Chapter 7: Unit 4. Solubility

Solubility

Solubility is a term used to express the dissolution of solute in the solvent. Both of the solute and the solvent have different phases, i.e. solute the solid and the solvent is liquid, or the solute is solid and the solvent is a gas etc.

Miscibility on the other hand is used to express the dissolution of the solute and solvent with both solute and solvent have the same phases, i.e. solute is solid and solvent is solid or solute is liquid and the solvent is liquid or a solute is a gas and the solvent is a gas.

The solubility of inorganic ionic compounds in water is affected by the following factors:

1. Temperature:

The higher the temperature the higher the rate of the dissolution of the solute in the solvent

2. Surface area:

The higher the surface area of the solute, the higher the rate of the dissolution of the solute in the solvent. An example of this can be seen in the solubility of fine powder sodium chloride in water compared with the solubility of the coarse sodium chloride (rocky sodium chloride) in water. The fine powder sodium chloride has larger surface area and hence higher rate of dissolution in water.

3. Solute concentration:

The higher the concentration of the solute the higher the rate of dissolution in the solvent.

4. Agitation, shaking and Stirring:

Agitation, shaking and stirring are increasing the rate of the dissolution of the solute in the solvent.

A You Tube video illustrates some of the factors affecting solubility:

Temperature Effect:

The solubility of solids and in general liquids solutes fin a solvent is increased with increasing the temperature and a polar or none polar solution is obtained. This trend cannot be seen in case of gases solutes.

The solubility of gases in water decreases as the temperature increases. At higher temperatures the gases molecules acquire enough energy to escape the solution.

An example of such escape is the fuzzing CO₂ gas out a soda can when it is open to atmospheric the temperature and when the soda is warmed up to room temperature after it was refrigerated. This effect is illustrated below:

Reference: https://www.ausetute.com.au/henryslaw.html

The partial pressures of gases on the other hand have more effect on their solubility in water.

The solubility of gases in water increases as their partial pressures over the solution increases. At higher pressures more molecules of gases are available and can enter the solution and can be dissolved. This is known as Henry’s law.

Henry’s Law:

Henry’s Law has the formula:

P = K_H x C

Where:

P = partial pressure of a gas above the liquid in Pascal (P) or in kilopascal (kPa).

K_H = Henry’s constant in [kPa / (mol/L)]

C = concentration of the solution in (mol/L)

The figure below illustrates the Henry’s law of gases solubility in liquids:

Reference: https://www.ausetute.com.au/henryslaw.html

A You Tube video illustrates the Henry’s law in some details:https://www.youtube.com/embed/9JtTpPEesOk?feature=oembed

Inorganic Compounds Solids Formation

Some solutes are not soluble in some solvents. The attractive forces within the solute ions in the crystal network are greater than that with the solvent and hence the solvent will not be able to penetrate through the crystal network of the solute and the solute remains in the solution undissolved.

The solubility table is one of the methods used to predict the possible solubility of ionic compounds in water. [Reference: http://mrsspencerslab.weebly.com/semester-2/double-replacement-reaction]

Examples:

Ionic compounds (solutes)	Solubility in water (solvent)
KNO3	soluble
NH4Cl	soluble
AgBr	insoluble
PbSO4	insoluble
CaCO3	insoluble
LiCO3	soluble
Al(OH)3	soluble
FeS	insoluble
Ba3(PO4)2	insoluble
NaClO3	soluble

Types of Writing Ionic Chemical Equations

There are three types of Writing Ionic Chemical Equations:

1. Molecular Chemical Equation:

In this type of writing chemical equation, all the ionic compounds are written intact and not taken apart in ions.

Example:

2 Na₃PO₄(aq)  +  3 Ca(NO₃)₂(aq) → Ca₃(PO₄)₂(s)   +  6 NaNO₃(aq)

In this example Na⁺ and Ca²⁺ are exchanging their anions.

2. Complete Ionic Chemical Equation:

In this type of writing chemical equation, all the ionic compounds with aqueous symbol are written as ions. All solids, liquids and gases are not taken apart.

2 Na₃PO₄(aq)  +  3 Ca(NO₃)₂(aq)   → Ca₃(PO₄)₂(s)   +  6 NaNO₃(aq)

All ions will be taken apart, except solids, liquids or gases:

6 Na⁺(aq)   +  2 PO₄^3-(aq)  +  3 Ca²⁺(aq)  + 6 NO₃^–(aq)    → Ca₃(PO₄)₂(s)   +  6 Na⁺(aq)   +  6 NO₃^–(aq)

3. Net Ionic Chemical Equation:

In this type of writing ionic chemical equation, all spectator ions will be discarded.

Spectator ions are ions appear on both sides of the chemical ionic equation and they are said to be chemically inactive and they do not participate in the actual chemical reaction.

6 Na⁺(aq)   +  6 NO₃^–(aq) are spectator ions and can be discarded.

6 Na⁺(aq)   +  2 PO₄^3-(aq)  +  3 Ca²⁺(aq)  + 6 NO₃^–(aq)    → Ca₃(PO₄)₂(s)   +  6 Na⁺(aq)   +  6 NO₃^–(aq)

The net ionic chemical equation is written as follows:

2 PO₄^3-(aq)  +  3 Ca²⁺(aq)  → Ca₃(PO₄)₂(s)

Net ionic chemical equations are considered to be very important because they are expressing the actual chemical species that participate in the chemical reactions.

Example:

Mg(OH)₂(aq)   +  2HCl(aq)  → MgCl₂(aq)  +  2H₂O(l)

1. Molecular Chemical Equation:

Mg(OH)₂(aq)   +  2HCl(aq)  → MgCl₂(aq)  +  2H₂O(l)

2. Complete Ionic Chemical Equation:

Mg²⁺(aq) + 2OH^–(aq)   +  2H⁺(aq) +  2Cl^–(aq)  → Mg²⁺(aq)  +  2Cl^–(aq)  +  2H₂O(l)

3. Net Ionic Chemical Equation:

2OH^–(aq)   +   2H⁺(aq)   → 2H₂O(l)

A You Tube video illustrates the writing of the net chemical equations:https://www.youtube.com/embed/EQIqcT9a7DY?feature=oembed

Molecular Workbench illustrates a simulation of dissolving salt in water. The simulation requires Java software to run:

http://mw.concord.org/modeler/showcase/simulation.html?s=http://mw2.concord.org/public/student/solution/dissolve.html

This simulation demonstrates that dissolution occurs because the electrostatic interactions between water and salt ions compete with the electrostatic interactions among the salt ions themselves and therefore cause the structure of ionic crystal to fall apart.

Try this simulation and make your notes regarding the dissolution of the salt in water and the effect of the electrostatic interactions and answer the questions below. To have better knowledge about the dissolution of the salt in water, your will need to use the buttons showing the electrostatic attractions as well as charges.

Questions:

Why is water able to penetrate the salt crystal network?

Describe the electrostatic attraction forces orientation in the solution i.e. positive and negative charges of the solvent water versus the positive and negative charges of solute sodium chloride.