2. GASES THE THIRD STATE OF MATTER

3. We live at the bottom of an ocean of air – the ATMOSPHERE The highest pressures occur at the lowest altitudes. If you go up a mountain, atmospheric pressure decreases because the depth of air above you is less.

4. One of the first instruments used to measure gas pressure was designed by the Italian scientist Evangelista Torricelli (1608-1647). Measuring Atmospheric Pressure He invented the barometer, an instrument that measures the pressure exerted by the atmosphere. The height of the mercury column measures the pressure exerted by the atmosphere

5. The Barometer One unit used to measure pressure is defined by using Torricelli’s barometer. The standard atmosphere (atm) is defined as the pressure that supports a 760- mm column of mercury.

6. The Barometer This definition can be represented by the following equation. Because atmospheric pressure is measured with a barometer, it is often called barometric pressure.

7. The Barometer A barometer measures absolute pressure; that is, the total pressures exerted by all gases, including the atmosphere.

8. Other Pressure Units Atmospheric pressure is the force per unit area that the gases in the atmosphere exert on the surface of Earth. The SI unit for measuring pressure is the pascal (Pa), named after the French physicist Blaise Pascal (1623-1662).

9. Pressure Units Because the pascal is a small pressure unit, it is more convenient to use the kilopascal. 1 kilopascal (kPa) is equivalent to 1000 pascals. One standard atmosphere is equivalent to 101.3 kilopascals.

10. Pressure Units Because there are so many different pressure units, the international community of scientists recommends that all pressure measurements be made using SI units, but pounds per square inch continues to be widely used in engineering and almost all nonscientific applications in the United States.

11. The Gas Laws No kinetic energy is lost when gas particles collide with each other or with the walls of their container. All gases have the same kinetic energy at a given temperature. Gas particles are in constant, random motion.

12. Boyle’s Law: Pressure and Volume Robert Boyle (1627-1691), an English scientist, used a simple apparatus pictured to compress gases.

13. Boyle’s Law: Pressure and Volume After performing many experiments with gases at constant temperatures, Boyle had four findings. a) If the pressure of a gas increases, its volume decreases proportionately. b) If the pressure of a gas decreases, its volume increases proportionately.

14. Boyle’s Law: Pressure and Volume By using inverse proportions, all four findings can be included in one statement called Boyle’s law. c) If the volume of a gas increases, its pressure decreases proportionately. d) If the volume of a gas decreases, its pressure increases proportionately.

15. Boyle’s Law: Pressure and Volume Boyle’s law states that the pressure and volume of a gas at constant temperature are inversely proportional. Click box to view movie clip.

16. Boyle’s Law At a constant temperature, the pressure exerted by a gas depends on the frequency of collisions between gas particles and the container. If the same number of particles is squeezed into a smaller space, the frequency of collisions increases, thereby increasing the pressure.

17. Thus, Boyle’s law states that at constant temperature, the pressure and volume of a gas are inversely related. In mathematical terms, this law is expressed as follows.

18. Applying Boyle’s Law A sample of compressed methane has a volume of 648 mL at a pressure of 503 kPa. To what pressure would the methane have to be compressed in order to have a volume of 216 mL? Examine the Boyle’s law equation. You need to find P 2, the new pressure, so solve the equation for P 2.

19. Applying Boyle’s Law Substitute known values and solve.

20. Charles’s Law When the temperature of a sample of gas is increased and the volume is free to change, the pressure of the gas does not increase. Instead, the volume of the gas increases in proportion to the increase in Kelvin temperature. This observation is Charles’s law, which can be stated mathematically as follows.

21. Charles’s Law Click box to view movie clip.

22. Applying Charles’s Law A weather balloon contains 5.30 kL of helium gas when the temperature is 12°C. At what temperature will the balloon’s volume have increased to 6.00 kL? Start by converting the given temperature to kelvins.