mart360

When a liquid changes to a solid, there is often a very short transient stage where the viscosity of the liquid increases greatly and an almost treacle like stae is reached before it solidifys.

This transient stage can be very rapid.

one i know of is when wallpaper paste is added to water, theres a thickening stage, and then it changes to a semisolid, but very viscous.

Question.

Do / does this transient / thicking stage occur when water freezes?

is there a point just before it turns to ice, that the water changes its viscosity?

or is the change to ice a different set of rules


mart

Jamo

iTrader: (23)

where's Chaos when you need him?

jods

Water freezing is not simply the reverse of ice melting. Melting is a single step process that occurs at the melting point as ice is heated whereas freezing of liquid water on cooling involves ice crystal nucleation and crystal growth that generally is initiated a few degrees below the melting point even for pure water. Liquid water below its melting point is supercooled water. It may be expected that the directional hydrogen bonding capacity of water would reduce its tendency to supercool as it would encourage the regular structuring in cold liquid that may lead to a crystalline state. Liquid water, however, is easily supercooled down to about -25°C and with more difficulty down to about -38°C with further supercooling possible, in tiny droplets (~5 μm diameter), down to about -41°C under normal atmospheric pressure. Water, supercooled down to -37.5°C, is sustained in storm clouds and the condensed clouds formed by aircraft at high altitude. Rather strangely, at the limit of this supercooling (also known as the homogeneous freezing point) the water activity is always 0.305 lower than that of water melting at the same temperature [457]. Where salts or hydrophilic solutes are present, the homogeneous freezing point reduces about twice as much as the melting point [663].


Liquid water may be maximally supercooled to about -92°C and 210 MPa. It should be noted that bulk water never forms a glass as the glass transition temperature (Tg, = ~136 K) for water is far lower, relative to its melting point (Tm, 273 K), than expected; Tm/Tg ~ 2 rather than Tm/Tg ~ 1.3-1.5 as for more typical liquids. Thus supercooled bulk water (i.e. not affected by surfaces or solutes) always crystallizes before its temperature can be sufficiently lowered, whatever the cooling rate [558]. Water glass may only be produced by extremely rapid cooling (105 K s-1) of tiny volumes of water (<~100 μm diameter).

As water is cooled, the cluster equilibrium shifts towards the more open structure (e.g. ES ) with higher viscosity. In order for crystallization to occur at least 3 - 4 unit cells worth of water molecules have to come together in the correct orientation.b The formation of whole or part icosahedral clusters interferes with this process whilst not allowing cluster crystallization due to their five fold symmetry. Lowering the temperature further, which should encourage crystallization, is partially counteracted by the increase in icosahedral clustering. The presence of ES clusters is, in principle, in agreement with computer simulation studies requiring the presence of metastable states [216]. Methods that break the hydrogen bonding in these clusters, such as ultrasonics [296], cause the supercooled water to immediately freeze

escott

iTrader: (1)

Blimey, was going to answer this then saw the rather comprehensive post above!

As an old ex-chemist, from my poor memory (and other caveats such as me just possibly being wrong) it is:

water turns to ice instantanously, however in a body of water this will not happen at the same time across the whole body due to slight temperature differences. This means there will be a perceptable 'thickening' and increased viscosity during the freezing process, but only because of timing rather than there being an intermediate state.

Happy to be corrected by more knowledgeable people.

Odds on

Yes. Just before it freezes it goes through a molecular change. There's actually several changes it can go through though.

The change is known as the 'Slush Puppy' stage. Personally my favorite is the orange stage.

PeteBrant

One of the most fascinating states is when you cool an absolute pure sample of a liquid is cooled below its freezing point it will become a superliquid - better known as a Bose Einstein condensate - A state that is not entirely understood. What is apparant is that the "liquid" will act in extremely odd way, due to their being almost zero kinetic movement amongst its atoms. In practical terms it means that this stuff can basically defy gravity, or pass completely through a glass container when it is poured into it.

If you ever get a chance to see a programme on BBC called "Absolute Zero" you should watch it.

jaytc2003

iTrader: (1)

ahh yes the slush puppy stage, my favourite is the blue stage (raspberry)

Leslie

There is also the latent heat due to a change of state to consider, ie it requires a loss of heat energy of 80 calories/gram for water to change into ice. This will slow things up a bit depending on the energy absorbent qualities of the ambient surroundings.

Les