usando sabão, as caudas envolvem o óleo apolar para formar micelas solúveis em água, permitindo que o óleo seja lavado.

solubilidade

Solução, solubilidade e equilíbrio de solubilidade

Uma solução é uma mistura homogênea composta de um solvente, o componente principal, e um soluto, o componente secundário. O estado físico de uma solução, sólido, líquido ou gasoso, é normalmente o mesmo do solvente. As concentrações de soluto são frequentemente descritas com termos qualitativos como diluída (de concentração relativamente baixa) e concentrada (de concentração relativamente alta).

Em uma solução, as partículas de soluto (moléculas, átomos e/ou íons) estão intimamente cercadas pelas espécies de solvente e interagem através de forças de atração. Esse processo de dissolução é chamado de solvatação. Quando a água é o solvente, o processo é conhecido como hidratação. Para a solvatação, as interações soluto-solvente devem ser mais fortes do que as interações soluto-soluto e solvente-solvente. A precipitação é o oposto da solvatação e ocorre devido a fortes interações soluto-soluto.

Solubilidade é a medida da quantidade máxima de soluto que pode ser dissolvida em determinada quantidade de solvente. Temperatura, pressão e polaridade molecular são alguns dos fatores importantes que afetam a solubilidade. O equilíbrio de solubilidade é estabelecido quando a dissolução e a precipitação de uma espécie de soluto ocorrem em taxas iguais.

Semelhante dissolve semelhante

Para prever se um soluto será solúvel em determinado solvente, a regra prática é “semelhante dissolve semelhante”. Solutos polares ou iônicos dissolvem-se em solventes polares devido às interações íon-dipolo ou dipolo-dipolo resultantes com as moléculas do solvente. Tal interação não será possível com um solvente apolar. Solutos apolares dissolvem-se em solventes apolares através de forças de dispersão intermoleculares.

Compostos hidrofílicos, hidrofóbicos e anfifílicos

A água é um solvente polar. Os solutos que são solúveis em água são chamados de “hidrófilos” ou “que gostam de água”. Por exemplo, quando KCl sólido é adicionado à água, a extremidade positiva (hidrogênio) das moléculas polares de água é atraída pelos íons cloreto negativos, e as extremidades negativas (oxigênio) da água são atraídas pelos íons positivos de potássio. As moléculas de água cercam individualmente os íons K+ e Cl−, reduzindo as fortes forças que mantêm os íons juntos e permitindo que eles se movam para a solução como íons solvatados.

Um soluto que é insolúvel em água é denominado “hidrofóbico” ou “que repele a água”. Tais solutos, como o óleo, são incapazes de formar ligações de hidrogênio com as moléculas de água circundantes devido às fortes interações soluto-soluto. Como resultado, as partículas de soluto se agrupam e permanecem não dissolvidas.

Compostos que possuem grupos polares e apolares são chamados de 'anfipáticos' ou 'anfifílicos', por exemplo, sabões, que são sais de ácidos graxos. Eles têm uma cauda hidrofóbica de hidrocarbonetos apolares e uma cabeça hidrofílica polar. A ação de limpeza dos sabões e detergentes pode ser explicada em termos das estruturas das moléculas envolvidas. A extremidade de hidrocarboneto (apolar) de uma molécula de sabão ou detergente se dissolve ou é atraída por substâncias apolares, como óleo, graxa ou partículas de sujeira. A extremidade iônica é atraída pela água (polar). Como resultado, as moléculas de sabão ou detergente ficam orientadas na interface entre as partículas de sujeira e a água, atuando como uma espécie de ponte entre dois tipos diferentes de matéria, apolar e polar. Como consequência, as partículas de sujeira se tornam suspensas como partículas coloidais e são facilmente removidas pela lavagem.

Este texto foi adaptado do<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>.

Solubilidade

Solução, Solubilidade e Equilíbrio de Solubilidade
Uma solução é uma mistura homogênea composta de um solvente, o componente principal, e um soluto, o ...

Education

JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Covalent Bonding and Structure

Ligação Covalente e Estrutura

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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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Carbon Compounds in Everyday Life

Organic chemistry focuses on carbon compounds called organic compounds. These compounds may come from living things or be made by chemists. Carbon is the main element in them, and it bonds with other carbon atoms as well as hydrogen, oxygen, nitrogen, and sulfur. Carbon can form up to four strong bonds. That ability lets carbon atoms build chains and rings of many sizes, shapes, and levels of complexity. Not every carbon-containing substance is classified as organic. Carbonates, cyanides, and...

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Atomic Orbitals and Quantum Numbers

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chemical bonds

Chemical Bonds: Types and Strength

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lewis structures and formal charges

Lewis Structures, Bonds, and Formal Charge

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valence bond theory and hybridized orbitals

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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

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
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Factors Influencing the Rate of Chemical Reactions

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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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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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Solution Equilibrium and Saturation

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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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