States of Bromine
A sealed Kjeldahl flask containing bromine is placed in a dry ice/ethanol bath. The maroon bromine vapor pales rapidly as the vapor pressure decreases. Pale yellow bromine crystals form along the side of the flask. This demonstration illustrates the states of matter, the equilibrium nature of their relationship, and the triple point of bromine. For maximum effect, this demonstration should be video projected.
This is a good demo to use when talking about states of matter or equilibrium concepts as applied to phase transitions. Visibility is problematic with this demo. The projected image really doesn't do justice to the beauty of the bromine crystals. Ideally students should file past and see it if time allows. Allow about 10 minutes to perform this demo.
At room temperature, bromine has a vapor pressure of about 30 kPa. The vapor is in equilibrium with the liquid bromine. Molecules of bromine are continuously leaving and reentering the liquid state at the interface. After the bromine is immersed in the dry ice/ethanol bath all three states are present. It is interesting to observe that the crystals deposit directly from the vapor phase on the sides of the flask above the liquid surface. This deposition continues even after the color of the bromine vapor is no longer visible. The continued deposition indicates that even though the concentration of bromine vapor has been substantially reduced, it still plays a role in the deposition process. It is also interesting to note the persistence of a "puddle" of liquid bromine on the surface of the freezing bromine in the bottom of the flask. This demonstrates both the inefficiency of cooling a fluid from the bottom and the low thermal conductivity of bromine.
- 2 sealed 1 L Kjeldahl flasks containing about 20 mL of liquid bromine
- 2 cork rings to hold the flasks upright
- a dry ice/ethanol bath in an insulated container
- Hold the sealed Kjeldahl flask containing liquid bromine up in front of the class for their examination. Ask the class to explain what is happening on the molecular level inside the flask.
- After the class has responded, submerge the bottom part of the flask in the dry ice and ethanol bath. The intensity of the maroon color of the bromine vapor quickly fades and after a minute or two. Pale, yellow needles of solid bromine crystals form along the walls of the flask.
- Hold the flask up again for the class to examine. Ask them what is now happening on the molecular level.
- The dry ice and ethanol bath is very cold. Avoid touching the bath and the bottom of the flask when it is removed from the bath.
- Though the very cold temperature of the bath decreases the vapor pressure of the ethanol above the bath, ethanol and ethanol vapor are quite flammable. The dry ice and ethanol bath should be kept away from sources of open flame. A fire extinguisher should be kept nearby when this demonstration is performed.
- The bromine and bromine vapor inside of the flask are very corrosive. If the flask should rupture, avoid breathing the vapors. Cover the liquid bromine with dry sand and neutralize with 5% sodium thiosulfate solution if possible and evacuate the room.
Be sure to include the bromine spill kit (dry sand and 5% sodium thiosulfate solution) when you prep and deliver this demo.
