absolute zero- temperature at which molecular motion ceases; 0 K = 273.15 °C.Common Student Misconceptionsbarometer- device used to measure atmospheric pressure.
Boyle’s law- states that the volume of a fixed amount of gas at a constant temperature is inversely proportional to its pressure.
Charles’ law- states that the volume of a fixed amount of gas at a constant pressure is directly proportional to its absolute (kelvin) temperature.
gas- form or state of matter in which a material assumes the shape of its container and expands to fill the container (thus having neither definite shape nor volume).
gas constant- (R) numerical constant appearing in ideal gas equation (PV = nRT).
ideal gas- one whose behavior can be predicted by the ideal gas equation.
ideal gas equation- relates the pressure, volume, temperature, and amount of gas through the expression PV = nRT.
manometer- device used to measure the pressure of a gas, usually by comparing the gas pressure to barometric pressure.
non-ideal gas- departs from the behavior predicted by the ideal gas equation. Its behavior can only be predicted by other equations of state.
partial pressure- pressure exerted by an individual gas in a mixture, independently of other gases.
Misconceptions such as those listed are best dispelled with demonstrations followed by class discussion.1. “There cannot be any pressure around us, since we cannot feel it.”
a. In an airplane you can feel the pressure in your ears, particularly during take offs and landings. When opening a soda can you can hear the change in pressure as the gas escapes.
b. Demonstrate the breaking of a ruler covered with one sheet of newspaper by placing a cheap yardstick on a table with about half of it extending past the edge. Carefully spread a full page of newspaper over the portion of the yardstick on the table. (Press paper against the yardstick to expel air, smoothing the paper flat against the tabletop.) Then, with a karate-like blow, chop off the portion of the yardstick that extends beyond the table. Students can calculate amount of pressure on newspaper.
2. “Gases do not weigh anything.” (See Demonstrations section of module.)
a. Wrap a 250-mL Erlenmeyer flask with strips of masking tape to prevent injury in case of an implosion. Fit the flask with a one-hole stopper, a 5-cm piece of glass tubing, and a 5-cm piece of rubber tubing with a clamp. Measure the mass of the assembly when full of air. Evacuate moderately with a vacuum or hand pump. Fasten clamp tightly and quickly weigh again to show the loss of mass. Demonstrate that the assembly returns to its original mass when the clamp is opened.
b. Set up a balloon balance and add air to one balloon and CO 2 or methane to the other. The balance will tip to the side containing the higher-density gas sample.
3. “Hand me an empty glass.”
This demonstration shows that the air will be displaced by water flowing into the flask; thus, the flask was not really “empty”. This demonstrates also how gases can be collected “over water,” provided the gas is not very soluble in water. (Hydrogen and oxygen can be collected this way; chlorine, ammonia and hydrogen chloride cannot.)
4. “When a gas expands the particles get larger.”
The molecules of a gas sample stay the same size, but distances between the molecules get larger when the sample expands.
