1. The Nature of Gases Objectives: 1. Describe the nature of gases and recognize that gases have mass, can be compressed, exert pressure, occupy the space available to them, and diffuse. 2. State the six postulates of the Kinetic- molecular theory and explain how they account for the physical properties of gases. Key Terms: Diffusion, elastic, Kinetic-molecular theory

2. Nature of Gases Molecules exist as monatomic, diatomic and polyatomic molecules. Below are some basic properties of gas molecules. Gases have mass Gases are compressible Gases will fill the space available to them Gases diffuse through each other very rapidly Gases exert pressure The pressure of a gas depends on its temperature

3. Kinetic Molecular Theory The Kinetic-molecular Theory attempts to explain the behavior of gas molecules based on the nature of gas. The theory is grounded on three basic assumptions: 1.All gases have mass but are very far apart – Gas molecules are separated from each other by extremely large distances gases can be compressed gases have low density 2.Gases particles are constantly moving at a very rapid rate – explains why gases immediately fill a container – gases move in a strait line until colliding with an object – gases freely diffuse through each other 3.Collisions between gas molecules are perfectly elastic – there is no loss or gain in kinetic energy – kinetic energy (1/2 mv 2 ) of the gas molecules increases with added temperature explains the expansion of gases when heated it is important to remember that the temperature of a substance is the average KE of the molecules

4. Gas Pressure – closed container force exerted by the collisions of the gas molecules over the surface of an object – force/unit area

5. Gas Pressure – atmosphere Atmospheric (air) pressure is the result of the mass of the molecules of gas and the gravity of the Earth. – Pressure of gas at sea level is 760 mmHg - 1atm - 101.3kPa The millibar is used by the National Weather Service - (1013 millibars = 1 atm at sea level) – Air pressure decreases with altitude – Can be measured using a mercury barometer