Liquid Nitrogen

We started with an insulated foam box, about half filled with liquid nitrogen

• The Chemistry Department has liquid nitrogen delivered since we use it for cooling in some chemistry laboratories
• It's quite cold (Our temperature gauge read about -200^oC in class)
  • It needs to handled carefully to avoid frostbite
• it looks like a fairly normal liquid (clear and colorless, pours easily, about same density as water)
• being very cold it forms a fog in the area
  • cools the air nearby and causes water vapor in the air to condense into tiny liquid droplets
  • that's what fog is -- tiny droplets of water, suspended in air
  • also forms frost on outside of the container (condenses water vapor and freezes it)
• it's actually at its own boiling point-- so it looks more like boiling water than a cup of water
  • any hot source causes it to boil quickly-- almost burst into a cloud of fog.
  • You could see this when we threw some on the floor (to liquid nitrogen the floor is very hot.)
• new term: cyogenics (the science or technolgy associated with very cold)
• uses of liquid nitrogen if you are interested
  • as a very fast way to cool substances
    • it's an exciting way to make ice cream in about 20 seconds
    • it's portable-- sometimes used with refrigerated trucks to avoid expensive mechanical refrigerators.
  • to store biological substances-- at these temperatures they won't decompose or spoil / bacteria and fungus are inactive / normal development ceases
    • used to maintain pure cultures of bacteria, seeds for long periods of time
    • used to store sperm, eggs, embryos in animal breeding and in human fertility applications.
    • used by those who believe / hope that science will find a way to bring them back to life many years from now--
  • also used as a convenient way to store and ship nitrogen gas-- the liquid is a very compact form of the element and no high pressure tanks are required.
  Balloon collapses when dunked in nitrogen
  • volume of a gas decreases when the temperature drops
  • also the oxygen (20%) condenses completely
  • the gas is still there-- balloon reinflates as it warms up

Dry Ice -- Solid Carbon Dioxide

• also very cold (we measured about -50^oC in class)
• also produces fog by cooling air
• when thrown in water it sinks (dry ice is more dense than water), forms bubbles and lots of fog.
  • we added it to a blue dye solution
  • the dye is an acid-base indicator (you may be more familar with litmus as acid color test)
    • changes color to green/yellow as the solution becomes acidic
    • color changes tell us CO₂ acts as an acid (weakly)
  • You didn't see any liquid-- under normal conditions CO₂ sublimes
    • goes directly into a gas without forming a liquid ; the process is called sublimation
    • the liquid does exist (but only under pressure)
    • CO₂ fire extinguishers are filled with pressurized CO₂

The Blue Bottle

• Contains sugar and water, a dye and a strong base (potassium hydroxide)
  • The air space contains 20% oxygen and the water contains dissolved oxygen
    • Oxygen dissolves in water-- that's how fish live and "breathe"
  • The sugar reacts with oxygen, removing it from solution
• The dye, called Methylene Blue has two forms-- the blue form has oygen that is easily lost
  • When the solution loses oxygen, the dye loses its oxygen to the solution and turns colorless
  • The form without oxygen is a colorless species
  • Shaking the bottle dissolves more of the oxygen from the air, and we get our blue form of the dye back again.
    • you are not responsible for knowing the details of this stunt.

Fire and Air

• The candle needs the oxygen in air to burn
  • In a closed container, the candle goes out as the oxygen is consumed.
  • Since CO₂ has a higher density than air it can be poured into the beaker with our fire.
  • CO₂ extinguishes the fire simply by displacing the air and thus removing much of the oxygen.
  • We were able to pour nitrogen gas from our liquid nitrogen container and also extinguish the fire.
• We also added oxygen (from a small tank) and the fire responded by burning brigher and hotter