Factors affecting Solubility

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This is a series of lectures in videos covering Chemistry topics taught in High Schools.
What are the Factors that affect Solubility?

Three Factors Affecting Dissolving
The rate of dissolving is a very important property of solutions.
The rate of dissolving depends on several factors:

  • Temperature
  • Agitation
  • Particle size

For most solid solutes, the rate of dissolving is greater at higher temperatures.
At high temperatures, the solvent molecules have greater kinetic energy and collide with undissolved solid molecules more frequently.
Agitating a mixture by stirring or shaking the container increases the rate of dissolving.
Agitation brings fresh solvent into contact with undissolved solute.
Decreasing the size of the particles increases the rate of dissolving.
When you break up a large mass of solute into smaller pieces, you increase the surface area that is in contact with the solvent.

Solubility and Particle Attractions
The reasons why a solute may or may not dissolve in a solvent are related to the forces of attraction between the solute and the solvent particles.
When the forces of attraction between different particles in a mixture are stronger than the forces of attraction between like particles in a mixture, a solution forms.
The 3 step process of dissolving at the Molecular Level.
Step 1
The forces between the particles in the solid must be broken - this step always requires energy.
In an ionic solid, the forces that are holding the ions together must be broken.
In a molecular solid, the forces between the molecules must be broken.
Step 2:
Some of the intermolecular forces between the particles in a liquid must be broken - this step also requires energy.
Step 3:
There is an attraction between the particles of the solid and the particles of the liquid - this step always gives off energy.

Factors Affecting Dissolving Dipole Attractions
Solubility and Intermolecular Forces
Water molecules are polar.
Molecules such as water, have charges separated into positive and negative regions - they are considered to have a permanent dipole.
Dipole-dipole attractions are attractions between the opposite charges on two different polar molecules.
These attractions are intermolecular.
If the force is greater between two different dipoles on the same molecule, then the solute will dissolve in the solvent.
The positive end of the polar molecule attracts the anion (negatively charged particle); the negative end of the dipole attracts the cation (positively charged particle)
Hydration of ions - when water molecules completely surround an ion.
Electrolyte is a solution that conduct electricity.
Not all ionic compounds are soluble in water because the force of attraction within the ionic compound are too strong.

Factors Affecting Solubility Polar and Non Polar
Predicting Solubility
Use differences in electronegativities to determine ionic nature of the compounds to help predict solubility.
If the elements are polar or ionic then the compound will probably dissolve in water.
If the elements are non-polar then the compound will probably not dissolve in water.
Covalent compounds do not have dipole charges - not soluble in water.
Exceptions are methanol, ethanol & sugars.

  • able to form hydrogen bond with water.
  • allows them to behave like ionic compounds.
    When a molecular compound dissolve in water, the solid breaks up into molecules which are coordinated by the water.

Insoluble Covalent Compounds
Lack ions or polar bonds therefore unable to dissolve in polar solvents.
If a compound possesses both polar and non-polar components, it may dissolve in both types of solvents.

Three Factors Affecting Solubility

  1. Molecule Size
  2. Pressure
  3. Temperature

Temperature and Solubility of Solids
Increased temperature usually increases the solubility of solids in liquids.
According to the Second Law of Thermodynamics, increased temperature means a greater average velocity for the particles.
This allows them to move from one position to another more easily.
The greater freedom of movement allows the system to change its state more easily.
The change in solubility with change in temperature can be used to create solutions with more solute dissolved than is predicted by the solubility of the substance.

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