Mythbusters

mythbusters

With the meeting drawing towards a close, I chose this morning to attend a session in the division of chemical education. It was the symposium title that drew me in – ‘Busted: myths of a chemical nature’.

Inflating your tyres with nitrogen is better than using air

busted stamp

busted stamp

Seth Rasmussen from North Dakota State University, US, became aware of this myth when one of his colleagues had problems with her car tyres deflating overnight in the harsh winter. The dealer told her that filling the tyres with nitrogen might solve the problem.

Looking into things a bit further, Rasmussen found a number of dealers offering nitrogen filling of tyres, charging around $30 a time, and claiming a number of benefits:

1. Nitrogen is ‘bigger’ than oxygen, so tyres stay inflated at the right pressure for longer. While it is true that N2 is lost from tyres more slowly than O2, a simple analysis of the sizes of the molecules quickly shows that size cannot be responsible for the difference. In fact this is due to the differences in polarisability of the two molecules – oxygen is more polarisable, so dissolves more readily in the rubber compounds used to make the tyre, so escapes faster. However, given that this is the case, if you keep topping up your tyres with air, the O2 will escape faster, leading to nitrogen enriched air inside. Equally, if you fill your tyres with nitrogen, oxygen will diffuse INTO the tyre to establish an equilibrium concentration after a couple of months.

burn rubber

burn rubber

2. Lack of oxygen stops chemical degradation of the tyre. While this might be true to some degree, to the average person it makes absolutely no difference – it only really makes a difference if you plan to re-tread tyres several times like truckers do in the US.

3. Nitrogen is inert so prevents autoignition of tyres. This is never going to be a problem for normal cars, and even racing car tyres don’t get hot enough for this to be an issue. The only place where there is any risk of tyres getting hot enough to spontaneously ignite is in aerospace tyres on jumbo jets and the space shuttle, where tyres can get to the required temperatures of over 250°C.

So, what’s the real reason dealers might offer to fill your tyres with nitrogen? The membrane nitrogen generators they use to separate N2 from air cost about $4000 dollars to buy and 25 cents to run each time, so if they charge $30 to fill your tyres it quickly adds up to a massive $Ker-ching!

busted stamp

busted stamp

The ‘staircase line’ dividing metals and non-metals on the periodic table

Greg Girolami from the University of Illinois at Urbana-Champaign, US,  described his investigations into the basis of the dividing line that is ubiquitous on periodic tables, starting underneath boron and stepping down in a staircase diagonally to divide polonium and astatine.

periodic_table

Girolami found that the first mention of such a division between metals and non-metals was made by John Walker, a professor of Chemistry at the University of Edinburgh, UK, in 1891, based on vague empirical observations. In his words ‘this has been sustained by mindless copying and its intrinsic aesthetic appeal’.

He looked at the physical and chemical properties of the elements to see if there was any basis for the division. One of the properties he looked at was electrical resistivity and how it changes with temperature – metals have low resistivity that increases with temperature, whereas non-metals have high resistivity which decreases with temperature as electrons are promoted to conduction bands.

From this he has come up with a different line – which classifies germanium as more non-metallic and arsenic as more metallic than the traditional line (see picture – the black line is the traditional ‘staircase’, the red line is Girolami’s proposal based on electrical resistivity of the room temperature allotropes). However, he pointed out that if you look at chemical properties and other allotropes of the elements, the distinction becomes a lot more blurry.

The moral – if you’re going to draw a line dividing metals and non-metals, make sure it’s based on some kind of fact and say what you’re basing it on. But in the back of your mind remember that, to paraphrase Neo in the matrix, there is no line…

That’s it from me in the Walking City – now that the sun has finally appeared, I’m off for a quick look around before I get on a plane back to Blighty.

Phillip Broadwith

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