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The Stars and Us

Hydrogen is the smallest and lightest of all elements, but it plays a central role in physics, chemistry and biology. 74% of everything in the universe is hydrogen, by mass; it's more like 90% of all atoms, but each atom is very light, consisting only of one proton and one electron. Another 24% of the mass is helium, which is four times as heavy. In the early universe the proportions were even higher, but some of the hydrogen and helium has been used up in stars, making everything else in the universe. The 2% that isn't hydrogen or helium, in other words. Hydrogen is many stars' single biggest source of power, indeed it's the only source that smaller stars are able to use.

Floating Protons

In chemistry, all standard acids contain hydrogen, and the main measure of acidity is pH, which is determined by the concentration of hydrogen ions (H+) in a solution. A hydrogen ion is an atom of hydrogen that's lost its one electron, leaving behind a single proton: it's a subatomic particle that will turn back into a real atom as soon as it manages to attract a new electron. Acids work because the hydrogen ion engages in a two-pronged attack along with the rest of the acid, which is always a negative ion. For every point that pH increases, the number of hydrogen ions goes down by a factor of ten, so something with a pH of 5 has ten times as many hydrogen ions as something with a pH of 6.

In biology, hydrogen ions are widely used for transporting and storing energy, playing a fundamental role in both respiration and photosynthesis. This 'proton pump' mechanism pushes the hydrogen ions from one side of a membrane to the other, and energy is released when they go back again, like a tiny boulder rolling back down a hill. There's more to it than that, but hopefully you get the gist.

The fact that hydrogen is prone to losing electrons and forming positive ions is one of the things that makes it rather odd, as an element. Usually, only metals do that. But hydrogen is an odd case in many ways; in chemistry teaching I often find myself saying '...except hydrogen.'

Organic Chemistry

It's not only in the form of free protons that hydrogen is important, of course. Hydrogen is found in a huge range of compounds, including almost all organic compounds; it's so ubiquitous that some styles of representing organic compounds just leave out the hydrogen altogether. Instead, places where there isn't hydrogen, but there could have been, are marked out. We're left to fill in the gaps ourselves. That's what's meant by 'unsaturated', when people talk about fats and hydrocarbons - there's some hydrogen missing somewhere, so carbon atoms have to form double bonds with each other, making them less stable and frankly, a whole lot messier. Which is why unsaturated fats aren't solid at room temperature: they don't stack so neatly, so the molecules don't stick together as well.

The hydrogen contributes to the energy density of organic compounds, and having it there enables carbon to form stable chains, rings and so on. Left to its own devices, carbon would mostly only form graphite, diamond and fullerenes, which are all perfectly nice chemicals in their own way, but none of them is anywhere near interesting and versatile to give rise to the complex chemistry of life.

Hydrogen Bonding

The most familiar of all hydrogen compounds is of course water (H2O). It's so much a part of everyday life that it's easy to lose sight of what an odd compound it is. It has the extraordinary property of getting bigger and less dense when it freezes, so unlike almost any other substance its solid form floats on top of its liquid form. If not for this anomaly, Earth's water would almost certainly freeze over, and life would never have had a chance to develop.

Even the fact that water is a liquid at all, at the temperatures we're used to, a little surprising. Most small molecules boil at far lower temperatures. The reason water doesn't has to do with the fact hydrogen is not very good at holding on to electrons, whereas oxygen has exceptional powers of electron-grabbing (it's electronegative, in chemist-speak). That makes for a very uneven molecular partnership, with the oxygen taking the lion's share of each electron for itself. So a water molecule consists of one oxygen atom with the negative charge of almost two whole extra electrons, and two near-naked protons hanging onto one side, with their positive charges exposed to the world. Opposite charges attract, so the hydrogens of one water molecule naturally tend to pull in the oxygen of another. This attraction, known as hydrogen bonding, is why water has such a high boiling point. It's also part of the reason why ice floats. Thanks to hydrogen bonding and the way the hydrogen atoms are arranged around the oxygen, for water to freeze the molecules need to line themselves up in a hexagonal honeycomb pattern, with one water molecule at each vertex, leaving a big gap in the middle.

Hydrogen bonding happens whenever you've got a hydrogen atom attached to oxygen or another electronegative atom, like nitrogen, fluorine or chlorine. It's the single strongest kind of intermolecular force, and it not only explains many of the properties of water, but also those of any other molecule where it features. So ethanol, the alcohol found in booze, is a liquid because it has a hydrogen atom attached to an oxygen (we call this a hydroxyl functional group); but it only has one, so it boils at a lower temperature than water. Glycerol, the alcohol that holds fat together, has three separate hydroxyl groups, so it's got a higher boiling point, and it's very viscous. Glucose has six hydroxyl groups, which is why it's solid at room temperature. All of these are soluble in water, and to some extent also in each other, because hydrogen bonds work between molecules of different types. The same kind of electrostatic forces make water the world's greatest solvent, allowing it to overcome the ionic bonds that hold together many crystals.

Hydrogen Economics

Hydrogen gas is of huge economic importance. The use with the most profound consequences for humankind is the Haber-Bosch process, which turns hydrogen and nitrogen into ammonia (NH3). Nitrogen fixation is such a limiting factor in growing food crops that without this one chemical reaction, we'd only be able to feed about half of the humans currently on this planet.

Producing hydrogen is straightforward, but unfortunately not very efficient. Any acid reacting with a metal will produce hydrogen bubbles, which is how they inflated dirigibles like the Hindenburg, but even the cheapest metals are not that cheap. Electrolysis can be used to split water into hydrogen and oxygen, but that takes quite a bit more energy than you'll ever get back out of it. So most industrially produced hydrogen is currently made from fossil fuels. The usual method is to heat methane with steam in the presence of a catalyst, producing carbon monoxide and hydrogen gas.

For decades people have been talking up the possibility of a hydrogen economy, in which fuel cells would take the place of petrol, and cars run on electrical power, emitting only water. If only we had plentiful hydrogen and good ways to store it and transport it, this would be a very appealing prospect. However, hydrogen is only a liquid at temperatures below about -252°C, and storing it in gas form is impractical unless it's under massive pressure. The challenges of maintaining such temperatures and pressures efficiently may yet be overcome, but given the competition from the developing technologies of algal fuels and battery power, each requiring far less infrastructure investment to take off, it seems likely that hydrogen cells will never fulfil their promise.

More on Hydrogen & Water

Sodium acetate (also known as sodium ethanoate) is currently my favourite chemical. That's only partly because it gets bloody cold in my lab sometimes, and it's nice to have a hand warmer at the ready.

One thing I like about it is how mundane it is. When you react vinegar with sodium bicarb (baking soda) in the classic kitchen chemistry 'volcano' demonstration, it's the chemical left behind once all the carbon dioxide has bubbled away. As you might expect from something easily produced from common food ingredients, it's not particularly harmful. It is a mild irritant though, and we want it much more concentrated than you'd normally get in the kitchen, so wash your hands if you get it on your skin.

The really fun thing about sodium acetate is that you can dissolve far more of it in hot water than in cold - but once it cools down it stays dissolved, until it has something to crystallise around. That's because any change of state requires a nucleation point, a place to get started. In this case, the beginnings of a crystal structure provide a scaffold for more particles to crystallise around. Without that, the solution stays supersaturated, containing more of the chemical than it could normally dissolve. Once a seed crystal is introduced, crystals quickly spread out in all directions, like a cloud of smoke made out of needles of ice. You may have seen this in action, in those re-usable hand warmers - sodium acetate is the main chemical used in them. It only takes a few seconds for the crystals to silently spread through a whole hand warmer or flask, radiating a pleasing warmth as they go.

If you have seen reusable hand warmers in action, you may have wondered where that heat comes from. The answer is that when crystals form, particles are attracted together - electrostatic forces pull them into place, and that gives them energy, which becomes heat. A shoe will drop to the the floor with a thump for essentially the same reason: forces cause things to move, and that movement energy has to go somewhere. So crystallisation is almost always exothermic. The energy it releases is called the latent heat of fusion. By the same token it takes energy to break a crystal's bonds, whether by melting a solid or dissolving it, because that means tearing apart mutually attracted particles. With sodium acetate, this isn't too hard - you can re-dissolve the crystals by just heating the container in boiling water or on a Bunsen burner for a couple of minutes. Anything that heats it up past 58°C for long enough should do the trick.

There are at least two very satisfying ways of demonstrating sodium acetate's power of rapid crystallisation. One way is to introduce a seed crystal to the container, which is what commercial hand warmers do when you pop the little metal thing inside. In the lab I've found that shaking the flask usually triggers the process, thanks to imperfections in the bung, but it's probably more fun to start it off by sprinkling magic dust on top. The other way to do it is to pour the supersaturated solution out onto a seed crystal, creating an instant stalagmite which just keeps growing as long as you keep pouring.

The chemical has various other uses besides making hand warmers and great chemistry demonstrations. It has antifungal and antibacterial properties. It's used as a sealant for concrete. Together with acetic acid, it acts as a buffer, helping keep a solution's pH fairly steady when acids or bases are added to it. Perhaps most excitingly, compounding it with acetic acid allows the formation of sodium diacetate crystals, suitable for flavouring salt and vinegar crisps.

Further reading

The Rosetta Mission was a 10-year long project by the European Space Agency (ESA) to place a lander on a comet. It succeeded with the placement of the Philae lander on the Comet 67P/Churyumov-Gerasimenko (AKA Chury) on November 12, 2014.

The mission was originally aiming for a much smaller comet, 46P/Wirtanen1, but difficulty with the Ariane 5 engine postponed liftoff from January 2003 to March 2004, causing Rosetta to miss the launch window for Wirtanen. Chury was identified as a good backup target, and the mission was back on.

The goal of the mission was to find clues about the formation of the Solar System. Chury's composition should reflect the composition of the pre-solar nebula out of which the Solar System formed over 4.6 billion years ago. The ESA will also be looking at the isotopic profile of the cometary ice to see what percent of the water in Earth's oceans may have come from early comet impact, and will check for organic molecules that may have affected abiogenesis.

The media make a point of telling us that Rosetta traveled 6.4 billion kilometres (42.78 AU) to reach Chury, but this is potentially misleading. Chury is not nearly this far out2, and the distance traveled includes four trips around the sun, with three gravity assists from Earth (in 2005, 2007 and 2009) and one from Mars (2007). But any way you spin it, ten years of weaving through the inner solar system to hit a target the size of Cruithne is impressive.

Rosetta spent about nine months gathering data on the comet through photographs and sampling the various off-gassings. Finally, it moved within 10 km of the comet and released the Philae lander3, a clunky cube about the size of a washing machine, which was intended to land gently on the comet and take some serious rock samples and high-resolution photos. Chury is an odd shape, often compared to a rubber duck, consisting of a larger and a smaller lobe connected by a thick 'neck'. Philae was targeted to land on a large open area on the smaller lobe.4

In order to land on the comet in microgravity, Philae was supposed to rely heavily on a harpoon anchor system, but this failed to fire. The lander bounced off the surface, arching a full kilometer up and a kilometer to the side before coming back to the surface, bouncing lightly once more, and finally landing in shadow and at an odd angle. All systems continued to work properly, but Philae is solar-powered, and will run out of power shortly if it can't be moved. Fortunately, the drills were still able to take subsurface samples despite the tilt of the lander, and all of the planned initial experiments were completed successfully.

The lander has run out of power, but the solar panels have been re-positioned, and it is possible that it may be able to power back up at some point in the future. There is not any great expectation in this area, however.

In the meantime, here is the ESA's photo gallery.


1. Wirtanen is much smaller than Churyumov-Gerasimenko, with an estimated diameter of 1.2 kilometres v. Chury's 4.1. We know comparatively little about it, but it will be coming quite close to Earth in December of 2018, passing within a mere 0.0777 AU (Chury will only come within 3.3 AU of Earth, but we will only have to wait until 2015). We may yet pay Wirtanen a visit; NASA's proposed Comet Hopper mission would like to make comet-fall in the early 2020s, if funding comes through. It doesn't seem very likely at this point.

2. Aphelion of 5.6829 AU; Perihelion of 1.2432 AU. As the crow flies, Rosetta reached a max distance of 6.68 AU from the Earth (a point reached in 2012), before its orbit moved back in to align with Chury's.

3. Named after an island in the Nile region of Egypt, the place of discovery for the Rosetta stone.

4. The original landing site, originally known as Site J, was renamed Agilkia as the result of an international naming competition. Ironically, it is named for Agilkia Island, the island that many buildings, including the famous Temple of Isis, were moved to when Philae flooded during the building of the Aswan dams in the 1960s. This would be a very appropriate name for an unfortunate, unplanned landing site, but odds are that the new landing site will get some other title.

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