Lower melting point or increase the melting point?

Question

Well, when you apply electrolysis, you often add some other substances to lower the melting point of the reactant,

But adding salt to ice can makes ice melts slowly, right?

I wonder why. Could anyone please help me?

Answer 1

Lancenigo di Villorba (TV), Italy

ELECTROLYTICAL MOLTEN MASS
As you wrote, Metalurgical Industries often have to apply Electrolytical Processes.
Sometimes, these latters interests Very Active Metals so the processes cannot involve water or moist phases : I think to Aluminium's Production which need Liquid Phases to permit Matter's Transfer since Solid Matters avoid it.
Thus, the processe need to melt the Solid Mass to form Molten One : ALUMINA was involved as Aluminium's Source at the beginnings of the XIX century (e.g. H. Davy and J. J. Berzelius) but F. Wohler and H. S. C. Deville shown that ALUMINIUM CHLORIDE results a better raw.
In the second half of XIX century, H. P. T. Heroult begun to involve a different Aluminium's Compound (e.g. CRYOLITE) which resulted able to modify the ALUMINA Properties.
Instead ALUMINA alone, the new mixture is characterized by a very lower Melting Point, so Heroult led to a very cheaper process.

KITCHEN SALT UPON ICE
In the winter season, European Routes may breeze during the very cold nights...As our scientist stated, Kitchen Salts results very effective in order TO AVOID ICE FORMATION UPON COLD AND WET SURFACE.
Now, you may state that KITCHEN SALT ACTS IN ORDER TO DIMINUTE THE MELTING POINT.
How it happens?
In the former half of the XIX century, a french scientist (H. M. Raoult) executed some very interesting experiments : he remarked that WATER EXERCITES A VAPOUR PRESSURE's VALUE GREATER THAN EVERY SALT's AQUEOUS SOLUTIONs.
Taking over this trials, Raoult began a Experimental Series leading to state another interesting Principle :
"ONCE ASSIGNED THE SOLVENT's NATURE (e.g. water), TEMPERATURE AND SOLUTE's CONCENTRATION ARE THE LONELY PARAMETERs ABLE TO DETERMINE THE PHASE EQUILIBRIA OF THE SOLUTION".
This is the most general statement known as "COLLIGATIVE PROPERTY's LAW" where scientist may interpretate all the experimental remarks related to the following cases :

-) VAPOUR vs. SOLUTION equilibria named as EBULLIOSCOPIC EFFECT (e.g. HEAVY SOLUTEs ENHANCE BOILING POINT when compared against Pure Solvent) ;
-) SOLID vs. SOLUTION equilibria named as CRYOSCOPIC EFFECT (e.g. HEAVY SOLUTEs DIMINUTE MELTING POINT when compared against Pure Solvent).

The latter case shows you that Kitchen Salt diminute the Melting Point....ICE WILL GET SALT so ICE WILL MELT!

I hope this helps you.

Answer 2

I think you may have two things mixed up. One adds other ionic substances (electrolytes) such as sodium chloride to water to increase the conductivity.

It is true that sodium chloride and other soluble ionic and nonionic solutes also lower the freezing/melting point of water. There is a complicated explanation for this that involves the lowering of the vapor pressures of solvents by solutes.

Answer 3

It is just due to increase in the entropy. As a result, the melting point otherwise called as freezing point depression occurs.