Why do liquids freeze instead of slowly forming into a cluster?

[kirbsrob]

Shouldn't liquids slowly get thicker than instantly freezing into a solid? The more you freeze an object, the more the molecules start to slow down, cluster and get thicker. So why is it that you freeze a liquid the process of molecule clusterization is instantaneous?

[swansont]

Liquids don't generally instantly freeze into a solid. The closest to instant freezing I'm aware of is when you have a supercooled liquid which freezes quickly (but not instantly) when perturbed.

[studiot]

This is a good question, but it would be helpful to know some background to establish what level to pitch an answer at.

 

What do you know of

 

The kinetic theory of matter

The Molecular theory of matter

Cooling curves for liquids. These are plots of temperature v time

 

It is useful (and easier) to understand how solids melt before tackling how liquids freeze.

Neither process is instantaneous.

Dendrites can cause flash freezing at the speed of sound in the material. that is the maximum

These can be very pretty

 

 

http://www.msm.cam.ac.uk/phase-trans/dendrites.html

 

https://www.google.co.uk/search?q=dendrites+freezing&tbm=isch&hl=en-GB&biw=&bih=&gbv=2&oq=dendrites+freezing&gs_l=img.3...21656.25188.0.25578.9.3.0.6.0.0.110.282.1j2.3.0....0...1ac.1.34.img..7.2.220.l7cM9r1kHL4

Edited March 15, 2014 by studiot

[rktpro]

Like in lakes, ice formation takes time and layer thickness increases with time. You can even calculate its rate!

[Enthalpy]

Freezing takes time because it releases heat, which raises the temperature of the liquid hence prevents further freezing. Time is necessary to remove the heat.

 

A supercooled liquid may be cold enough that a significant portion can freeze before the remaining liquid is too warm to proceed further.

[studiot]

 

Freezing takes time because it releases heat, which raises the temperature of the liquid hence prevents further freezing. Time is necessary to remove the heat.

 

This is a nice theory, but is against the experimental evidence that is gained in the secondary school experiment to plot cooling curves (temperature - time graphs).

 

Unless you have enormous quantities, for a pure substance the temperature remains constant at the melting point temperature until the substance is either melted or solidified.

For a mixture the plateau is still present but is no longer parallel to the time axis ie is tilted.

 

This is the basis for significantly used industrial chemical analysis and quality control techniques.

Edited March 15, 2014 by studiot

[John Cuthber]

"Shouldn't liquids slowly get thicker than instantly freezing into a solid?"

They do.

Syrup is probably one of the better known examples that shows this effect strongly, but most liquids become more viscous when they are cooled.

[Enthalpy]

 

This is a nice theory, but is against the experimental evidence that is gained in the secondary school experiment to plot cooling curves (temperature - time graphs).

 

Unless you have enormous quantities, for a pure substance the temperature remains constant at the melting point temperature until the substance is either melted or solidified.

For a mixture the plateau is still present but is no longer parallel to the time axis ie is tilted.

 

This is the basis for significantly used industrial chemical analysis and quality control techniques.

 

Unless you have a supercooled liquid.

But this exceeds the level of secondary school, sure.