Pressure is a commonly used word. It is applied to psychological stress and mechanical conditions. Pressure has a very specific meaning in chemistry. It is defined as force divided by area. It may be worht while to recall that a force is a "push" or a "pull". Pounds per square inch is a pressure unit seen on inflated objects like basket balls, inflated boats and car tires.

This unit has the qualities of force (weight is a measure of force)in the numerator and area, inches x inches = inches², in the denominator.

When you stand on the florr you exert a force against the floor. The pressure you exert is your body weight in pounds divided by the surface area of your feet or shoes. It snow country you can see people wear snow shoes. They do this to avoid sinking in the snow. This trick spreads a person's body weight over a larger area. The constant body weight and larger area produces a lower pressure on the snow surface.

Pressure units are varied. Commonly used pressure units are listed here.

The normal reference for pressure is the earth's atmosphere. One atmosphere is related to these units.

Pressure measurements

A list of pressure units is good as a reference but how is pressure measured.

The barometer is the simplest instrument for measuring atmospheric pressure. The earth's atmosphere has a weight of 14.7 pounds over a square inch of surface. This is the weight of a column of air that extends from the earth's surface to the edge of the atmosphere. This weight changes as the temperature and composition of the air mass changes. A barometer uses a substitute column of mercury fluid in place of the air. A cheaper barometer can be mae using a column of water.

Mercury is the most dense fluid available. It has a density of 13.6 g/mL. This is 13.6 times the density for water. Original barometers where made using water. This was a nuisance because the length of the mercury column is 76 cm fluid

The manometer is the simplest tool for measuring gas pressure differences. A manometer is a u-tube. One side of the "U" is open to the aair and the other side is connected to a closed container. The "U" is filled with a fluid. If both sides of the "U" have the same liquid levels then the pressure inside and the pressure outside are he same. The difference between the liquid levels equals the pressure difference between inside and outside. The liquid level will be lower on the side with greater pressure.

Gas Laws

There are two types of gas equations. There is the class of equations describing a single state and the class of equations that compare two states.

The single state equations are

Single state

Gas properties change with conditions. A refenece set of conditions are identified as standard temperatur and pressure, STP. STP is 273.16o Kelvin and 1 atmosphere pressure.

All gases at STP have the same molar volume, 22.4 Liters.

Molecular attractions

Atractions between molecules are classified as

Dipole-dipole interactions

London forces also known as vanderWaals forces

Hydrogen bonding

The relative size of these interactions is important so the relative effects are understood.

Relative strength of forces

Covalant bonds > Hydrogen bonding > Dipole-dipole interactions > London forces van derWaals force

400 kcal > 12-16 kcal > 2-0.5 kcal > less than 1 kcal

Dipole-dipole interactions exist between molecules that are polar. This requires the presence of polar bonds and a unsymmetric molecule.

London forces exist in nonpolar molecules. These forces result from temporary charge imbalances. These temporary charges in one molecule attract similar charges in other molecules.