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Water is strange. Unlike most compounds, its solid form is (normally) less dense, and of a larger volume than its liquid form. Because of this, its very difficult to compress water, because normally there isn't really anything to compress it into. But the story of ice is a bit different from the snow and hail we see falling outside of our windows during these winter months. In fact, ice has many different forms, depending on the conditions it forms in. The ice we commonly know is called Ih - a common ice type with a hexagonal structure. But as you can see from the picture, there are many different types of ice. Ic is also a (relatively speaking) common ice type, with a cubic structure that can be present in the upper atmosphere. In total there are 15 different types of ice, all forming at different pressures and temperatures, all with different crystal structures, densities, and electrical properties. For example, while water is hard to compress, when put under great enough pressure at normal temperatures, can form into ice IV (not pictured), a denser form of ice. While most variations are just density and structure based, certain forms (like ice XI) have ferroelectric properties, which is something I looked up and failed to understand, but it sounded interesting. And noticing the lower pressures, below ~1 kPa (about 1/100 of normal sea pressure), liquid water fails to exist, and water vapour will undergo deposition straight into ice below this point. As we head into winter, it's interesting to note the complexities of such a common substance. It can take on many forms with many properties, and I think that's pretty cool.
While I was pouring ice cold lemonade for myself, I wondered-- "What would happen over time if I waited for a cup filled completely to the brim with ice to melt? Would the water spill over the cup as the ice melted? Or would the ice just melt leaving the cup still completely filled to the brim with no spills?" Huh. I had to test this out. I decided to use a cup filled with ice, and slowly poured water to the exact brim of the cup, and left a napkin under to see if the water would spill over after the ice melted. This was not enough for me. What if the cup were filled with ice and grape juice? Or ice cube grape juice filled with water? Or ginger ale? Or milk? I was curious. I tested these all out, only to find I was wrong in my original hypothesis. I was sure I'd come back to my kitchen a pooling mess of water, milk, grape juice, and ginger ale, but I was very wrong. I had three cups of perfectly filled glasses completely filled to the very very tippy top, like no other cup has even been. It was amazing. I realized something was up with water. These things called hydrogen bonds really mess with us chemist and physicists. Why? Because they can. In liquids, molecules slip, side, bond, break and reform. However when the water turns to ice, the molecules are rigidly bonded. This creates more empty space between the molecules when the hydrogen atoms bond together so rigidly and thus frozen water occupies more room. It is also less dense than liquid H2O because of this space. This is why ice floats in your Sodas. Or why in the winter-- better known as the constant weather in Rochester-- lakes and ponds freeze at the top and not on the bottom. Because ice is less dense due to H2O's molecular structure of Hydrogen bonding (positive to negative --oppositely charged ends of the water molecules-- creating space). Solid ice takes up more space than the liquid state of H2O. You would think that water would behave like every other substance from liquid to solid-- that the molecules would become denser and more compacted-- but no, it does the exact opposite. Because water is tricky, and that's why we drink it. You may be wondering why the milk and grape juice? Those are mostly water based as well, that is why. Due to the change in thermal energy, we all know that the water transferred energy from the high temperature (water) to the low temperature (ice). This is the second law of thermodynamics. It is also considered an energy heat flow. As we know, this happens so that this water glass can reach a happily balanced equilibrium. This is why ice melts. Even milk ice. The energy in the glass is never destroyed; the first law of thermodynamics tells us energy is conserved. Here are some cool links (pun intended) on ice and why it is less dense than its liquid state of H2O. (Also why it would not spill over a glass even when filled to the brim and left alone for an hour or so.) Not all science experiments have to be messy. http://www.word-detective.com/howcome/waterexpand.html