SOLUTION TO EXERCISE 11


Ice cubes float in water.  Dry ice cubes sink in liquid carbon dioxide.
The slopes of the solid-liquid borderlines of the two phase diagrams tells
you this information.  How?  

For water, the slope is negative; that means 
that for a given temperature, you need less pressure to contain the same
number of molecules of solid water than for liquid water.  So when liquid
water becomes ice, its volume increases - and thus its density decreases.
The density of ice is lower than that of liquid water, so ice will float
in water.  

On the other hand, the slope is positive for carbon dioxide;
the exact opposite reasoning applies, and dry ice will be denser than
liquid carbon dioxide.  Thus the dry ice sinks.