# How does a liquid melt a hotter solid?

I don't have a degree in anything, but I do know basic rules of science I believe. It seems pretty intuitive that heat will always flow from something hotter into something colder in order to reach an equilibrium.

My understanding of temperature is that it is a measure of how fast something's particles are vibrating.

I've been very interested in the mechanics of salt water and ice spontaneously dropping in temperature because it seems counter intuitive and goes against everything I seem to know.

As far as I know, pure ice begins to melt at 0°C and above. If I were to place an ice cube at -2°C in oil that is -5°C, it wouldn't melt because the oil has less energy than the ice cube. If anything, energy would leave the ice cube to warm the oil and the cube of ice would get colder.

So, if I have salt water at say -2°C, and I put an ice cube in the mix that is pure ice and is 0°C, how is that colder water going to impart energy into something that's warmer (the ice cube). This seems extremely bizarre and seems to go against basic rules of thermodynamics.

I know that adding more salt would lower the melting point even further, but isn't a temperature a temperature? Does the material matter? Do two substances with the same temperature contain the same kinetic energy on the molecular level? If not, then what is temperature even a measurement of?

Thank you.

• As you said, things want to be in equilibrium. The question has already been answered, but about your temperature thing: Temperature isn't really something that exists on a molecular level. The classic example is: If all particles of a system move with high kinetic energy into the same direction, they still have a temperature of 0K. One can argue that temperature is more of a measurement of how broadly states are distributed rather than something like kinetic energy. Temperature is one of the most complicated concepts we have and every time you try to define it, you run into problems. – AMT Mar 15 '17 at 10:25
• PS: (ran out of space) Rather than wondering what something says about temperature, ask the other way around: How does temperature affect this? Because temperature is something we observe very well as humans and thus have an intuitive understanding of. And btw, I'm not saying we do not understand temperature, it's just that it's not simply kinetic energy and "vibration" – AMT Mar 15 '17 at 10:28

Dissolving a solid in a liquid, say salt in water, can cause a drop in temperature (provided the container is insulated) because it takes energy to dissolve compounds and this can only come from the liquid. (The ions or molecules in the solid must be attracted to one another, by ionic or van der Waals forces, and so to be separated takes energy). The measure is called the heat of fusion (or melting), or enthalpy of fusion which is a positive quantity, for common salt it is $\approx 30 \pu{~kJ mol^{-1}}$ and of ice $\approx 6 \pu{~kJ mol^{-1}}$.