Gases have many properties that are interrelated; one important property of a gas is its pressure. A gas exerts pressure perpendicular to any surface it is in contact with. Mathematically, pressure is equal to force divided by area. According to the Kinetic Molecular Theory, gas particles are in constant straight-line motion. When gas particles collide with each other or with a surface, it is an elastic collision, so total kinetic energy is conserved. These collisions with a surface are the cause of the pressure exerted by a gas. Since pressure is caused by collisions of gas particles, pressure depends in part on how many gas particles are in a particular volume. More gas particles mean more collisions and a higher pressure, all other things being equal. We have many different units for pressure. One unit for pressure is millimeters of mercury. This unit originated with the mercury barometer. In a simple mercury barometer, we have a glass tube with a vacuum standing straight out of a reservoir of mercury. The atmosphere is pushing down on the surface of the mercury, which forces the mercury up the glass tube until the height of the mercury pushes back down with the same pressure. This is the same principle behind using a straw to drink from a glass. When you use a straw, you create a vacuum in your mouth and gases in the atmosphere push down on the surface of the liquid, forcing it up the straw. We use mercury in a barometer because it is very dense, so the height of the barometer can be relatively short. If we use water in a barometer instead of mercury, the water would rise to a height of about 10 meters at atmospheric pressure. Another unit of pressure is the atmosphere. 1 atmosphere equals 760 millimeters of mercury. We can also use units of torr, Pascals, pounds per square inch, or bar for pressure. We can interconvert between pressure units using dimensional analysis and these conversion factors.