When water and methanol mix,

[wonders]

what causes the temperature increase?

[insane_alien]

water and methanol are mutually soluble. essentially it is the same thing that happens when you add sulphuric acid to water but with a liquid instead.

[wonders]

I don't really get what you mean. What does solubility have to do with change in temperature?

[YT2095]

look up Hydration enthalpy.

[insane_alien]

well, technically it is the hydration enthalpy. a couple of water molecules will form stronger than average intermolecular bonds with the methanol molecules. the formation of these bonds releases energy which we detect throuhg the temperature rise.