The Naked Scientists

The Naked Scientists Forum

Author Topic: How does salt melt snow/ice?  (Read 1740 times)

Offline confusious says

  • Jr. Member
  • **
  • Posts: 38
    • View Profile
How does salt melt snow/ice?
« on: 16/05/2013 11:17:34 »
Hi everyone, I just want to know how salt actually melts snow and ice, what is the chemical action or reaction going on? :)


 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4096
  • Thanked: 244 times
    • View Profile
Re: How does salt melt snow/ice?
« Reply #1 on: 16/05/2013 11:44:15 »
Salt is mostly Sodium Chloride (Na+Cl-), mixed with some other ionic solids like Magnesium Chloride (MgCl2).

Water (H2O) is a "polar" molecule, with a slight negative charge on the Oxygen end, and a slight positive charge on the Hydrogen end.

When water mixes with salt, about 8 water molecules surround the positive Na+ ion, with the negative oxygen side facing inwards.
Similarly, about 6 water molecules surround the negative Cl- ion, with the positive hydrogen side facing inwards.

Water is more strongly attracted by these Na+ and Cl- ions than it is by other water molecules, so the salt water does not tend to form ice crystals until lower temperatures than fresh water.

A strong solution of salt can decrease the melting point of water by as much as 20C; more dilute solutions are enough to prevent ice forming on the road when the temperature drops slightly below 0C/32F.

See: http://en.wikipedia.org/wiki/Sodium_chloride#Road_salt
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: How does salt melt snow/ice?
« Reply #2 on: 16/05/2013 12:38:03 »
Hi everyone, I just want to know how salt actually melts snow and ice, what is the chemical action or reaction going on? :)
For a reaction to happen, the Gibbs free energy G have to decrease, that is Delta G < 0.
At constant temperature Delta G is = Delta H - T Delta S
where H is the enthalpy and S the entropy. The melting of snow/ice is an endothermic process, that is Delta H > 0. But the dissolution of salt in water increases the system entropy S, so Delta S > 0. If this Delta S is high enough, given a certain temperature, the entire process can have negative Delta G, as in this case, so the reaction is "spontaneous" that is, "happens".

 

The Naked Scientists Forum

Re: How does salt melt snow/ice?
« Reply #2 on: 16/05/2013 12:38:03 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length