Ice Ice Baby

                Ice floats in water.  Ice is less dense than water.  Ice has a more open structure than water.

                What in the world does all that mean aside from making my summer beverage more refreshing?

                I’m going to tell you.  No!  Wait.  I’m going to show you.

                Let’s start at Figure 24.4 from my Play Ball! post (GO PHILLIES!).  It shows how atoms/molecules/ions (blue circles in the figure) line up in the solid, liquid and gas phases.  You’ll notice that the atoms/molecules/ions are very close together in the solid phase, move further apart in the liquid phase and are further apart still in the gaseous phase.  This is generally how close atoms in each phase can be described.  Key word: generally.

Now let’s discuss density.  Sure, we all learned the equation in third grade – mass divided my volume – but what does that mean in real words?  Mass is directly related to the number of atoms/molecules/ions (also called particles) present.  So, if you look at the same volume of a solid, liquid or a gas, how many particles can you find there?  Figure 69.1 shows that for a particular volume (orange square) the solid has 9 particles; the liquid has 4 and the gas has 1.  The solid’s density is greatest (most amount of atoms/molecules/ions in the volume), the gas’s density is least and liquid is in the middle.  This means that if we had a cube of solid nitrogen and a glass of liquid nitrogen (brr!) and we dropped the cube in the liquid, the cube would sink to the bottom.  Why?  Because the cube is MORE dense than the liquid.  Again, this is generally true for most substances.

When particles come together to form a solid, many things need to be considered such as the size and charge of the particles.  The atoms/molecules/ions need to align themselves as close together as possible in the solid form, but without ticking each other off.  For example, two positive charges close together would be repulsive and unfavorable.  How do particles deal with this balancing act?

                To explain this, I’m going to show you two examples: solid NaCl (table salt) and solid water (ice).

NaCl: Solid NaCl is ions of sodium and chloride all packed very closely together.  A sodium ion is positively charged while a chloride ion is negative charged.  Obviously two chlorides aren’t packing directly next to each other because they both have the same charge.  Instead, the ions arrange themselves such that a chloride ion is surrounded by sodium ions in a very ordered array.  This way, the negatively charged chlorides are pacified by the positively charged sodiums enough such that the ions can pack close together (Figure 69.2).

Water:  I’ve discussed water and its fun properties a few times on this blog: Soap! and What Does Water Look Like? come to mind so you should be familiar with what a molecule of water looks like and how parts of it are positively and negative charged (Figure 69.3).  Similar to NaCl, water must not place two positive or negative charges too close together in the solid form.  Water’s shape, however, is quite different than ions of Na⁺ and Cl^- so its shape comes into play when ordering itself in the solid phase.  Figure 69.4 shows how water molecules line up when forming ice.  Compare it to solid NaCl (Figure 69.2) and the generic example of solids I showed in Figure 24.4.  Do you notice the differences?

Water molecules adopt a very open conformation.  Yes, the water molecules are close together, but there’s a lot of room between the molecules, which is not true in NaCl.  This kind of arrangement minimizes the steric and charge clashing of water molecules the most so it is the most favorable way that water molecules can pack together.  It is not air between those water molecules, it is empty space (I discussed air and its properties in my Play Ball! Post).

                If we now return the idea of Figure 69.2, but redraw the solid phase so it more accurately represents water, we can see that liquid water, not solid water, has the most molecules per volume.  

                As the ice is warmed to form water, the lovely ordered structure falls apart.  Instead of being held in an array, the molecules fall into a disordered jumble which results in more water molecules being present in a particular volume than would be true for ice.  More water molecules in the same volume of space means a higher density (Figure 69.5).  Solid water is LESS dense than liquid water and therefore floats in it.     

                Exceptions exist for every rule and, in the case of decreasing density from solid to liquid to gas, water is the exception.

Ions: Atoms that have either more or less electrons than the number of protons in their nuclei.

Density: the measure of how much mass is present per unit volume.  Less dense items float in more dense items.

Particles: an alternative name for atoms, molecules, or ions

REFERENCES

Zumdahl, Steven S. “Chemical Principles, 4^th Edition” (2002) Houghton Mifflin Company, Boston, MA.