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THMG058 – Phase Changes


In this episode, we discuss the fundamentals of phase changes (known and physical properties) and how they affect our operations.

Complete Show Notes

3:05 3 Major Phase Changes

  • We’re also talking about a change in the state of matter – always occurs with a change in heat
  • Heat either goes into or out of the material – endothermic vs. exothermic
  • Solids
    • Particles are packed very closely together, which means they aren’t able to move very much
    • Particles in solids have very low kinetic energy, which means they want to go and do things – electrons in each atom are in motion almost constantly
    • This means the atoms have a small vibration, but they’re fixed where they are
    • Solids have a definite shape and don’t conform to the shape of the container they’re in
    • They also have a measurable volume and are pretty simple overall – one of the easiest substances to deal with
  • Liquids
    • Particles in the substance have a little bit more kinetic energy than in a solid – correlates directly with temperature
    • Particles aren’t held together in any definite arrangement – they move around and past each other while loosely interacting
    • They have a defined volume but do not have a defined shape – this means they conform to their container
    • There’s almost zero compression when it comes to liquid – volume won’t change as long as you maintain the same temperature
  • Gasses
    • Particles have a great deal of space between them and very high kinetic energy – there’s a lot of movement
    • If unconfined, particles will spread out almost indefinitely – when confined, gasses spread to fill the entire container
    • Gasses will spread out until they’re so diluted that they aren’t even a concern anymore – as volume expands, your parts per million (PPM) measurement goes down

8:45 2 Other Phases in Chemistry

  • Plasma
    • Not a common state of matter seen on Earth, but it’s very common in the universe on the whole
    • Plasma consists of very highly charged particles with extremely high kinetic energy – all of the electrons are stripped away, so all we have is a nucleus and charged particles
    • You can manipulate plasma with electromagnets
    • Common plasmas include helium, neon, argon, radon, etc. – used all over the place (i.e. neon signs)
  • Bose-Einstein condensates (BEC)
    • New state of matter created by scientists Eric Cornell and Carl Wieman in 1995
    • Combines lasers and magnets to cool substances to within a few degrees of absolute zero (-273 degrees Fahrenheit)
    • At these insanely low temperatures, molecular motion comes to a standstill and atoms come together into a giant mass
    • Used to study quantum mechanics on a macroscopic level – light appears to slow down as it passes through these BECs, allowing the study of particle and wave paradoxes
    • BECs also have many of the properties of a superfluid, which means they’re flowing without friction – they’re frozen beyond recognition and flow like they’re made of grease
    • Used to simulate conditions that might apply in a black hole

15:00 Melting, Freezing, Boiling, and Condensation

  • The point where we change phases is most interesting and relevant for us – no reaction has taken place, but a phase change has
  • Phase changes are generally accompanied by a change in matter (other than water)
  • Melting point
    • The temperature at which a substance heats up from a solid to a liquid
  • Freezing point
    • The temperature at which a substance cools down from a liquid to a solid
    • Super-cooling doesn’t necessarily correspond with freezing point
  • Boiling point
    • The point at which vapor pressure equals atmospheric pressure/temperature and/or the temperature at which boiling occurs under the pressure of 1 bar
    • Heat of vaporization – amount of energy required to convert or vaporize a saturated liquid into vapor (changes based on location)
    • Adding pressure also affects the boiling point of each substance
    • The addition of salt (or any impurity) elevates the boiling point for anything that dissolves in a liquid
    • Evaporation – surface phenomenon in which molecules at the surface of a liquid escape into the atmosphere as vapors
    • Evaporation rate – the amount or the speed at which those molecules are moving from a liquid to a gas
    • Evaporation is different than boiling because only the molecules at the edge of the liquid are going into the atmosphere – with boiling, molecules anywhere in the liquid escape
    • Liquids that evaporate much more readily than others and quickly change into a gas, we refer to it as being volatile
    • If you increase the surface area of a volatile substance, you increase the volatility
  • Condensation point
    • This is when vapors or gasses return back to the liquid phase (i.e. dry ice and iodine)
    • Sublimation – this is when a material converts from a solid straight to a gas without any interference whatsoever with no middle phase

28:30 More Important Critical Points

  • Critical temperature
    • The minimum temperature at which a gas cannot be liquefied, no matter how much pressure is applied
    • A compressed gas handbook is very helpful when determining critical temperatures – as hazmat techs, we need to know this stuff
    • For example, if you’re dealing with a liquid that reaches its critical temperature and cannot be kept from becoming a gas, you’ll have a BLEVE – stands for a boiling liquid expanding vapor explosion
  • Critical pressure
    • The pressure needed to liquefy a gas – every temperature range has its own critical pressure – this is where we start to get into cryogenics and temperature curves
  • Auto-ignition temperature
    • Lowest temperature at which a substance will ignite in air when there’s no ignition source
    • We deal with these when we handle diesel trucks that are involved in a fire
  • Self-accelerating decomposition temperature (SADT)
    • Temperature above which decomposition of an unstable substance will continue unimpeded, regardless of the ambient or external temperature
    • In other words, it’s the point at which something that’s ready to go is going to go regardless of what you do – very dangerous point in a lot of chemistry
    • Mentioned frequently in fixed facilities with different mixing tanks and different SADT points – crucial to keep these substances cool to avoid endo- or exothermic reactions
    • This isn’t technically a phase change, but it’s still very important for us to know about
  • Maximum safe storage temperature
    • Maximum temperature at which a product can be safely stored – especially important with peroxides and oxidizers
    • Usually well below the SADT
  • Polymerization
    • Chemical reaction that involves monomers that react with themselves to form long chains of chemicals that are basically unstoppable once they start

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Author: The HazMat Guys