Latent Heat

Discussion

discussion

Name the 6 major phase changes (including synonyms).

Heat absorbed or released as the result of a phase change is called latent heat. There is no temperature change during a phase change, thus there is no change in the kinetic energy of the particles in the material. The energy released comes from the potential energy stored in the bonds between the particles.

• exothermic (warming processes)
  • condensation
    • warmer in the shower
    • steam radiators
  • freezing
    • orange growers use ice to stop oranges from freezing
  • deposition
    • snowy days are warmer than clear days in the winter
• endothermic (cooling processes)
  • evaporation/boiling
    • sweat
    • alcohol is "cool"
  • melting
    • melting ice in drinks
  • sublimation
    • cooling with dry ice

Q = mL

Scattered thoughts…

• Steam burns are far, far worse than boiling water burns for water or steam at 100 °C (or whatever your local boiling point of water is).
• Under extreme conditions of heat and exercise, an individual may sweat more than a liter of liquid per hour.
• The interior of roasted meat can never reach temperatures higher than the boiling point of water until all the water is cooked out of it, at which point it would resemble shoe leather. The outside is quickly dried out, however, and can reach the temperature of the surrounding cooking medium.
• Cocoa butter is unique among the fats in that it is very regular in composition; whereas most other fats are actually mixtures. This gives it a definite melting point; unlike butter, which softens gradually. As it melts in your mouth, it absorbs latent heat. This makes chocolate bars taste "cool". Cocoa butter is remarkably uniform in composition and structure: only three fatty acids form the majority of its triglycerides, with the same one occupying the middle position. Pure cocoa butter is quite brittle up to about 34 °C (93 °F), at which point it melts quite quickly.
• Condensing furnaces extract energy from the water vapor produced during combustion. When you burn a fuel like natural gas or propane one of the the byproducts is water vapor — the others are carbon dioxide and (hopefully, not much) carbon monoxide. A standard furnace exhausts all of these to the outside world — but that water vapor is hot and, more importantly, is a source of latent heat. When it condenses into a liquid, as it will inevitably since the outside temperature is well below the boiling point of water, it releases that heat. A condensing furnace forces that process to take place inside the home instead of outside. Why waste that energy on the environment? Use it to your advantage. A condensing furnace comes with a second heat exchanger after the first. (The first is used to extract the heat of combustion directly.) This cools the exhaust gases, but more importantly it condenses the water vapor to extract its precious latent heat. This allows furnaces that would normally extract 80 to 85% of the heat of combustion to reach efficiencies of 90 to 95%.