Home Lesson for Students
If we think of gasses as many tiny ball, when we imagine mixing a hot gas with a cool gas,  it seems like the fast molecules might slow down (get cooler) and the slow molecules might speed up (get warmer.)  It's like the fast motion of the hot molecules gets "shared" among all the molecules.  Indeed, something does get shared, or transferred - the kinetic energy.  Kinetic energy is the energy of motion. Some kinetic energy is transferred from the fast moving, hot molecules to the slow moving, cool molecules. After awhile, all the molecules will be moving at about the same speed and have the same kinetic energy - in other words, they will have about the same temperature.  When two gasses (or liquids, or solids - or a gas and liquid etc.) reach the same temperature like this, we say they are in equilibrium.

Go to Activity 6 (outside site) to explore equilibrium.


People often talk about heat flow.  What does that mean in in terms of what you have just learned about molecules sharing kinetic energy?


Sometimes you also hear people talk about cold flowing, for example, from the ice in a glass of water.   What do you think about this?  


Which would melt faster - an ice cube in a cup of room temperature water, or an ice cube in a empty cup?  Can you explain why? (Hint: think about the density of air compared to the density of water, click here if you need a reminder.)


Imagine you have left two metal spoons in a hot pot of soup on the stove. In order to cool the spoons off you put one in a glass full of room temperature water and the other in a glass of very cold water. Which will cool off faster? Can you explain why using the Billiard Ball Model and the idea of equilibrium?


Now that you know something about gasses - let's go back to thinking about the gas in the ISM. 


How is the gas in the ISM different than the air we breathe (which is also a gas?)  If you need a hint, click here.


Scientists know that stars are formed when clouds of gas and dust in the ISM cool off and collapse.  Use what you've just learned about gasses to explain how these clouds can cool off.