Solutions to HW 2 (p 6)

1.         What happens during evaporation at the molecular level?


Liquid molecules vibrate and rotate. As they absorb energy they begin to rotate faster, and eventually the attractions between liquid molecules are overcome. At that point molecules translate (and continue to vibrate and rotate); we have a gas.

2.         Explain melting at the molecular level.


Solid molecules are arranged in an orderly, crystalline fashion. The heat causes solid molecules to vibrate faster, loosening the bonds. Eventually the crystal structure breaks up and the molecules begin to rotate. We now have a liquid.


3.         A student at a fashion show forgot to bring a camera. Since he was unable to take pictures, he decided to do some science. He applied the limewater test to the "smoke" coming from the stage and found it to be CO2.


a.         Explain how solid carbon dioxide turns into gas.


The solid CO2 molecules are arranged in an orderly, crystalline fashion. They absorbed heat. For some reason, the heat was enough to get the solid molecules to pass directly form a vibrations-only state to a state where the molecules have all three types of motion: vibrations, rotations and translations.


b.         Why does a smoke machine, which uses dry ice, create "smoke"? Isn't carbon dioxide gas invisible? Hint: What other gas can be in a room that is filled with people?


The coolerCO2 and air above the dry ice removed heat from the water vapour in the air, causing it to condense into tiny water droplets that we see as clouds.


4.         Why does a balloon burst if too much air is blown into it? Avoid three-word answers.


            You suddenly have more air molecules colliding in about the same space since the balloon cannot stretch indefinitely. The collisions eventually exert enough force to break the bonds between the balloon’s material.


5.         Explain why water rises into a jar soon after the candle's flame is extinguished.


            Remember there is atmospheric pressure pushing on the water both in and out of the beaker initially. Then burning the candle changes things (how?): the forces no longer cancel out. There is less pressure once the oxygen is consumed and the carbon dioxide escapes. For more details see candle.


6.         Which of the following is the slowest molecule? Assume constant temperature.


a.         Ne                   b.         F2                    c.         Ar                   d.         CO2



7.         The average velocity of four gases was measured at the same temperature and pressure.


The four gases are: He, Ne, CH4, and CO2.


In which of the following are the gases arranged in increasing order of their average velocity?





He, Ne, CH4, CO2




CO2, Ne, CH4, He




CO2, CH4, Ne, He




Ne, He, CH4, CO2