Wonders of Life. Andrew Cohen

water than Earth. This interstellar reservoir was part of the cloud that, 4.5 billion years ago, condensed into our Solar System and formed the oceans and rivers that cover our blue planet today.

      Pond skaters can walk on water. However, it is not just their anatomical adaptations, but also the physical properties of water, that enable them to occupy this unique environmental niche.

WALKING ON WATER

      Of all the creatures on Earth, few exploit the unique characteristics of water as overtly as the family of insects known as Gerridae. You may also know them as pond skaters, water striders or Jesus bugs.

      Gerridae are successful and vicious killers, piercing the body of a captured spider or fly with a specially adapted mouthpart and finishing it off by sucking out its insides. The 1,700 known species around the world are found in a large range of water habitats, from the pond in your back garden to slow-flowing rivers in the deepest recesses of the Mexican jungle. But it is not their killing methodology or diversity that makes these animals so interesting to school children and a physicist dabbling in biology; it is their ability to walk on water, like Jesus. Next time you look at a common pond skater, you’ll be observing a creature that exhibits an exquisitely balanced relationship between its anatomical features and the physical properties of water, because the Gerridae is beautifully adapted to life at the interface between water and air. Its short front legs are used for capturing prey, while its middle legs propel it through the water. Its back legs are long and slender, spreading the animal’s weight over a larger surface area. These gangly appendages contribute to the pond skater’s ability to walk on water, but alone they would not be enough to keep it afloat. Every square millimetre of its body is covered with a cohort of tiny hairs that increase the surface area still further. These hairs are also hydrophobic, making the whole animal water-resistant. Without this adaptation, a single drop of rain would be enough to weigh the creature down and sink it below the surface. Even if the animal is pushed under, the tiny water-repelling hairs trap air, adding buoyancy and returning the creature to the surface. All of these anatomical features combine to allow the pond skater to live out its life in this unique environmental niche, moving around the water’s surface at speeds of up to 1 m per second – remarkably fast for such a small creature. Yet all these clever adaptations alone would not keep a pond skater afloat if it wasn’t for the especially strong bonds that exist between the water molecules themselves, and it is ultimately these bonds that make water so vital for life. This is why we chose this common but fascinating little animal as our introduction to the wonder of water.

      This image of pond skaters was taken from underneath the water, looking upwards. The strong bonds between the water molecules help to prevent the insects from breaking the surface.

      The pond skater’s back pair of legs spreads the animal’s weight over a wider area, while the middle pair propels it through the water.

      Look at a common pond skater, and you’ll be observing … an exquisitely balanced relationship between its anatomical features and the physical properties of water.

TREETOPS TO TEARDROPS: THE MAGIC OF HYDROGEN BONDS

      Water is colourless, tasteless, odourless and has a simple chemical formula – H₂O – but this simplicity is deceptive, because the geometry of the water molecules themselves means that their collective behaviour is tremendously subtle and complex. The diagram below shows a series of different molecules, each consisting of hydrogen atoms covalently bonded to different elements. The simplest is hydrogen fluoride, which forms a linear structure as there are only two atoms – one of fluorine and one of hydrogen. Fluorine bonds with only one hydrogen atom because it has only a single electron in its outer shell. Water has two electrons available for bonding, but it also has two pairs of electrons sitting inertly in its outer shell. Inert they may be, but they still have to ‘fit’ somewhere, and their presence means that water molecules are not linear. The hydrogen atoms sit on one side of the oxygen, at an angle of 104.45°.

      This has a very important consequence. Electrons are negatively charged, and the nuclei of hydrogen, being single protons, are positively charged. Water’s angled geometry means that the region surrounding the hydrogen atoms has a slight positive charge, and the region away from the hydrogen atoms has a slight negative charge. This means that water is a ‘polar’ molecule – one side is slightly negatively charged, and the other is slightly positive, although the molecule itself remains electrically neutral. This is the reason for the unexpected behaviour of water in a classic school science experiment. Take a Perspex rod (one of those perplexing objects found in every science laboratory but nowhere else) and rub it against a fleece. This gives the rod an electric charge, in much the same way that you might get charged up by walking across a carpet and discharged uncomfortably by grabbing a door handle. If you move the rod next to water flowing out of a tap, the stream of water will bend because the positive and negative sides of the water molecules are either attracted to or repelled by the electric charge on the rod.

      It is the polar nature of water molecules that gives this seemingly innocuous liquid an array of complex properties so vital for life on Earth. The molecules are not only attracted to or repelled by external electrically charged objects, they also attract each other, forming weak bonds known as hydrogen bonds. Water isn’t the only liquid to do this – hydrogen fluoride and ammonia also exhibit hydrogen bonding for the same reason – they have a negative and positive side to them because of their geometry and the distribution of the electrons around their component atoms.

      HYDROGEN BONDS

      Water bends towards a Perspex rod that has been rubbed against a fleece. The rod has an electric charge, caused by the rubbing, and the water bends because of its own polar nature.

      One of the most immediate consequences of hydrogen bonding is a dramatic rise in the boiling point of these substances. Methane, which is a symmetric molecule because it has four hydrogen atoms surrounding its central carbon atom (see diagram), is not polar and does not exhibit hydrogen bonding. This means that methane molecules are only very weakly bonded together in the liquid state, and it doesn’t take much energy to split them apart from each other and turn liquid methane into a gas. This is why the boiling point of methane is a chilly -162°C. Ammonia, on the other hand, with only one hydrogen less than methane and a very similar molecular size and weight, exhibits hydrogen bonding because it is polar, and its boiling point is a fairly warm -33°C, a temperature regularly reached in cold areas on our planet. Hydrogen fluoride is also polar, because of fluorine’s voracious appetite for electrons, and it boils at room temperature. And water, of course, boils at 100°C at room temperature and standard atmospheric pressure, because of its strong hydrogen bonds. We can get an idea of the importance of hydrogen bonds by comparing water to hydrogen sulphide, a very similar molecule in terms of weight and size, but with an atom of sulphur replacing the oxygen atom at its heart. H₂S does not exhibit hydrogen bonding, because the sulphur atom does not drag the electron cloud around it as effectively as oxygen. This is because it has an extra inner shell of electrons shielding the positive electric charge of its nucleus. As a result, H₂S boils at -60°C. Without hydrogen