2.2.3 Pressure

Pressure is force per unit area. The more force, the greater the pressure. The smaller the area to which the force is applied, the greater the pressure. The larger the area subject to a pressure, the larger the total force on that area.

Reread that paragraph. Folks often confuse the concepts involved.

There are too many units in use for the measurement of pressures.^2.4 In chemistry three are often used. These are the atmosphere, the torr, and the bar. There is also the standard unit, the pascal, which can't be ignored.

The pascal is a standard MKS unit defined as one kilogram per meter per second squared. It is used in calculations because of its compatibility with other standard MKS units.

But the Pascal is too small a unit for ordinary work. A more meaningful unit is the bar which is 100,000 pascals.

The most common pressure unit in ordinary chemistry is the atmosphere. It is defined today as exactly 101325 pascals, or, what is the same thing, 1.01325 bars. Thus bars and atmospheres are almost, but not quite, the same thing.

Lastly, the torr is defined as 1/760 atmospheres.

Chemists (and many others) have found it convenient to define a standard pressure. Even this has its difficulties because, for years it used to be 1 atm. Today, by international agreement, the standard pressure is 1 bar. Sadly, many data tables are still presented using 1 atm as the standard. While the difference is often small, it pays to pay attention to that.

There is a special symbol for the standard pressure. It is a superscript circle with a line through it: $p{ \circ\hspace*{-0.5em}-}$.

The pressure is an intensive property