Water

Rajesh R. Parwani

Most of the world has probably seen the movie ‘Titanic’. I know of people who have seen it multiple times. I have never seen it. I refused to see it when it came out and I avoided seeing it even when it reached the small screen. Nonetheless, given the immense publicity and trailers, I couldn’t avoid knowing that it made Leonardo an idol among teenage girls and launched Kate’s career. But in my opinion the real star, ignored completely by the media and the Oscars despite a remarkable performance, was the iceberg.

The ship sank because it struck the iceberg. But how is it that the iceberg was floating in the first place? Oh, you say, we all know ice floats, just look at all those ice-cubes in a glass of soft-drink, or look at the skaters on frozen lakes that sometimes fall into the chilly water below if the surface cracks, or recall those documentaries that show a polar bear or Eskimo fishing through a hole in frozen landscape. It is well known that ice floats.

Yes, but it is highly unusual from the point of view of physics. Most substances are denser in their solid state than in their liquid state. This has a simple explanation in terms of the atomic nature of matter. In a solid state the atoms are in relatively fixed locations, held there by their bonds to other nearby atoms. As the temperature is raised, the atoms get more energetic, break free from their constrained environments, and roam over a larger volume. Thus clearly a substance should be less dense in its liquid state. So in this simple picture, ice should be heavier than liquid water: the iceberg should have been at the bottom of the sea, posing no threat to the Titanic.

Here is an analogy illustrating the general argument why colder substances should be denser. Imagine a school hall where all the children have assembled to listen to a speech by the principal. The whole school can fit in there, with the students sitting quietly in orderly rows and closely packed. While the density of students is very high, their energy level, or effective temperature, is low. Now imagine the same hall when it is used by the students during recess. The children will be running and playing games. Fewer students can be accommodated in the hall for such energetic activities. Thus the density of energetic students in the latter case is much less than the density of quiet students earlier. Now think of the hall as a volume of substance, the children as atoms, their energy level as the temperature, and you have the explanation of why as the temperature rises things become less dense. But of course water is a remarkable exception to this fact. Water’s unusual properties are due to the shape of its molecule and the hydrogen bond.

Each molecule of water consists of a large oxygen atom covalently bonded to two smaller hydrogen atoms (as everyone knows, “H-2-O”). In the shell model of atoms, each neutral oxygen atom has six electrons in its outermost shell, but requires eight to be chemically inactive. Similarly each neutral hydrogen atom has a single electron but desires two to become chemically satisfied. In a water molecule the oxygen atom shares two of its electrons with two hydrogen atoms so that all atoms fulfill their needs, but since the oxygen atom is bigger and greedier (it has a larger positive core), it pulls the electrons from the hydrogen atoms closer to itself. The sharing is thus unequal, leaving the hydrogen atoms slightly positively charged and the oxygen atom slightly negatively charged.

The slightly charged ends of a water molecule give rise to an attraction between the hydrogen atom of one molecule with the oxygen atom of another. This interaction between different molecules of water is called the hydrogen bond. Now, each water molecule is tetrahedral in shape, with the center of the tetrahedron occupied by the oxygen atom, two corners occupied by the hydrogen atoms and each of the remaining two corners occupied by pairs of electrons (called lone pairs). This tetrahedral shape can be understood as desire of each electron pair, the ones in the covalent bonds and the lone pairs, to be as far apart as possible so as to minimize their energy.

The three-dimensional shape of the water molecule, together with the hydrogen bond, result in water and ice being huge three-dimensional lattices, with each molecule being hydrogen-bonded to perhaps four others. In liquid water the lattice structure is highly dynamic, with the hydrogen bonds breaking and reforming, allowing the individual atoms greater freedom of movement. However as the temperature is lowered and ice formed, the hydrogen bonds become more stable and the lattice structure more rigid. The rigid lattice has an open porous structure, with the atoms located at vertices and large empty ‘cages’. Thus the molecules in ice end up occupying more space than in liquid water, making ice less dense than liquid water! (Apparently bismuth is the only other common substance that is less dense as a solid).

The fact that ice floats has profound consequences for biology and our climate: deep parts of a lake are the last to freeze in winter, allowing aquatic life-forms to survive; while the ice-caps in the Arctic and Antarctic, would clearly result in a very different Earth if water froze from the bottom. The hydrogen bonds of water also allow it to resist changes in temperature by redistributing heat energy among the many bonds. This results in water having a relatively high specific heat capacity. That means a relatively large amount of heat is required to warm water and conversely a large amount of heat must be removed to lower its temperature. This resistance of water to a change in temperature also has biological and climatic consequences: The oceans and other water sources act as heat reservoirs, providing a relatively stable environment for aquatic life and moderating the weather.

The fact that water expands when it freezes also implies that ice melts under pressure. For when pressure is applied to ice, it is desirable for the molecules to get closer, and this is achieved by ice changing to the denser liquid water. So pressure applied to ice causes a thin layer of liquid to form on its surface, making it slippery and a boon for ice-skaters. (Heat generated by friction between the ice-skates and ice is apparently also important in melting the ice).

Water has so many other unusual physical and chemical properties (such as being a very good solvent and chemically very reactive), compared to other substances, which might have only one or two of those uncommon attributes, that it is not surprising that it is considered by many to be a prerequisite for life to form and flourish anywhere in the cosmos. Astronomers tell us that oxygen is the third most abundant element in the universe after hydrogen and helium. Since helium is inert, the next simplest compound that can be formed arises from a combination of hydrogen and oxygen --- water! So there might be life elsewhere.

Here is a final teaser. Water is a very good absorber of most radiation, be it the short wavelength ultraviolet or beyond, or the longer infrared and microwave frequencies (microwave ovens work by exploiting water’s resonance in that range). But there is a very narrow range of frequencies to which water is relatively transparent: This is precisely what is known as the visible spectrum, from the red to the blue. Light, in contrast to all other electromagnetic radiation, can travel large distances in water before being diminished.

Think about that last fact for a moment and try to ‘see’ what it means. Now, don’t you want to plunge into physics to understand how Nature works?