Melting points (in blue) and boiling points (in pink) of the first fourteen alkanes (temperatures in C, number of carbon atoms along the horizontal axis)

Melting points (in blue) and boiling points (in pink) of the first fourteen alkanes (temperatures in degrees Celsius, number of carbon atoms along the horizontal axis).

In addition to reactivity, the physical properties of an organic substance, such as melting point, boiling point and solubility, are amont its most important traits. The physical properties of an organic substance can often be predicted from its structure. In most cases, a substance's molecular weight and the functional groups are sufficient information to allow for a good estimate of the melting point, boiling point, and solubility to be estimated. Comparing molecules of similar size, the greater the strength of intermolecular force, the more equilibrium will favor the condensed phase at a given pressure and temperature.

Understanding how functional groups affect the physical properties of organic compounds is of primary importance to laboratory organic chemistry, and, furthermore, one of the bridges from organic chemistry to biochemistry and cell biology. The student must obtain this form of understanding to be able to grasp protein folding, for example, or the structure of biological membranes. Every MCAT makes this area a priority. You can expect both direct and indirect questions. Straightforward questions asking for a comparison of solubilities are common, for example, as are questions involving similar reasoning in a more advanced biological context.

WikiPremed Resources

Physical Properties of Organic Compounds Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Organic Physical Properties
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals

Proficiency 

Understand the relative strength of intermolecular force deriving from various functional groups.

Be prepared to rank organic compounds in order of melting point or boiling point.

Account for the effect of degree of saturation on boiling or melting point of compounds with long hydrocarbon chains.

From its structural formula, be able to predict the solubility of an organic compound in water.

Suggested Assignments

This is not a large formal topic. Perform the crossword puzzle to get oriented a bit. Here is the solution to the puzzle.

Read the discussion on physical properties at the Virtual Textbook of Organic Chemistry and the discussion of solubility at Indiana University Southeast.

Conceptual Vocabulary for Organic Physical Properties

Hydrophobicity
Hydrophobicity refers to the physical property of a molecule that is repelled from a mass of water
Hydrocarbon
A hydrocarbon is an organic compound consisting entirely of hydrogen and carbon.
Hydrophilicity
Hydrophilicity refers to a physical property of a molecule that can transiently bond with water through hydrogen bonding.
Miscibility
Miscibility is a term in chemistry that refers to the property of liquids to mix, forming a homogeneous solution.
Lipophilicity
Lipophilicity refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene.
Aliphatic compound
Aliphatic compounds are organic compounds in which carbon atoms are joined together in straight or branched chains or in rings, that can be either saturated or unsaturated, but not aromatic.
Paraffin
Paraffin is a common name for the group of alkane hydrocarbons.
Volatility
Volatility is a measure of the speed at which a substance turns into a vapor from a solid or liquid state.
Volatile organic compound
Volatile organic compounds (VOCs) are organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere.
Amphiphile
Amphiphile is a term describing a chemical compound possessing both hydrophilic and hydrophobic properties.
Partition coefficient
A partition or distribution coefficient is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium.
Protic solvent
A protic solvent is a solvent that carries a hydrogen bond between an oxygen as in a hydroxyl group or a nitrogen as in an amine group.
Liquid-liquid extraction
Liquid-liquid or solvent extraction, also known as partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent.
Hygroscopy
Hygroscopy is the ability of a substance to attract water molecules from the surrounding environment through either absorption or adsorption.
Multiphasic liquid
A multiphasic liquid is a mixture consisting of more than two immiscible liquid phases.
Relative volatility
Relative volatility is a measure of the vapor pressure differences of the components in a liquid mixture of chemicals.
Advanced terms that may appear in context in MCAT passages
Bolaamphiphile
Bolaamphiphiles are amphiphilic molecules that have hydrophilic groups at both ends of a sufficiently long hydrophobic hydrocarbon chain.
Lipophobicity
Lipophobicity is a term which literally means fat rejection and describes compounds which are not soluble in lipids or other non-polar solvents.
Flory-Huggins solution theory
Flory-Huggins solution theory is a mathematical model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecular sizes in adapting the usual expression for the entropy of mixing.
Lyotropic
A material is called lyotropic if it forms liquid crystal phases because of the addition of a solvent.