Average kinetic energy = Here the bar over the KE indicates average kinetic energy. The "m" is the mass in kilograms and the "v" is the velocity in meters / second.

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 worth 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 floor, you exert a force against the floor. The pressure you exert is your body weight in pounds divided by the surface area under your feet or shoes. In 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. This is another reason why large vehicles have such big tires. The larger mass must be distributed over more area or else the higher pressure on the road surface would mechanically break down the road.

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

Unit	symbol
Millimeters of mercury		mm
Pounds per square inch		PSI
torr		torr
Pascal	Pa
atmospheres	atm

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

	76 cm
1 atmosphere	760 mm Hg
	101,325 Pa
		14.7 psi
		29.

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

Single state equations
Dalton's law	Ptotal = P1 + P2 +		Boyle's law		P1V1 = P2V2
Universal gas law	PV = nRT		Charle's law		V1/T1 = V2 /T2
			Gay-Lussac's law		P1/T1 = P2 /T2
		Combined gas law		P1V1/T1 = P2V2 /T2
		Avogadro's law		n1/V1 = n2/V2
All these equations are follow assumptions of the kinetic molecular theory for gases.

Gas properties change with temperature and pressure. A reference standard temperature and pressure, are set at STP, 273.16^o Kelvin and 1 atmosphere pressure.

All gases at STP have the same molar volume, 22.4 Liters. This naturally comes about when the Ideal / Perfect Gas law is used to calculate volume for 1 mole at STP.

Terms to know