14.1. Gas Properties http://www.ck12.org
In the chapter opener, we saw a hot air balloon traveling through the air. The walls of the balloon are pushed
outward by the hot air inside. On the molecular level, this outward force is due to countless tiny collisions between
gas molecules and the walls of the balloon.Pressureis a measure of the total amount of force divided by the surface
area over which it is exerted. If the total force of the collisions inside the balloon is greater than the force of collisions
exerted by the atmosphere on the outer surface of the balloon, it will expand. Equivalently, if the pressure inside the
balloon is higher than the pressure outside, the balloon will expand.
Pressure can be measured with an instrument called abarometer(Figure14.2).
FIGURE 14.2
Diagram of a mercury barometer.The word barometer comes from the Greek rootsbaro, meaningpressure, and–meter, meaning tomeasure. Evange-
lista Torricelli (1608-1647) is credited with inventing the barometer in 1644. The invention consisted of a mercury-
filled container attached to a glass tube into which mercury flowed. As shown inFigure14.2, atmospheric gases exert
a downward force on the fluid reservoir, and the fluid rises in the tube. The height of the fluid is one way to measure
atmospheric pressure. If the tube is connected to an enclosed sample of gas instead of open to the atmosphere, the