Why do ice cubes get old ?

[McCrunchy]

Why is it that, after some time in the freezer, ice cubes (in a usual open ice-cube tray) get white, porous, and start taking up all the bad smells in your fridge ?

 

Couldn't find an answer on the web ...

 

Nicolas

[swansont]

Sublimation, which is essentially evaporation but from a bulk solid (which always has some vapor on its surface). The impurities don't escape at nearly the same rate (and the remaining ice might also be absorbing impurities). This is why old ice shrinks and tastes bad.

[CaptainPanic]

Sublimation and also deposition change the shape of the ice cubes, although most of the deposition will come from water from outside the fridge (it enters when you open the fridge).

 

About the nasty smells in the ice: Perhaps the old ice has a much larger surface area, and is therefore much more capable of absorbing the nasty smells? Just an idea.

[UC]

Porous ice would be very good at adsorbing smells, not absorbing them

[swansont]

Porous ice would be very good at adsorbing smells, not absorbing them

 

Good point.

[CaptainPanic]

Porous ice would be very good at adsorbing smells, not absorbing them

 

Ah, yes... it is indeed adsorption, not absorption.

 

In deposited ice (which resembles snow), what percentage of the water molecules will find themselves on the outside of a crystal, and what percentage is completely surrounded? (I know that it will depend a lot on many parameters - humidity, temperature, time - but does anyone feel like taking a shot here?)

[McCrunchy]

I'll take a shot :

 

considering a cubic lattice with 5 Angstrom atoms in diameter ( lattice constant for NaCl), you get for

a 10*10*10 atom crystal - ie, 5 nm large - that

 

Ns/Nv = 6*10*10/(10*10*10)=60 % of atoms are on the surface

 

6 % for 50 nm, 0.6 for 500, 0.06 for 5000 nm, that's 5 um, which should be about the typical sizes of crystals in ice : I'll go for it, 0.06 %.

 

McCrunchy

 

PS : so, but why is it that sublimating ice gets porous ?

[swansont]

 

PS : so, but why is it that sublimating ice gets porous ?

 

This is a guess, but impurities, whether exposed by sublimation or present from adsorption, might interfere with further sublimation — a kind of mask. Once you have a pit, the different surface area might also affect the sublimation rate.

[Mokele]

If the OP lives in florida, there is often a large quantity of Hydrogen Sulfide in the groundwater. When frozen, it comes out of solution, resulting in a foul smell.

[UC]

This is a guess, but impurities, whether exposed by sublimation or present from adsorption, might interfere with further sublimation — a kind of mask. Once you have a pit, the different surface area might also affect the sublimation rate.

 

Well, this is also just a guess, but aside from what swansont said, wouldn't water molecules along crystal grain boundaries and in crystal defects be slightly higher energy than those in regions of perfect crystallinity? Rate of sublimation would therefore be slightly higher along the crystal boundaries (which are extremely numerous unless you go to great lengths to freeze high purity water very slowly).

[Mr Skeptic]

Out of curiosity, am I the only one who has repeatedly misread the title by adding a "c" in front of the "old"?

[JKSFNet]

Isn't this "cracking"? The edges of the cube are turning into "snow", i.e. smaller crystals form as the moisture further condenses.