Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Why can adding salt to ice water make the ice melt slower?  (Read 10722 times)

0 Members and 1 Guest are viewing this topic.

npeters

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 3

I just read an article at this address

http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-cools-icewater.shtml

and I am now thoroughly confused.

 I understand that adding salt to an ice/water mix will lower the freezing point, thereby making the ice melt. Melting ice absorbs heat from the surroundings, thereby lowering the temperature of the solution, in turn slowing the melting of the ice. The ice/water mixture without salt will also be melting and thereby lowering its temperature. In addition, as this pure ice/water solution gets colder, the ice in it will refreeze more easily than in the salt/ice/water solution because in the pure mixture there are no solute particles to interfere with refreezing. Thus, the salt/ice/water solution should have a greater rate of melting compared to refreezing than the pure solution.

Why on earth would the ice in the salt/ice/water solution melt more slowly?
Logged

Mitch

  • In Vivo Chemist
  • Administrator
  • Sr. Member
  • *
  • Mole Snacks: +352/-81
  • Offline Offline
  • Gender: Male
  • Posts: 5003
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
Re: Why can adding salt to ice water make the ice melt slower?
« Reply #1 on: April 28, 2009, 08:39:31 AM »

Logged
Most Common Suggestions I Make on the Forums.
1. Always start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

npeters

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 3
Re: Why can adding salt to ice water make the ice melt slower?
« Reply #2 on: April 28, 2009, 09:26:38 AM »

I have looked into colligative properties. Based on what I read, ice melting slower in salt water just doesn't make sense. That's why I posted to begin with.
Logged

syd

  • Regular Member
  • ***
  • Mole Snacks: +1/-0
  • Offline Offline
  • Posts: 18
Re: Why can adding salt to ice water make the ice melt slower?
« Reply #3 on: April 29, 2009, 05:31:06 AM »

I may be wrong here:

Adding salt to the water will lower its freezing temperature.  Therefore, the water will be liquid at a lower temperature.

We can agree that water at a temperature of 40 degrees centigrade will melt ice slower than water at a temperature of 50 degrees centigrade.

It probably depends on your setup, but if you had enough ice in the water, and added salt, the water temperature would be able to get colder without freezing.. while the water that is locked up in the ice cubes will have no salinity, and a normal melting point.

Something like that?
Logged

typhoon2028

  • Full Member
  • ****
  • Mole Snacks: +18/-12
  • Offline Offline
  • Posts: 251
Re: Why can adding salt to ice water make the ice melt slower?
« Reply #4 on: April 29, 2009, 08:06:01 AM »

Consider the following:

1.)  Ice floats in water, due to lower density of the ice.  A salt water solution will have a higher density.  A pure ice cube should float more in a salt water solution.  This could change the surface area of the ice exposed to the salt water.

2.)  Is the salt solution being mixed?  The melting ice will form a concentration gradient.

3.)  What is the quantity of ice compared to quantity of salt solution?

4.)  How much salt is in solution? 

5.)  Will the salt solutions temperature drop to a point, where salt will come out of solution?
Logged

ARGOS++

  • Sr. Member
  • *****
  • Mole Snacks: +199/-56
  • Offline Offline
  • Gender: Male
  • Posts: 1493
Re: Why can adding salt to ice water make the ice melt slower?
« Reply #5 on: April 29, 2009, 08:52:57 AM »


Dear npeters;

IMHO: I agree with you that slower melting ice doesn’t make any sense to me too.

Why?:
The force that controls the speed for melting is the energy-flux from surrounding to the ice/water mixture.  And the measure of this flux is the Temperature Difference/Gradient (TDG) between surrounding and the ice-mixture, at least as long as we hold the active surface constand (a little more precise: dE/dt da  =   k * dT/dt  * A).

Conclusion:
The DTG for ice/water/salt (25°C - -21°C) is nearly twice as much as for only ice/water (25°C – 0°C), and that means, that the ice in ice/water/salt must melt faster and not slower.
And that’s also what my experience tell me.
(IMHO: The opposite is against second thermodynamic law)

Good Luck!
                    ARGOS++
Logged
Formulas, in its best sense,  ARE ONLY Recipes for “A Picture”,  —
     If you DON’T catch “The Picture”, you are lost, - for ever!      (A++)

There is ONLY one correct Formula for the “Hydrogen”:  —
     The Atom/Molecule, ITSSELF!                          (Dr. R. Mory  1968)

"Make it AS SIMPLE AS possible,  — BUT NEVER SIMPLER!"  (A. Einstein)

npeters

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 3
Re: Why can adding salt to ice water make the ice melt slower?
« Reply #6 on: April 29, 2009, 10:22:22 AM »


1.)  Ice floats in water, due to lower density of the ice.  A salt water solution will have a higher density.  A pure ice cube should float more in a salt water solution.  This could change the surface area of the ice exposed to the salt water.


I bet that's it. It would be interesting to observe the melting rates if both cubes were kept under water the whole time
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.066 seconds with 23 queries.