Portrait of a molecule part 1 - Oxygen

Every other week, if I get the time, there will be a portrait of a molecule, adapted from John Emsleys "Molecules at an Exhibition" just for your reading pleasure.

This weeks molecule is the most important gas in the atmosphere, making up 21% of the volume of dry air. In it's absence or where it gets depleted we would die, as it is essential for respiration.

This weeks molecule is Oxygen.

Early mountain explorers thought that there was less oxygen as we get higher up, but there is still 21%, but the pressure is too low to extract it from the air. If we remember back to our lesson on pressure, where you lot got very confused and picture the particles of air up a mountain and at the bottom of a mountain the ones at the bottom will be "squashed" closer together by the weight of all the air particles above it so would be closer together. As we get higher up there is less pressure from above and hence the particles move further apart. When we breathe air at low pressure there is therefore less particles per breath and so up a mountain we take in less oxygen (and other molecules) with every breath so it becomes harder to breathe. (Can you explain why mountaineers have to take tea that brews at a lower boiling point when they climb mountains using the particle theory and the universal gas law?)

ON May 29, 1953 Tenzig Norgay and Edmund Hilary became the first men to climb Mount Everest, which they did with the help of oxygen cylinders. 40 years later Harry Taylor, a 33 year old ex-SAS officer climbed to the summit alone, without extra oxygen. In 1975 the first woman to scale the peak, Junko Takei of Japan took an oxygen sylinder and in May 1996 the late Alison Hargreaves became the first woman to achieve this feat without the aid of oxygen.

Oxygen is vital for our body but equally it can be lethal. There is a lower and upper limit to the amount of oxygen in the air if it is to be considered safe. If we are not to suffocate, the amount must be above 17%. If we are not to burst into flames the oxygen level must be below 25%. When we consider that the amount on Earth is 21% it is really quite a fortunate coincidence that we are here.

This upper limit of oxygen was used as the plotline by the most amazing comics writer of modern times, the British writer Alan Moore. In his now-classic epic "Swamp Thing", which I highly recommend as one of the most groundbreaking novels across any media, the villain of the piece takes control of the the worlds flora and fauna and holds the world to ransom by pushing the oxygen level of the world up through photosynthesis, while the worlds superheroes look on powerless. The world suddenly becomes a very flammable place and it is up to the hero of the piece to save the day.

We can breathe in oxygen-enriched air, as many sick people do but it does come with associated risk. Hospital patients inside oxygen tents have suffered horrific burns when they have tried to light a cigarette (duuuuuuuuuuh! I have no sympathy for smokers...) The three astronauts destined for the first manned Apollo flight in Earth orbit were burned alive in minutes in their spacecraft on January 27, 1967, when fire started in the oxygen-enriched air of the cabin.

But generally it is too little oxygen that poses the biggest threat to life. The Biosphere project in Arizona came to a premature end in January 1993 when 8 people sealed in a glass-walled ecosystem were left gasping for breath. The project was to investigate if it was possible to sustain human life in a space station or on the moon. Somehow, over a few weeks 30 tons of oxygen had disappeared and the oxygen level fell below the critical level of 17%. It is thought that it had probably reacted with iron in the soil.

This property of iron, that it will readily react with oxygen, is utilised by our bodies. You will learn all about this in Biology. Oxygen is attracted to Haemoglobin in our blood, which contains iron, and is thus efficiently transported to where it is needed. Most, but not all, species use iron as the oxygen carrier. Spiders and lobsters use copper, which is why their blood is blue. Because of iron, a litre of blood can dissolve 200ccs of oxygen, fifty times more than will dissolve in the same volume of water.

Molcules of oxygen gas consist of 2 oxygen atoms, but the bond still puzzles chemists. It appears to be a double bond, and yet the molecule still has 2 rogue electrons which means that it is a so-called 'free radical'

Oxygen gas will liquefy at -183 Celsius and the liquid is magnetic, as Michael Faraday discovered in 1848 when he spilled some and watched it run towards the poles of a magnet; it behaves like this because of the 2 free electrons. (it's observations like these that make good chemists and this should be a lesson to everyone - make a note of everything you observe as it may be useful)

In theory it should react instantly with anything it touches, and yet we know that oxygen is a fairly unreactive molecule, otherwise it would not have built up over millions of years to comprise a fifth of the Earth's atmosphere. Even when it enters our bodies it does not react chemically with its target molecules, but needs an enzyme to catalyse the reaction.

There are a million billion tons of oxygen gas circling the globe, and all of it is produced as a byproduct of photosynthesis in plants. The seven billion tons of fossil fuel we burn each year consumes around 24 billion tons of oxygen, which is only 0.00024% of the total, and plants replace most of it. Even if plants did not replenish the oxygen in the atmosphere, it would take over 2000 years at the present rate of depletion for the oxygen level to fall from 21% to 20%

Our brain must have oxygen to function, and without it this vital organ will begin to die within minutes. There is a critical time after someone stops breathing that you become brain dead, where doctors know that resucitation will probably do more harm than good.

Less well known is that too much oxygen will poison the brain. This threat is not appreciated by many sports divers, according to Kenneth Donald of Edinburgh University, Scotland, who has made a lifelong study of the subject. In his book "Oxygen and the Diver" Donald warns against breathing pure oxygen below 25 feet, since this can lead to convulsions and several divers have drowned. Instead of using compressed air, amateur divers such as undersea photographers, bounty hunters and archaeologists have takent o using so-called nitrox mixtures, which is air with a boosted oxygen content - but this too can be dangerous. Nitrox is a mixture of nitrogen and oxygen, and was developed by the British Navy in World War II for divers disposing of mines, because it allowed more time underwater while avoiding oxygen poisoning and decrompression sickness (the 'bends') Today professional divers use a costly mixture of oxygen and helium (remember your work from elements, compounds and mixtures?) which allows them to work safely at 500 metres and below.

Oxygen is produced industrially by distilling liquefied air, and is either made on site, or delivered via a pipeline, or transported in specially insulated tankers. In the USA production is 25 million tons a year, and in the UK it exceeds 4 million tons. Over a half goes to making steel, about a quarter to making ethylene oxide, which is turned into antifreeze or polyester for fabrics and bottles, and the rest is used as the gas itself, either in medical care, or to purify sewage and so prevent environmental disasters like the one in Paris in 1992. A violent storm caused raw sewage to flood the river Seine, and this rapidly used up the oxygen in the water and killed all the fish. There are now giant pumps to bubble 15 tons of oxygen gas a day into the Seine.

Who first discovered oxygen? The credit usually goes to Joseph Priestly, who was born in Leeds, England. He was a nonconformist preacher and left-wing intellectual who supported the aims of the French Revolution, and an amateur chemist who specialised in studying gases. He discovered oxygen in 1774 after he moved to Lord Shelbourne's estate at Calne in Wiltshire, and it was there that he heated mercury oxide and collected the gas which it gave off. He breathed his new gas (all in the name of science! What a hero! I don't suggest any of you do this in class....) and reported how light-headed it made him feel. He also noted taht a mouse could survive much longer in this new gas than in ordinary air. Priestly moved to Birmingham but there his house was ransacked by a right-wing mob. (That doesn't surprise me at all, as I lived in Birmingham for 3 years and that's the sort of place it is. Horrible. Avoid it like a plague, though just up the road, Stratford upon Avon is very beautiful and was the birthplace of Shakespeare.) Perhaps not surprisingly he eventually moved to the USA. Heck after living in Birmingham for 3 years I moved out here! See the effect that place has!

Little did Priestly know, that Carl Scheele at Uppsala, Sweden, had made oxygen a few months earlier, but had failed to gain the credit that was due because the publisher to whom he sent his manuscript did nothing to publish it. Neither Priestly nor Scheele was responsible for naming the new gas. 'Oxygen' was chosen by the great French chemist Antoine Lavoisier (look this guy up on the web people - he is a chemist of great importance). The name oxygen is actually wrong as it means 'acid-forming', because Lavoisier thought, wrongly, that this element was an essential component of acids. In fact the essential component of acids is Hydrogen.

But could there have been a previous discoverer of oxygen? There is evidence that oxygen was produced 150 years earlier. How else do we explain a remarkable event that happened in London in 1624, when King James and his subjects turned out in their thousands to watch a new wonder of the age: a submarine. This remarkable craft consisted of a wooden framework covered by a watertight, greased leather skin. It was manned by 12 rowers whose oars protruded through sealed ports. With its Dutch inventor, Cornelius Drebel, on board with a few other passengers, it sailed for two hours underwater from Westminster to Greenwich (near my house back home). The Admiralty was not impressed and advised against its adoption.

This mysterious journey was still being talked about 40 years later by no less a scientist than Robert Boyle (of Boyles Law fame) He wrote that one of the passengers, then still alive, had said that when the air in the submarine had been consumed, Drebbel was able to refresh it with pure air from a container. It has been suggested that this purer air must have been oxygen.

One explanation is given by Zbigniew Szydlo in his book "Water which does not wet hands", in which he says that Drebbel was conversant with the work of the Polish alchemist Michael Sendivogius, who lived from 1566-1636, and who knew of a gas which he referred to as 'the aerial food of life'. 'Water which does not wet hands' was Sendivogius' code name for nitre. Sendivogius had observed that when nitre (the old name for potassium nitrate) was heated, gases were evolved. Gentle heating of this salt produces oxygen. In those days nitre was collected from the walls of cellars and latrines (that's toilets to you and me. mmmmm...nice), where it grew as white crystals, or from the leachings of manure and soil. mmmmmmmm...niiiiiiiiiiiiice.... Nitre was gathered on a commerical scale because it was needed to make gunpowder.

Nitres curious ability to produce oxygen might have been known to John Mayow (1641-1679), an Oxford chemist and early fellow of the Roayl Society of London. This is the premier body for scientists and any of you who really want to be scientists should aspire to join these guys when you grow up! He wrote about 'nitro-aerial particles' which came from nitre when it was heated, and this phrase too is thought to refer to oxygen. It has even been suggested that the alchemists' Elixir of Life was not a liquid as popularly supposed, but might have been this secret gas, oxygen.