Film, Media & TV2 mins ago
No best answer has yet been selected by kermit911. Once a best answer has been selected, it will be shown here.For more on marking an answer as the "Best Answer", please visit our FAQ.
hot water contains a lot of thermal (heat) energy which, when the water hits your skin, is shared equally between the water and the skin. this doesn't do good things for your skin, since it can't handle this much energy at once and so you end up with a burn. i'm guessing that really hot water feels cold because it's probably too much sensory information for your body to work with & so the temperature doesn't filter through
this is all a barely-educated guess, but i'm probably not far off.
I'm imagining that very hot and very cold objects contacting the skin were just not commonly in the environment when men and human were being 'built' genetically (in prehistory and pre human). The tuning, I suppose, is for 'warm' and 'coolish' not 'hot' and 'cold', so it isn't properly tuned to send the right signal when the extremes are met.
I'm trying to think of environmental variables, but there aren't any? This tuning was built pre-fire, and when we were living in normalish climates. So we're just not designed for 'hot running water'.
Its to do with kinetics and the thermal energy. The molecules of water in hot water are moving much quicker than the particles in cold water. Should you allow these particles to contact your skin, then they transfer this kinetic energy to your skin. Should the particles of water have a high enough energy, by being at a high enough temperature, then the kinetic energy is sufficient to damage the skin and produce a burn, via the methods mentioned by Newtron above.
On your second question, very hot water feels cold because its causing you pain, the signals of which may be confused as its unlikely you'll be holding say, your arm, in extremely hot water for long enough to sense its "hot" and not just "painful".