ice melting at -9degrees

[alext87]

I test an experiment on Wednesday about melting ice. Would i did was put ice into a beaker and stick in it a data probe. i then added a salt eg. AlCl3. When I did this the temperature the data probe record suddenly went down by about 7degrees. However the ice melting extremely quickly. How can the salt make the temperture decrease but cause the ice to melt very quickly?

[5614]

taken from about ice: http://www.scienceforums.net/forums/showthread.php?t=4382

When you dissolve something in water' date=' the freezing point decreases. So the liquid can absorb more energy from the solid that allows it to melt. Under conditions where the ambient temperature is a little below 0 C, you can still melt the ice.

 

The freezing point decrease is related to the concentration of solute. Salt does a better job because it's ionic, and you get both Na and Cl contributing.

 

more on freezing point depression: http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/meltpt.html [/quote']

 

it shouldnt make the temperature decrease though...

[jdurg]

If the dissolution of the salt is an endothermic process, it will cause the temperature to depress. (However, that's more typical with ammonium salts and not aluminum, though I'm not 100% sure. I'd have to look that up).

[swansont]

The ice was at a temperature colder than -7 C (or whatever) but the water can't get any colder than the freezing point. Adding the salt lowered the freezing point, so the temperature of the water went down, cooled by the ice. Some energy to cool the water had to come from the ice in addition to any effect the dissolving had.

 

But you have to be careful - as cubes of ice get smaller, the melting rate increases anyway (all other factors being equal) because the surface/volume ratio increases. So you have to be careful what you are measuring.