Homework #2
SOLUTION

You can find information that you can use for this homework problem at the "Chemicool" web site (linked to the course schedule as Web database on the elements).  Continue your answers on the back if you need more room.

Whether an element or compound will be abundant in a solar system body depends very much on the element or compound's volatility.  An indication of a material's volatility can be obtained from its boiling point, that is, the temperature (at 1-atmosphere pressure) at which the material has its liquid and vapor phase in equilibrium.  The opposite of a volatile element or compound is a refractory element or compound -- this would be an element or compound that forms a vapor phase only at very high temperatures.

1.  This table gives the relative amounts of elements in the Sun.  List the first three noble (inert) gases, and for each, give its abundance in the Sun, relative to silicon:

element:                Helium                           number per Si atom:   2162

element:                Neon                              number per Si atom:    3.5

element:                Argon                             number per Si atom:     0.1

2.  For each of these three noble gases, state whether it would be considered volatile or refractory, and why:

element:                 Helium                           volatile or refractory?   volatile                               why?   boils at 4 K

element:                 Neon                              volatile or refractory?   volatile                               why?   boils at 27 K

element:                Argon                              volatile or refractory?   volatile                               why?   boils at 87 K

3.  Is silicon a volatile or refractory element, and why?

 Pure silicon boils at 3553 K and therefore would be considered refractory, at least in comparison to substances that vaporize at much lower temperatures.

4.  Is oxygen a volatile or refractory element, and why?

 Pure oxygen is a gas at 1-atmosphere pressure and room temperature.  It boils at 90 K (much lower temperature than room temperature, 300 K), and so it is very volatile.
  
5.  Is hydrogen a volatile or refractory element, and why? 

Pure hydrogen is a gas at 1-atmosphere pressure and room temperature.  It boils at 20 K (much lower temperature than room temperature, 300 K), and so it is very volatile.
 
6.  Hydrogen atoms can combine with oxygen atoms to form H2O molecules (water).  Is water more or less volatile than pure hydrogen?  Is water more or less volatile than pure oxygen?  Explain your answers.

Hydrogen boils at 20 K, and oxygen boils at 90 K (they are both cryogens).  In contrast, water boils at 100 Centigrade, or 373 K, so it's a lot less volatile than the elements of which it is composed.  This tells us that volatile elements can be a lot less volatile when they are in chemical compounds.  Similarly, Earth's rocks contain significant amounts of oxygen, a volatile element by itself.