Kitchen Chemistry 102: Whoa… Water Is Cool (And Frozen, And Boiling)


As the temperature of a mixed substance is raised, it will soften over a period of time, but a PURE substance has a sharp melting point and changes states immediately.

Pure substances have an either-or quality to them.  Only mixtures know degree.

This fact further supports my suspicion that –whatever the Universe is at bottom (pretending for the moment that it has a bottom)– it must be something possessing a strong binary nature… Yin or Yang, 0 or 1, yes or no.

I wonder, being the states of pure matter-types are abrupt in their phase-shifts, if the threshold between energy and matter, itself, is also abrupt?  Do matter-energy particles exist as hybrids?  Or rather, is the difference between pure matter and pure energy another either/or proposition?

And what of Life?  Is something either alive or not?

And while I’m on the subject of changes in state, I have to gush a little bit over the amazing substance known as water.  Perhaps I should add to my Self-Doctorate Reading List (“The 300”) a book just on water.  Water has so many, sometimes weird, properties essential for life as we know it…  For one thing… did you ever stop to consider that there are actually very few common and naturally liquid-state  elements on Earth?  Water is not one of the 100 or so chemical elements (although it is nearly just Oxygen with extra protons), but it is pretty elemental– and thankfully, it’s often found in liquid form!

Furthermore, water molecules just happen to be lighter than air when single, but heavier than air when they combine.  This works-out quite satisfactorily for Life, since it allows for the evaporation-rain cycle to continue.

Getting back to kitchen chemistry (I tend to stray into philosophical territory if my tether is long enough)…  because there is less atmospheric pressure pushing down on water at higher altitudes, single water molecules have an easier time there of rising into the air.  This is why the boiling point of water is lower at higher altitudes.  Therefore, if you are boiling foods at high altitudes, you will have to cook foods longer since the water won’t become as hot before it boils away.

But let’s push this to extremes (my general rule of life):  If lower air pressure allows water to boil away at lower temperatures– what if we take away ALL the air?  In other words, cook in a vacuum?  

This is precisely what happens with freeze-dried foods.  If you freeze food inside a vacuum, the water molecules (ice, actually) will escape due to the lack of pressure, leaving behind the food.  The resultant freeze-dried food will have “an airy crunch” says the authors of Culinary Reactions, due to the cavities left behind where the water used to be.  And without water to support life (read: bacteria), the food will stay preserved.

The reverse side of this phenomenon is the Pressure Cooker.  You can INCREASE the pressure on water and thus make it more difficult for it to boil away.  This will push the boiling temperature up, allowing you to heat foods hotter.  The hotter boiling temperature reached in Pressure Cookers is excellent for sterilization (killing tiny baddies)– which is why the procedure is used when canning.

You can also raise the boiling temperature of water by adding solutes which crowd-out water from the surface of the mixture, thus making it harder for the water to leave the mixture and join the atmosphere.  In this case, some of the water molecules which would normally evaporate are being partially blocked.  Sugar is good for this, and salt works, too, though less well.

As well as raising the boiling point of water, solutes in water can also LOWER the freezing point.  This is accomplished by the same method as above… the solute getting in the way of the water.  But in the case of anti-freezing solutes, like salt on icy sidewalks, the solute interferes with the ability of the ice to keep reforming and/or augmenting itself.

The ice on a frozen sidewalk is in a state of equilibrium, with some water molecules leaving the ice (melting) and some joining it (freezing).  When salt is added, it gets more in the way of the joiners than of the leavers, and so the frigid equilibrium is thrown off in favor of more melting… Thus, less and less ice forms and more and more water.

When making homemade ice cream, the most important facet of this phenomenon is that the new equilibrium of ice and water after salt is added will be LOWER– thereby lowering the temperature of the sweet milk you’re churning and quickening the freezing of your delicious homemade treat!


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