A molecule exerts a force when it collides with a surface. The result of many of these collisions is pressure. Pressure- the force exerted per unit area by gas molecules as they collide with the surface around them. Kinetic molecular theory- a model for understanding the behavior of gases. Kinetic molecular theory makes the following assumptions: a gas is a collection of particles (atoms or molecules) in constant, straight line motion. (they do not interact, gas particles do not attract or repel each other. As temperature increases, the particles move faster and therefore they have more energy. Kinetic energy- is the energy due to motion. Ex) a cylinder filled with a gas sample. You can tell this because the particles are separated by large distances. When you push the piecestone down you exert outside pressure. Ex) solids and liquids do not work this way.