Name: how water dissolves ionic compounds
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 42 s
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Chemistry

CH.5

CH.5.b

Aqueous Solutions and Heats of Hydration

Ionic solutes are held together by attractive interactions called Coulombic forces.
When ionic solutes are dissolved in water, the hydrogen bonds between the water molecules break and disrupt the Coulombic forces between the ions.
Breaking up an ionic crystal lattice into its constituent ions in this manner requires a large energy input and, thus, the enthalpy of the solute is an endothermic process.
The energy released when one mole of ionic solid is formed from the constituent gaseous ions is called the lattice energy and is always exothermic. Thus, the enthalpy to break one mole of the solute into its components is equal and opposite to the lattice energy.
However, when an ionic lattice is broken in an aqueous solution, each ion is surrounded and stabilized through ion-dipole interactions with the oppositely charged end of the water dipole.
This phenomenon is called hydration. The enthalpy change associated with the dissolution of one mole of ions in water is known as the heat of hydration. It is a combination of the enthalpy of the solvent and the enthalpy of mixing.
Since the ion-dipole interactions between a hydrated ion and the water molecules are much stronger than the hydr

aqueous solutions and heats of hydration

Water and other polar molecules are attracted to ions. The electrostatic attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. These attractions play an important role in the dissolution of ionic compounds in water.
When ionic compounds dissolve in water, the ions in the solid separate and disperse uniformly throughout the solution because water molecules surround and solvate the ions, reducing the strong electrostatic forces between them. This process represents a physical change known as dissociation. Under most conditions, ionic compounds will dissociate nearly completely when dissolved, and so they are classified as strong electrolytes. Even sparingly, soluble ionic compounds are strong electrolytes, since the small amount that does dissolve will dissociate completely.
Consider what happens at the microscopic level when solid KCl is added to water. Ion-dipole forces attract the positive (hydrogen) end of the polar water molecules to the negative chloride ions at the surface of the solid, and they attract the negative (oxygen) ends to the positive potassium ions. The water molecules surround individual K+ and Cl&#8722; ions, reducing the strong interionic forces that bind the ions together and letting them move off into solution as solvated ions. Overcoming the electrostatic attraction permits the independent motion of each hydrated ion in a dilute solution as the ions transition from fixed positions in the undissolved compound to widely dispersed, solvated ions in solution.
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/11-2-electrolytes">Openstax, Chemistry 2e, Section 11.2: Electrolytes</a>.

Water and other polar molecules are attracted to ions. The electrostatic attraction between an ion and a molecule with a dipole is called an ion-dipole ...

Education

JoVE Core

Solutions and Colloids

KEY TERMS AND CONCEPTS

how water dissolves ionic compounds

Water dissolves ionic compounds because its polar molecules attract ions. This attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. It helps explain why many ionic solids can separate in water and form aqueous solutions.

When an ionic compound dissolves, its ions separate and spread out evenly through the solution. Water molecules surround the ions and solvate them, which lowers the strong electrostatic forces between the ions. This separation is a physical change called dissociation.

Most ionic compounds dissociate nearly completely in water, so they are strong electrolytes. Even ionic compounds that are only sparingly soluble are still strong electrolytes, because the small amount that does dissolve dissociates completely. At the microscopic level, the dissolved ions can move independently once they are hydrated.

KCl shows this process clearly. The positive hydrogen end of water is attracted to chloride ions at the surface of the solid, and the negative oxygen end is attracted to potassium ions. The water molecules then surround the K+ and Cl− ions, weaken the forces holding the crystal together, and let the ions move into solution as solvated ions.

How Water Dissolves Ionic Compounds

solution formation

Why Some Substances Dissolve Better

Some substances dissolve easily in one solvent but not in another. A quick way to predict solubility is the idea that "like dissolves like." Polar substances, such as salt and sugar, dissolve well in polar solvents like water. Non-polar substances are more soluble in non-polar solvents such as carbon tetrachloride. This selective solubility depends on intermolecular forces, which are the attractions within and between molecules. Stronger forces between solute and solvent molecules lead to...

intermolecular forces in solutions

How Solutions Form Without Energy Change

A solution can form as a spontaneous process. That means it happens under the right conditions without energy from an outside source. When the attraction between solute and solvent particles is about the same as the attraction in the separate substances, the solution forms with no energy change. A solution like this is called an ideal solution. Mixtures of ideal gases are one example, and helium and argon behave close to that model. These gases do not have significant intermolecular...

enthalpy of solution

Dissolution and Heat Changes

Dissolution and heat changes help explain why some solutions form on their own. Two factors can favor the spontaneous formation of a solution. One is a decrease in internal energy, which happens in an exothermic change. The other is increased dispersal of matter, which means the solute spreads evenly through the solvent. When a substance dissolves, the internal energy often changes as heat is absorbed or released. This does not happen in every case, but it is common. A solution forms when...

Water dissolves ionic compounds because its polar molecules attract ions. This attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. It helps explain why many ionic solids can separate in water and form aqueous solutions. When an ionic compound dissolves, its ions separate and spread out evenly through the solution. Water molecules surround the ions and solvate them, which lowers the strong electrostatic forces between the ions. This separation is a...

solution equilibrium and saturation

Saturation Limits in Solutions

Saturation limits in solutions describe how much solute can dissolve in a solvent under given conditions. As sugar is added to water and stirred, it dissolves at first. More sugar can be added until the solution reaches its natural limit. That limit depends mainly on the relative strengths of three attractions: solute-solute, solute-solvent, and solvent-solvent forces. When no more sugar dissolves, even after long stirring, undissolved sugar stays behind. The concentration of sugar in the...

physical properties affecting solubility

Gas Solubility in Liquids

Gas solubility in liquids depends on the attractive forces between the gas and the liquid. When a gas dissolves, there is no strong solute-solute attraction to break apart first. Gas particles are already far apart, so their internal interactions are negligible. That makes solute-solvent interactions the main energetic factor that controls how much gas can dissolve. Examples show how these interactions matter. Oxygen is about three times more soluble in water than helium because oxygen has...

expressing solution concentration

Measuring Solution Strength

Solution strength can be described with terms like dilute and concentrated, and it can also be measured with several scientific units. A solute is the part of a solution present at a lower amount than the solvent. Different units are useful in different situations, especially in chemistry. Molarity, written as M, is a common unit for concentration. It tells how many moles of solute are in one liter of solution. Because solution volume changes with temperature, molarity can change too when the...

vapor pressure lowering

Raoult's Law and Lower Vapor Pressure

Raoult’s law explains why a solution with a nonvolatile solute has a lower vapor pressure than the pure liquid. Vapor pressure is the pressure from a liquid’s gas phase when vaporization and condensation happen at equal rates. When a nonvolatile substance dissolves in a volatile liquid, the liquid’s equilibrium vapor pressure drops. One reason for this drop is at the surface of the solution. Solvent molecules must be at the surface to vaporize, and added solute molecules reduce the surface...

ideal solutions

Raoult's Law in Real Solutions

Raoult's law describes how vapor pressure changes in a solution. For a solvent, the partial vapor pressure equals the vapor pressure of the pure solvent multiplied by its mole fraction in the solution. This relationship works best for ideal solutions. An ideal solution is one in which solvent-solute attractions are just as strong as solvent-solvent and solute-solute attractions. In that case, both components need about the same amount of energy to escape into the vapor phase as they do in...

freezing point depression and boiling point elevation

freezing-point-depression-and-boiling-point-elevation

Colligative Effects in Solutions

Colligative effects in solutions include boiling point elevation and freezing point depression. These properties depend on how many solute particles are present, not on the solute’s chemical identity. Because of that, a solution can behave differently from the pure solvent even when only a nonvolatile solute is added. Boiling point elevation happens because the vapor pressure of a solution is lower than the vapor pressure of the pure solvent. The boiling point is the temperature at which vapor...

osmosis and osmotic pressure of solutions

osmosis-and-osmotic-pressure-of-solutions

Semipermeable Membranes and Reverse Osmosis

Semipermeable membranes let some molecules pass while blocking others. They are found in biological cell membranes and in dialysis tubing used to remove metabolic wastes from blood. These examples show how selective permeation depends on factors such as size, shape, polarity, and charge. Osmosis is the net movement of solvent through a semipermeable membrane. In a U-shaped setup, pure solvent and a solution are separated by a membrane that allows only solvent molecules to pass. Solvent...

electrolytes: van't hoff factor

Electrolyte Ions and the van't Hoff Factor

Electrolyte ions affect colligative properties because these properties depend on the number of dissolved particles, not their identity. The concentration terms used for freezing point depression, boiling point elevation, and osmotic pressure include every solute species present in the solution. Nonelectrolytes dissolve without dissociation, so each molecule that enters solution counts as one particle. Electrolytes behave differently because they can form ions in water. If sodium chloride were...

colloids

Colloids and Everyday Mixtures

Colloids are mixtures with particles between the size of a solution and a suspension. They stay dispersed instead of settling out, even though their particles are larger than most simple molecules. This makes colloids different from cloudy suspensions like mud and water, flour and water, or tempera paint. It also makes them different from solutions, where dissolved molecules or ions are invisible and do not settle. Colloidal systems can be made in two main ways. In dispersion methods, larger...

key terms and concepts

See how polar water molecules attract ions, break apart ionic solids, and form solvated ions in aqueous solution through dissociation and ion-dipole forces.

How Water Dissolves Ionic Compounds | JoVE K12

chemistry

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Texas Essential Knowledge and Skills

High School

CHEM.11

CHEM.11.A

CHEM.11.B

Alabama Course of Study Science

9-12

Matter and Its Interactions

9-12.C.1.5.b

Physical Science

9-12.PS.3.6.a

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New York State Science Learning Standards

HS. Structure and Properties of Matter

HS-PS1-10.

Georgia Standards of Excellence

SC6.

Solvation vs. Dissociation in Dissolving

Solutions and Concentrations

Source: Laboratory of Dr. Michael Evans &#8212; Georgia Institute of Technology
A solution is a homogeneous mixture containing some components in small ...

determining-the-density-of-a-solid-and-liquid

Determining the Density of a Solid and Liquid

Source: Laboratory of Dr. Michael Evans &#8212; Georgia Institute of Technology
The ratio of the mass of a substance to its volume is known as the mass ...

determining-the-mass-percent-composition-in-an-aqueous-solution

Determining the Mass Percent Composition in an Aqueous Solution

Source: Laboratory of Dr. Neal Abrams &#8212; SUNY College of Environmental Science and Forestry
Determining the composition of a solution is an important ...

Determining the Empirical Formula

Source: Laboratory of Dr. Neal Abrams - SUNY College of Environmental Science and Forestry
Determining the chemical formula of a compound is at the heart ...

determining-the-solubility-rules-of-ionic-compounds

Determining the Solubility Rules of Ionic Compounds

Source: Laboratory of Dr. Neal Abrams &#8212; SUNY College of Environmental Science and Forestry
An ionic compound&#39;s solubility can be determined via ...

General Properties of Solutions

Many common substances around us exist as a solution, such as ocean water, air, and gasoline. All solutions are mixtures of substances that are composed ...

Solution Concentration and Dilution

The relative amount of a given solution component is known as its concentration. Often, though not always, a solution contains one component with a ...

Expressing Solution Concentration

A solute is a component of a solution that is typically present at a much lower concentration than the solvent. Solute concentrations are often described ...

The student will investigate and understand that solutions behave in predictable and quantifiable ways. Key ideas include molar relationships determine solution concentration;

CH.5.a

Solution Formation

There is no one solvent that can dissolve every type of solute. Some substances that readily dissolve in a certain solvent might be insoluble in a ...

Intermolecular Forces in Solutions

The formation of a solution is an example of a spontaneous process, a process that occurs under specified conditions without energy from some external ...

Enthalpy of Solution

There are two criteria that favor, but do not guarantee, the spontaneous formation of a solution:

	A decrease in the internal energy of the system (an ...

Solution Equilibrium and Saturation

Imagine adding a small amount of sugar to a glass of water, stirring until all the sugar has dissolved, and then adding a bit more. You can repeat this ...

Physical Properties Affecting Solubility

Solutions of Gases in Liquids
As for any solution, the solubility of a gas in a liquid is affected by the attractive intermolecular forces between solute ...

Vapor Pressure Lowering

The equilibrium vapor pressure of a liquid is the pressure exerted by its gaseous phase when vaporization and condensation are occurring at equal rates:
...

Ideal Solutions

According to Raoult&#8217;s law, the partial vapor pressure of a solvent in a solution is equal or identical to the vapor pressure of the pure solvent ...

Freezing Point Depression and Boiling Point Elevation

Boiling Point Elevation
The boiling point of a liquid is the temperature at which its vapor pressure is equal to ambient atmospheric pressure. Since the ...

The student will investigate and understand that solutions behave in predictable and quantifiable ways. Key ideas include changes in temperature can affect solubility;

Electrolytes: van't Hoff Factor

Colligative Properties of Electrolytes
The colligative properties of a solution depend only on the number, not on the identity, of solute species ...

Electrolysis

In a galvanic cell, the electrical work is done by a redox system on its surroundings as electrons produced by the spontaneous redox reactions are ...

Electrolyte and Nonelectrolyte Solutions

Substances that undergo either a physical or a chemical change in solution to yield ions that can conduct electricity are called electrolytes. If a ...

Solubility of Ionic Compounds

Solubility is the measure of the maximum amount of solute that can be dissolved in a given quantity of solvent at a given temperature and pressure. ...

The student will investigate and understand that solutions behave in predictable and quantifiable ways. Key ideas include extent of dissociation defines types of electrolytes;

CH.5.c

pH Scale

Hydronium and hydroxide ions are present both in pure water and in all aqueous solutions, and their concentrations are inversely proportional as ...

relative-strengths-of-conjugate-acid-base-pairs

Relative Strengths of Conjugate Acid-Base Pairs

Br&#248;nsted-Lowry acid-base chemistry is the transfer of protons; thus, logic suggests a relation between the relative strengths of conjugate acid-base ...

Common Ion Effect

Compared with pure water, the solubility of an ionic compound is less in aqueous solutions containing a common ion (one also produced by dissolution of ...

Buffers

A solution containing appreciable amounts of a weak conjugate acid-base pair is called a buffer solution, or a buffer. Buffer solutions resist a change in ...

Henderson-Hasselbalch Equation

The ionization-constant expression for a solution of a weak acid can be written as:

Rearranging to solve for [H3O+] yields:

Taking the negative ...

calculating-ph-changes-in-a-buffer-solution

Calculating pH Changes in a Buffer Solution

A buffer can prevent a sudden drop or increase in the pH of a solution after the addition of a strong acid or base up to its buffering capacity; however, ...

Using a pH Meter

Source: Laboratory of Dr. Zhongqi He - United States Department of Agriculture
Acids and bases are substances capable of donating protons (H+) and ...

The student will investigate and understand that solutions behave in predictable and quantifiable ways. Key ideas include pH and pOH quantify acid and base dissociation;

CH.5.d

freezing-point-depression-to-determine-an-unknown-compound

Freezing-Point Depression to Determine an Unknown Compound

Source: Laboratory of Lynne O&#39; Connell &#8212; Boston College
When a solid compound is dissolved in a solvent, the freezing point of the resulting ...

Osmosis and Osmotic Pressure of Solutions

A number of natural and synthetic materials exhibit selective permeation, meaning that only molecules or ions of a certain size, shape, polarity, charge, ...

The student will investigate and understand that solutions behave in predictable and quantifiable ways. Key ideas include colligative properties depend on the extent of dissociation.

CH.5.e

How Water Dissolves Ionic Compounds

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