Demonstration 30

Demonstration 30

Air - Liquid gases.

These demonstrations cover a variety of topics including colours of liquid gases, the effects of low temperatures on physical properties of various materials, expansion of gases with temperature, etc. Some physical properties of oxygen and nitrogen are listed in the following table.

	O₂	N₂
Normal boiling point at atmospheric pressure, ^oC	-183	-196
Ration of volume of gas to volume of liquid, measured at 15^oC and absolute pressure of 101.3 kPa	842	682
Relative density of gas at 101.3 kPa and 25^oC (Air = 1)	1.105	0.967
Liquid density at absolute pressure of 101.3 kPa, kg m^-3	1141	807
Latent heat of evaporation, kJ kg^-1	213	199

Liquid nitrogen is a colourless liquid but liquid oxygen is pale blue in colour and paramagnetic because it has two unpaired electrons. Nitric oxide (NO), the gas in the tubes in Experiment 22, when liquefied forms a deep blue liquid (b.p.-151.7 ^oC) and is also paramagnetic. Oxygen is the most abundant element on the Earth. It was first obtained by Scheele in 1771 and independently by Priestly in 1774. Oxygen is twice as soluble in water as nitrogen (oxygen solubility in H₂O is 3.125 % v/v at 20 ^oC), which is good news for obligate aerobes and bad news for obligate anaerobes.

Preparation. The following materials are required: two unsilvered (demonstration) Dewar flasks, rubber tubing and a small rubber ball,‡ three vacuum (Bunsen) flasks with stoppers and party balloons tightly fitted to the side-arms,† a model steam engine. The boiling point of liquid oxygen is 13^oC higher than that of nitrogen and so can easily be made by condensing the gas using liquid nitrogen as a coolant. To do this make a coil of small diameter copper tubing so that it fits easily into a Dewar flask. Fill the Dewar with liquid nitrogen, and using a suitable length of plastic tubing, attach one end of the coil to an oxygen gas supply, and pass the gas at a moderate rate through the coil. Oxygen will condense in the coil and can be collected in a second, pre cooled, Dewar (or Thermos flask) via flexible tubing or by having bent the copper tube to a suitable shape at the distal end.

Demonstration. Show to the liquid nitrogen and oxygen by pouring them into clear Dewar flasks. Point out that the liquid gases are constantly boiling at room tempeature and pressure as it is almost 200 degrees above their boiling points. Compare the colours of liquid nitrogen and oxygen. Set the liquid oxygen aside (it will be used later in Experiment 33). Immerse a piece of rubber tubing and a rubber ball into liquid nitrogen. Point out the extra vigour with which it boils when an object at room temperature is placed in it. This is comparable to putting a hot poker into water. After several minutes (when boiling stops) take the rubber tubing from the liquid nitrogen and demonstrate that it goes brittle on cooling, for example by shattering it with a hammer. Then take the Dewar flask containing the remaining liquid nitrogen and the rubber ball and pour the liquid on the floor. When rubber ball drops on the floor it shatters into pieces.

Pour some liquid nitrogen into the vacuum flasks connected to the balloons and stopper them tightly. As the nitrogen boils the gas expands and inflates the balloons. If there is enough liquid nitrogen in the flasks the pressure built there will cause the balloons to explode. If you have a model of a steam engine you can pour some liquid nitrogen into the boiler and make the engine run on boiling liquid nitrogen instead of steam.

There are considerably more experiments that can be done using liquid nitrogen and oxygen [1-3]. Liquid nitrogen can be obtained from universities, higher education institutions, hospitals and industry. It is very inexpensive (about 10 p per litre) and can be stored and transported in ordinary vacuum flasks. A 1.5 litre vacuum flask full of liquid nitrogen will still be about 2/3 full after 24 hours [1]. Polystyrene cool boxes can also be used to store the liquid for shorter periods of time. Vacuum flasks containing liquid nitrogen cannot be stoppered and a good way of transporting liquid nitrogen in a car is to close the flask using the drinking cup/stopper in which a small hole is made. A plug of mineral wool can be used to improve the insulation [1]. It is also a good idea to drill several holes in the bottom of the outer (metal or plastic) case of the flask - these will allow the quick release of pressure should the glass Dewar inner flask be accidentally broken; ours have three 25mm diameter holes. It is needless to say that the flask must be kept upright during transporting. Remember that the increase in volume from liquid to gas approaches a factor of 700

Safety. Because of the low temperature of liquefied atmospheric gases the liquid or even vapour or gas can produce damage to the skin similar to heat burns. Unprotected parts of the skin coming in contact with uninsulated items of cold equipment may also stick to them and the flesh may be torn on removal. Cold vapours or gases from liquefied atmospheric gases may cause frostbite given prolonged or severe exposure of unprotected parts. A symptom is local pain that usually gives warning of freezing but sometimes no pain is felt or it is short-lived. Frozen tissues are painless and appear waxy, with a pale yellowish colour. Thawing of the frozen tissue can cause intense pain. Shock may also occur. The immediate treatment is to loosen any clothing that may restrict blood circulation and seek immediate hospital attention for all but the most superficial injuries. Do not apply direct heat to the affected parts, but if possible place in lukewarm water. Sterile dry dressing should be used to protect damaged tissues from infection or further injury, but they should not be allowed to restrict the blood circulation.

References

1. T. Lister, Classic Chemistry Demonstrations, ed. C. O'Driscoll and N. Reed, London, Royal Society of Chemistry, 1995, pp. 58-62.

2. M.A. Ivanova and M.A. Kononova, Chemical Lecture Experiment, 2-nd edition, Moscow, Vyschaya Shkola, 1984, pp. 29-34 (in Russian).

3. Tested Demonstrations in Chemistry, ed. L. Gilbert, et al., Granville, OH, Denison University, 1994, vol. 1, pp. D-36 - D-41.

‡ It is rare now to come across these cheap hollow rubber balls. MH found these in Seahouses, Northumberland, last year and immediately bought up their entire stock of twentythree at 20 pence each! From the ‘Teacher Notes’ which accompany our ‘Chemistry is Fun’ video, we can see that some nine years earlier Mike’s predecessor Don Whitely also had similar problems, but found some in Whitby, North Yorkshire, about 90 miles south of Seahouses. One wonders whether this northwards migration of hollow rubber balls, at ca. 10 miles per year, could be taken as an indicator of global warming!

† An easy way of doing this is to put a short length of thick vacuum tubing on the glass side-arm of the flask, pull the neck of the balloon on the rubber tubing and secure the connection using a plastic self-fastening clip or a piece of string.