SolubilityPolar Molecules and Solubility

Learning Goals Students will understand that physical

properties such as the polarity of molecules are related to a compound’s solubility.

Students will be able to predict the solubility of compounds in water

Students will understand how intramolecular and intermolecular forces affect

Success Criteria

Compounds with covalent bonds usually form non-polar molecules since neither atom gains a positive or negative charge.

There is a small electronegativity difference between the atoms.

In some molecules, there are polar bonds, but the atom is symmetrical leading to an overall non-polar molecule.

Review: Polar Molecules

Water (H2O) is the classic polar molecule. Oxygen has a higher electronegativity (3.5) than hydrogen (2.1). The electronegativity difference is 1.4 and a polar covalent bond forms. This means that the oxygen atom gains a negative charge and the hydrogen gains a positive charge. Water is a polar molecule.

A similar effect is seen in Hydrofluoric Acid, but carbon tetrachloride is symmetrical and is therefore non-polar!

Water (H2O)

HCl

NH3

BF3

CCl4

CH3Cl

Which Molecules are Polar?

Solutions are homogenous mixtures of substances composed of at least one solute and one solvent

Homogenous mixture is a uniform mixture of only one phase

Solute is a substance that is dissolved in a solvent (e.g. NaCl)

Solvent is the medium in which a solute is dissolved; often the liquid component of a solution (e.g. water)

An aqueous solution is a solute dissolved in water (e.g. NH3(aq))

Solutions - terminology

Electrolyte is a compound that, in aqueous solution, conducts electricity

Nonelectrolyte is a compound that, in aqueous solution, does not conduct electricity

Acid is a substance that, in aqueous solution, turns blue litmus paper red

Base is a substance that, in aqueous solution, turns red litmus paper blue

Neutral is a substance that, in aqueous solution, has no effect on either red of blue litmus paper; neither acidic or basic

Solutions - terminology

Polar molecules only dissolve into polar liquids. For example, salt dissolves into water.

Salt (NaCl) is ionic and water is a polar molecule (polar covalent bonds)

Solubility

Non-polar molecules only dissolve into non-polar liquids

Grease dissolves in gasoline Oil paint dissolves in paint thinner

Solubility

A solution is a homogeneous mixture of two substances. It is composed of a solvent (the substance that does the dissolving) and a solute (the substance that gets dissolved).

For example Kool Aid is a solution with the drink crystals being the solute and water being the solvent.

Solubility

Water dissolves Sodium Chloride, the polar water molecules surround the NaCl crystals and rip it apart! This causes the Ionic Crystal (NaCl) to Dissociate or Separate in Na+ ion and a Cl- ion.

In cases where water is the solvent, the solution is considered to be an aqueous solution.

Polar solutions conduct electricity. We call these solutions electrolytes.

Solubility

Solute in solvent

Example of solution

gas in gasgas in liquidgas in solidliquid in gasliquid in liquidliquid in solidsolid in liquidsolid in solid

Types of Solutions

Students will review concepts such as electronegativity, properties of matter and bonding.

Students will understand that intramolecular forces such as covalent bonding, polar covalent bonding and ionic bonding affect the solubility of a substance

Student will learn the terminology related solutions.

Intermolecular Forces

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Predicting High and Low Solubilities (pp. 275-277)

Highest Solubility - Acetic Acid and Methanol Molecules are polar Formation of hydrogen bonds with water molecules

High Solubility - Dimethyl ether Molecule is polar Presence of oxygen atom means some hydrogen bonding

with O-H ends of water molecules

Slightly Soluble - Carbon dioxide and Oxygen Should be non-polar, but oxygen presence means that

there is a possibility of some hydrogen bonding with water molecules.

Insoluble - Propane and Tetrachloromethane Nonpolar molecules Little or no solubility in water, cannot participate in

dipole-dipole forces and hydrogen bonding

When a solute and a solvent do not mix, the solute is said to be insoluble. The solute and solvent are immiscible.

When the solvent and solute do mix they are soluble and miscible.

Miscibility

Why is it important to know about polar and non-polar molecules?

Polar molecules dissolve into other polar molecules

Non-polar molecules dissolve into other non-polar molecules but often not as easily!

BUT polar molecules do not dissolve non-polar molecules – they form mixtures.

LIKE DISSOLVES LIKE!

Miscibility

Oil and water do not mix. They are immiscible. (Oil is insoluble in water). Oil is non-polar and water is polar.

Salt dissolves in water. They are miscible. (Salt is soluble in water).

Both salt (NaCl) and water are polar molecules.

Miscibility

Colloids are mixtures that look a lot like solutions. They are very fine particles that are non-polar and mix into other non-polar compounds and sometimes water.

Colloids

Milk

JelloMayonnaise

Whipped Cream

Solid stick deodourant

Properties of Mixtures:Solutions, Colloids and Suspensions

Properties

Miscible or Immiscible?