2
$\begingroup$

I did an online test for physics and one of the questions was: Salt was added to ice. Because of it the ice melted. What was happening with temperature DURING the process of melting? The right answer was that temperature was lowering.

Can please someone explain why? I always thought that during melting the temperature is constant and after all ice is melted then the temperature changes.

Thank you very very much

$\endgroup$
5
  • $\begingroup$ You have to account for freezing point depression of salt water. $\endgroup$ Aug 29 '20 at 19:21
  • $\begingroup$ I understand that salt lowers the freezing point of water but why is the temperature changing during melting. Why isn't working the rule that during melting energy is used for state transition and the temperature stays same? $\endgroup$
    – Sanvo
    Aug 29 '20 at 19:27
  • $\begingroup$ That rule is only true for the melting of a homogeneous substance. $\endgroup$
    – Karl
    Aug 29 '20 at 19:33
  • $\begingroup$ Oh and could be so nice please and explain me a bit more what happens during the melting in this case? Where is the energy used in this case when temperature changes? $\endgroup$
    – Sanvo
    Aug 29 '20 at 19:40
  • $\begingroup$ Salt water freezes below 0°C, precipitating ice with significantly lower salt content. Hence, if ice melts in salt water, it cools down to a lower temp. While it melts, the salt concentration in the water would decrease of course, and the temperature would slowly rise towards 0°C again. The energy goes into melting the ice, obviously. The opposite of heat of fusion. $\endgroup$
    – Karl
    Aug 30 '20 at 12:34
1
$\begingroup$

This is called a "Colligative property" - which include depression of the freezing point and elevation of the boiling point (plus 2 others). These properties increase in effect as more solute is added. I believe adding salt to ice only melts it if there is already liquid water present, which there usually is (it might be a very thin coat of water on the ice). This thin layer dissolves some salt, which reduces the freezing point of the liquid water (salt solution), leading to the surrounding ice to melt, which dissolves more salt, melting more ice etc. These properties are more related to the amount of solute, rather than the type of solute (at least in an ideal solution).

Reference

  1. Colligative Properties of Simple Solutions, Frank C. Andrews, Science 05 Nov 1976: Vol. 194, Issue 4265, pp. 567-571
$\endgroup$
2
$\begingroup$

Here is a naive interpretation, that may help you understand this phenomena.

In the presence of salt, water is attracted and "wants" to be adsorbed around $\ce{Na+}$ and $\ce{Cl-}$ ions. But this operation requires some energy for water molecules to separate the positive and negative ions. This is why the dissolution of $\ce{NaCl}$ in water is endothermic.

Now if you put salt in contact with ice, water in ice "wants" also to be attached to surrounding ions. But it cannot move. It first have to be a liquid. And there is no heat in the surroundings to make ice melt. Nature has found a way. Every piece of ice is divided in two parts. One part gives heat to the second part, that melts, makes water to dissolve the salt. But the remaining part has lost heat. So it gets cooler and cooler. The temperature of the remaining ice decreases from $0$°C to about -$20$°C.

$\endgroup$
2
$\begingroup$

After reading the wording carefully, I believe the question and the OP are asking, during melting, how does the temperature of system change (if any)? The question is not asking what happened to melting point of ice.

The temperature decreases is simply because melting of ice is endothermic (and assuming the system is adiabatic so the energy of system but not of surrounding is used for melting). It melts because the system is moving towards most thermodynamically favourable composition.

Considering pressure at 1 atm,

  1. if you consider melting of pure ice at constant temperature (isothermal), the melting is endothermic too but the net effect is that energy used for melting is taken from surrounding not from system, so the temperature of system does not change, all ices melt into liquid water if that constant temperature > 0 °C;

  2. else if you consider melting of pure ice adiabatically (no heat enter nor leaving the system), the temperature of system will drop as melting goes because the system uses its own energy for melting, until equilibrium temperature (0 °C) reached, and then the melting stop. Solid and liquid present at equilibrium, not all ices melt;

  3. else if you consider the melting of ice in the presence of salt isothermally, melting happens if that constant temperature > -21.1 °C.1 Why -21.1 °C but not 0 °C ? That is a thermodynamic effect due to mixing, and apparently this effect is more significant than colligative freezing point depression. Of course the system temperature will not change, same reasoning as (1);

  4. else if you consider the melting of ice in the presence of salt adiabatically, the temperature will drop to -21.1 °C as melting goes, same reasoning as (2).

I think the question is asking for (4), and you are confused because treating (4) as (3).


Reference

1 Peter Atkins, Julio de Paula. Physical Chemistry (8th Edition). OUP. 2006. Page 189, (a) Eutectics

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.