Water (H2O): A Mysterious Molecule

Water may be one of the most familiar substance on the planet, but it certainly isn't ordinary. In fact, water's unique chemical properties make it so complicated that even after decades of research, scientists still have much to learn about this remarkable and versatile substance.

If we drop an ice cube into a glass of water, it floats. This happens because water expands as it freezes, which makes the solid form less dense than the liquid. But most other liquids do just the opposite; they shrink and become more dense as they freeze, so the solid form sinks. If water behaved that way, ice would accumulate on the bottom of lakes and oceans during the winter, and would have difficulty thawing in the spring. If possible, this would have consequences for aquatic life.

 Another surprising characteristic of water is that it boils at a very high temperature—100 degrees Celsius at sea level—compared to similarly sized molecules. If water behaved like other liquids, it would exist as a gas at the temperatures and pressures found on Earth, and life as we know it couldn’t survive.

Water is all around us—in the sky, on the ground, in the air—continually changing form. Water's unique chemical properties make it the only natural substance that can be found in all three states: liquid, solid (ice) and gas (steam). The continual movement of water around the globe is known as the hydrologic cycle. Several processes take place within this cycle, including evaporation, condensation, precipitation, runoff and collection. During the cycle, water will change form many times.

 

That’s water, as in the clear, sparkling fluid that covers three quarters of the Earth’s surface—not to mention the basis of life as we know it, and possessor of the world’s most recognizable chemical formula (H2O). Water is everywhere. And yet, scientists are still learning about its properties.  

Unique & mysterious properties: 

1. Enthalpy of vaporization:

Water has a very high specific heat capacity – the second highest among all the heteroatomic species (after ammonia), as well as a high heat of vaporization (40.65 kJ/mol or 2257 kJ/kg at the normal boiling point), both of which are a result of the extensive hydrogen bonding between its molecules. These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature. According to Josh Willis, of NASA's Jet Propulsion Laboratory, the oceans absorb one thousand times more heat than the atmosphere (air) and are holding 80 to 90% of global warming heat.

 The specific enthalpy of fusion of water is 333.55 kJ/kg at 0 °C. Of common substances, only that of ammonia is higher. This property confers resistance to melting on the ice of glaciers and drift ice. Before and since the advent of mechanical refrigeration, ice was and still is in common use for retarding food spoilage.

 2. Density of water and ice:

The density of water is approximately one gram per cubic centimeter. It is dependent on its temperature, but the relation is not linear and is unimodal rather than monotonic (see table at left). When cooled from room temperature liquid water becomes increasingly dense, as with other substances, but at approximately 4 °C (39 °F), pure water reaches its maximum density. As it is cooled further, it expands to become less dense. This unusual negative thermal expansion is attributed to strong, orientation-dependent, intermolecular interactions and is also observed in molten silica.

Water also expands significantly as the temperature increases. Water near the boiling point is about 96 percent as dense as water at 4°C.

These properties of water have important consequences in its role in Earth's ecosystem. Water at a temperature of 4°C will always accumulate at the bottom of freshwater lakes, irrespective of the temperature in the atmosphere. Since water and ice are poor conductors of heat (good insulators) it is unlikely that sufficiently deep lakes will freeze completely, unless stirred by strong currents that mix cooler and warmer water and accelerate the cooling. In warming weather, chunks of ice float, rather than sink to the bottom where they might melt extremely slowly. These properties therefore allow aquatic life in the lake to survive during the winter.

 3. Density of saltwater and ice:

The density of water is dependent on the dissolved salt content as well as the temperature of the water. Ice still floats in the oceans, otherwise they would freeze from the bottom up. However, the salt content of oceans lowers the freezing point by about 2 °C and lowers the temperature of the density maximum of water to the freezing point. This is why, in ocean water, the downward convection of colder water is not blocked by an expansion of water as it becomes colder near the freezing point. The oceans' cold water near the freezing point continues to sink. For this reason, any creature attempting to survive at the bottom of such cold water as the Arctic Ocean generally lives in water that is 4 °C colder than the temperature at the bottom of frozen-over fresh water lakes and rivers in the winter.

 In cold countries, when the temperature of fresh water reaches 4 °C, the layers of water near the top in contact with cold air continue to lose heat energy and their temperature falls below 4 °C. On cooling below 4 °C, these layers do not sink but may rise up as fresh water has a maximum density at 4 °C. (Refer: Polarity and hydrogen bonding) Due to this, the layer of water at 4 °C remains at the bottom and above this layers of water 3 °C, 2 °C, 1 °C and 0 °C are formed. Since ice is a poor conductor of heat, it does not absorb heat energy from the water beneath the layer of ice which prevents the water freezing. Thus, aquatic creatures survive in such places.