Arrg, the science nerd in me is coming out..... (insert picture of nerdy little guy ripping off a labcoat while turning green)
Originally Posted by Golddiggie
I think people believe that at 212F water magically changes into steam (at sea level) and you cannot go above that. Not so. That's where water starts to boil. A HARD boil will be above that in temperature. Add some pressure (via a 'pressure cooker') and you can go even higher.
While OT, this is completely wrong. Phase changes (i.e. liquid->gas) require energy, meaning that while you drive a substance through a phase change its temperature does not change, as the energy (heat) you are adding is being used to change the phase of the fluid, rather than contributing to heating it. This is called the enthalpy of vaporization, and for water this is 2.26J/g.
For example, if we were heating 10ml of water (0.3oz) using a heating source that provided 1 watt of heating power, the water would warm 1C every 10 seconds (water requires 1J of energy per ml to increase 1C; 1W = 1J/s). When it hit boiling temperature this increase in temperature would stop, and the water would convert to steam. It would take 2.26J/ml (22.6J) to convert our 1ml of water into steam; equivalent to 23sec of heating using our wimpy heat source.
In plain english this means is when you boil water, the temperature of the water literally stops at 100C/212F (or whatever your altitude-adjusted value is). It remains at this temperature until the phase change is complete. The difference between a simmer and a hard boil is small; less than 1C.
Originally Posted by neosapien
super off-topic, but i'm a nerd...in this chart, what is the "critical point" of water? what does it do?
somebody with science please enlighten me?
Phase changes are dependent on 2 things - temperature and pressure. For example, water will boil at higher temps in a pressure cooker because the pressure is higher; in contrast, water will boil in space at body temperature because there is no pressure. Critical points are points where temperatures/ pressures above them create a unique form of matter called a supercritical fluid. These "fluids" are a hybrid between a gas and a liquid - they will dissolve things like liquids do, but will engage in gas-like behaviours like effusion (moving through solids). Supercritical steam is used for power generation, so its not just a lab thing.
A similarly weird point is the triple point, where the combination of temperatures cause the substance to simultaneously exist as a solid, liquid and gas.