Chemists: Absorption is a puzzle!

Factual Questions
1

Ok, this has been bugging me for some time now. Recently, it has all come to a head, and I MUST get the SD! This probably has to do with how hydroscopic substances behave, but it also has to do with conservation of mass:

The main question is: What happens when a chemical absorbs water? Is the water really gone?

a) First, I read how absorption chillers use a hydroscopic salt create a vacuum (to enduce vaporization to cause a cooling effect). The absorption of water creates the vacuum. So, is the water really gone?

b) The Food Network mentions baking soda and cake flour, as I recall, both have a drying effect by absorbing more water from a recipe. The host mentioned this, in turn, decreases baking time. So, are they implying the water is really gone?

c) This isn’t hydroscopic, but a side item sent to Uncle Cecil asks how much deeper would the oceans be if not for sponges. This, too, implies the absorbed water is gone occupying zero volume. But, the water isn’t gone, so the total volume of water remains the same, right?

I WAG there’s a lot of misinformation on this. What’s the SD?

  • Jinx
2

The water is not gone. Generally speaking, a drying agent of this nature is a solid of some highly polar molecule. Because these molecules have exposed edges which have charged rather than electrically neutral atoms, the free energy goes down when they’re near another polar molecule, like water vapor, rather than a nonpolar one like diatomic oxygen or nitrogen. The water vapor that’s always in the air (as well as liquid water) is attracted to these solids, where it sticks like glue due to a decrease in free energy. In effect, each molecule of the drying agent becomes surrounded completely by molecules of water, generally inseperably. The water isn’t gone, but it’s been sequestered, and it’s tremendously difficult to get out again.

Many very common solids have this property, notably sugar. Look in the ingredient label of a container of salt, and you’ll notice that the number two ingredient by weight is typically sugar. You also see rice in salt shakers for the same reason. Another example is the silica gel packet you see packaged with tennis shoes and many other products (don’t eat silica gel!). Google “desiccant” to find more information.

3

It depends what the chemical reaction is, and what you mean by “gone”, but generally speaking, no, water doesn’t magically disappear during a chemical reaction. Water, owing to its dipole nature and ubiquity as both liquid and vapor on planet Earth (the solid form is chemically less interesting) makes for an excellent, often-stated universal, solvent. It does this primarily via hydrogen bonding, which leaves the water molecule intact, although a very strongly reactive substance can debond one or even both of the hydrogens leaving an aquous hydroxide or free oxygen, respectively.

The scenerios you list, however, don’t require any stong oxidizing reactions. Your hydroscopic chiller, for instance, just debonds individual molecules, making them much easier to evaporate into air. (Evaporative coolers, BTW, aren’t refrigeration cycles in a strict sense; what they do is allow the air to become saturated and thereby carry away more heat in convection than dry air, and are ineffective in humid climates where they simply make things more soggy.)

Ditto for your baking example; the denser and more continuous the confection, the more heat it will retain, and the faster it will cook, taking into account, of course, that there is more mass to heat up. A less mechanically continuous substance will transmit heat less effectively and have an uneven heat distribution, leading you to scorch the outside while the inside remains undercooked. Evaporating water escaping out of the center also serves to moderate the temperature in outer layers, carrying away extra heat. Controlled water content is key, I’m told, to getting a perfectly flakey crust or getting a cheesecake to set up. Since, despite my professional experience in the area of food preparation, I’m a nearly utter failure in the baking of sweet goods, I can’t speak from experience, but McGee’s On Food And Cooking (which sits on my bookshelf only occasionally perused rather than thourougly consumed) goes into extensive detail on this.

As for the question of sponges in the ocean…no, the Colonel will come in and end this thread, protesting that it’s too silly. Much too silly.

Stranger

4

Awesome! A Monty Python reference! Hmm, I wonder if that gave John Cleese the idea of the “Major” on “Fawlty Towers”?

Thanks for the details on absorption. Makes sense, but I’ll have re-read to absorb it all (no pun intended)! - Jinx