Lowering the freezing point of water is easy. All you have to do is add salt, sugar or any other solute. Going in the opposite direction and raising the freezing temperature of water is not nearly as easy. In fact, some scientists doubt that it even be done. However, even though it may be true that you can't raise the freezing point by adding a solute, researchers have discovered other ways to the freezing point of supercooled water. One is by using electricity, and the other is by adding alcohol or testosterone. These methods work only with pure water.
Start With Supercooled Water and Add Alcohol
The process by which water freezes is complicated by the fact that water is a polar molecule, which means that, even though its net charge is zero, it has a positive and negative end, like a magnet. Water molecules bind electrically to each other and to impurities in the water by forming hydrogen bonds, and they coalesce into ice more easily if the water contains impurities. If you can find a way to suspend a droplet of pure water in the air without having it touch anything, it could remain in the liquid state at temperatures well below 0 degrees Celsius (32 degrees Fahrenheit). Such supercooled water can stay in the liquid state until the temperature falls to -40 C (-40 F).
Adding alcohol to the water, however, changes its behavior. When cooled, alcohol forms ice-like hexagons, and the water droplets coalesce around these instead of floating freely around each other. The hexagonal structures provide the same type of stability as solid impurities. By adding alcohol, scientists have found they can raise the freezing point of pure water to 0 C.
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Electricity Can Also Raise the Freezing Point of Water
Israeli scientists tried a different approach to raising the temperature at which supercooled water would freeze. They created charged cells by placing pyroelectric crystals inside copper cylinders. They placed these cells in a humid room and turned down the temperature until water started to condense on the crystals. They continued lowering the temperature and found that the droplets froze at -12.5 C (9.5 F) on an uncharged surface, but on a positively charged surface, they froze at -7 C (19.4 C). On a negatively charged surface, the water froze at -18 C (-0.4 F).
The experiment yielded an even more surprising result. The researchers found that the water droplets remained liquid on a negatively charged surface for 10 minutes at -11 C (12.2°F), but when the charge dissipated, they could cause the droplets to freeze by raising the room temperature to -8 C (17.6 F). The reason is that raising the room temperature generated a positive charge on the crystals.
Soot and Testosterone Also Work
Scientists know that adding soot to pure water raises the freezing point by about 7 degrees Celsius, but that's nothing compared to the male hormone testosterone. It can raise the freezing point of pure water that has been supercooled from -40 C to as high as -1 C (30.2 F). Researchers aren't sure how this works, but they suspect the mechanism is similar to that of alcohol.
Lowering the Freezing Point
The amount you can lower the freezing point of water depends on the concentration of the solute you add, but you can't lower the freezing point indefinitely. In fact, the zero point of the Fahrenheit scale (-17.8 C) is defined as the freezing temperature of a saturated solution of salt water. No more salt will dissolve in a saturated solution, so 0 F is the lowest temperature to which you can lower the melting point of water with salt. However, it is possible to supercool water to get it to remain in the liquid state at even lower temperatures. University of Utah researchers have determined the temperature at which water absolutely has to freeze to be -48 C ( -55 F).