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Anomalous expansion of water

Question: Why does water exhibit anomalous expansion when heated? Why does its volume decrease until 4 degrees centrigrade and increase thereafter?

M. Sriharsha, Chennai

Answer: Water is an unusual substance because its solid phase namely ice, is less dense than the liquid. Ice floats on water, where as every other solid sinks in its liquid. Because of this anomalous property, marine life like fish can survive in winter under the floating ice while the ocean too freezes in the polar regions.

The pressure-temperature history of all the three phases of water are shown. The solid-liquid line slopes to the left because ice contracts when it melts. For most substances which expand on melting this solid-liquid interface line slopes forward to the right.

The reason for this behaviour of water is the polar interaction of the hydrogen bond in the crystalline structure of the ice depicted in the sketch. Here each oxygen atom (white ball) is hydrogen-bonded two others by means of its own hydrogen atoms (black balls), and bonded to two more oxygen atoms by means of their hydrogen atoms. The coordination is tetrahedral with a structure similar to that of diamond. Since this causes an open network structure occupying more volume, ice is less dense than water at the melting temperature. Upon melting, part of this open-cage structure collapses, resulting in more compact liquid than the solid ice.

During melting, part of the clusters of hydrogen-bonded structure of the solid persists in the liquid. As the temperature is increased these clusters break up and the volume continues to shrink. If the temperature is increased still more, the expected thermal expansion begins to take place. Consequently, liquid water has a minimum molar volume, or a maximum density, at 4oC.

B.S.Murthy, Chennai

Measuring earthquakes

Question: What is Richter Scale? How is it used to measure the severity of earthquakes?

Jafarullah, Chennai

Answer: Two basically different methods are used for describing the severity of an earthquake. One is Intensity and the other is Magnitude. Intensity is an estimation of the earthquake's effect on people, damages inflicted to structures and the changes caused to the earth's surface etc. In 1905, an Italian Seismologist, Guiseppe Mercalli devised an Intensity Scale, based on evidences such as human reactions, damages to structures, fissures in the earth's surface, landslides, floods etc. This Scale had 10 divisions - Roman Numbers I to X - and was known as the Mercalli Scale. In 1931, two Seismologists, Wood and Newmann modified this Scale and extended it to 12 divisions, i.e. upto XII. After this, the Mercalli Scale came to be known as Modified Mercalli Scale or simply, the MM Scale.

Of far more importance and of greater scientific value is to describe the severity of earthquakes by Magnitude. Magnitude is related to the amount of strain energy released at the focus or epicentre of the earthquake, as recorded by the Seismographs. This is where the Richter scale comes into relevance. American Physicist and Seismologist, Charles Francis Richter (1900-1985), while with the Carnegie Institute, USA, embarked on the project for developing a suitable scale to describe the degree of intensity of earthquakes in terms of Magnitude, by numerical values. The culmination of his efforts was the evolvement of a Scale in 1935. This was called the Richter scale, after his name. In this endeavour, Richter was supported by his colaborator, Professor Beno Guternerg of the California Institute of Technology.The Richter Scale is an open-ended numerical Scale that describes an earthquake independently of its effects on people, buildings or other objects. It begins with Zero, in which the greatest wave-amplitude registered on a seismograph at a distance of 100 kms. from the epicentre of the earthquake does not exceed one Micron (one thousandth of a millimetre).

K. Ramamurthy, Bangalore

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This Week's Questions

Why is the day very hot and night very cold in a desert?

M.Kammar Prakash,Gulbarga

Why does the boiling point of a liquid increase with increase of external pressure (as in a pressure cooker) and vice versa (as on hills)? Why does the heat required for vapourisation decrease and increase in the former and latter cases reapectively?

K. Rakesh,Erode

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