Suppose I have a densely packed bucket of ice chips / cubes made from pure $\ce{H2O}$. It has been at room temperature for days, so the interior temp of the ice is likely close to 0 °C / 32 °F. (I have a working model of this in a clear ice - ice maker in a kitchen that does not have refrigeration; the ice is kept cold solely by the continuous addition of more ice.)
To a measured quantity of that ice (think about an ice bucket to cool wine) if I add:
- 23% of the weight of the ice in $\ce{NaCl}$ (table salt);
- Some water to make a brine solution;
- Stir.
The temperature of the resulting solution declines to roughly -22 °C / -8 °F.
I know that the $\ce{NaCl}$ depressed the freezing point of the water and that is why it can be liquid at this temperature. I suspect that $\ce{NaCl}$ is forming ions in the solution.
My question is: Where does the thermodynamic energy come from to lower a mass of ice and water to a significantly lower temperature?