There is one substance today that still has the power to amaze and entertain me as it did as a child. I never lose a chance to sing its praises and or to expound on its powerful chemical characteristics to all my students, young and old alike.
What is this amazing substance you might ask, water of course. Is there any other compound that has such far reaching effects on every level of science yet is so readily available and enjoyed by everyone?!
Three questions or observations fueled my initial interest in chemistry way back in elementary school.

• The first was, how come ice disappeared out of the freezer but didn’t melt,
•  the second, why did the water freeze and always get bigger, (This was of personal interest because I used to have to change out our farm animal’s water each morning and evening.  Do you know how hard it is to get a frozen block of ice out of a trough!!!),
• third, where did the beautiful diamond like morning landscape come from after a foggy evening on very cold winter night?

Hopefully, each of you can imagine the circumstances of each of these observations and have asked yourselves the same questions. Unfortunately, the answer to my questions did not arrive (at least to my satisfaction) until my first chemistry course in college and was surprisingly simple.
The answer to these questions was found in the three states of matter and their phase changes. A simple phase diagram of water opened my eyes to the delicate balance they have with each other and their dependence on both temperature and pressure. A picture is truly worth a thousand words.

Chapter 5 of the Physical Chemistry textbook discusses the details of phases and their equilibrium but I will recap here using the phase diagram of water shown above, found in the figure 5.1 from the textbook.  First, notice that the triple point, the point at which all three phases exist at the same time, is found at fairly low pressures and temperatures.
Any temperature/pressure combination below it excludes the existence of liquid water and the equilibrium is now between the solid and gas phases. The transition between them is either called sublimation (solid to gas) or deposition (gas to solid).
This answers two of my questions:  where did the ice go from the freezer without melting?  It sublimed of course!  The evidence is when the freezer is opened and releases a cold blast of “wet” air, or as anyone with a non frost-free freezer can attest to, can combine with other water vapor to create a beautiful crystal pattern on the inside known as frost (it was re-deposited).
This also answers the question of the disappearing fog and the appearance of diamond like crystals on cold winter mornings.  The water vapor in the air was deposited on every possible surface with beautiful results.  If you have seen such a morning, you can picture the glittering brilliance of water crystals hanging on every surface from fence posts to trees to power lines.  It also explains why those very crystals usually don’t melt but mysteriously disappear on a slightly warmer day.  Yep, the temperature increases enough for them to sublime, a perfect dance between the two phases.
Another balance takes place between liquid and solid water, melting (fusion) and freezing.  Alas, this is the phase responsible for so many of my winter mornings and evenings.  Unfortunately, while it did explain the appearance of the ice (and my subsequent responsibility of removing it), it didn’t explain why it expanded.  Another simple explanation but one I’ll save for another post.
The third and final dance takes place between the liquid and gas phases which takes place at much more comfortable temperatures with condensation and vaporization.  I find it a much more enjoyable observation as I am relaxing with an ice cold drink with water droplets sliding down my glass.
Again, as with all explanations, there is the qualitative or descriptive as I have done above or the nitty gritty quantitative explanation that require a fluent understanding of the language of science, math.  Each is explored in the textbook.
Luckily, these phase changes are not limited to just water, although it is still my favorite, and are explored for many other substances and mixtures as well.