26 Properties of Water
Dr. C. P. Singh Chandel
Contents
- Introduction
- Hydrologic Cycle
- Distribution of Water
- Chemical Composition of water
- Unique Properties of Water
- References
Introduction
Water is one of the most important material on Earth. It has unlimited applications. It is therefore necessary to understand the distribution of water in different spheres. Due to its many unique physical property water has acquired importance. In the following section the property and processes are discussed.
Hydrologic Cycle / Water Cycle
Hydrologic cycle is related to the movement of water from the Earth’s surface through evaporation into the atmosphere. Its application is to model the storage and movement of water between different spheres such as the biosphere, atmosphere, lithosphere and hydrosphere. In environment, there are reservoirs, which store water. The important reservoirs are oceans, lakes, rivers, glaciers, soils, snowfields, and groundwater on the earth. It is found in liquid, solid or vapor / gas. It is interesting to point out that no beginning or end can we assigned to hydrologic cycle. The study of hydrologic cycle involves an understanding of the evaporation, condensation, precipitation, interception, infiltration, percolation, transpiration, runoff and storage processes.
Figure-1 The hydrological cycle
Evaporation
When the physical state of water changes from liquid state to vapors state, the process is called Evaporation. The natural evaporation is affected by the external factors such as solar radiation, atmospheric temperature, vapor pressure, wind, and atmospheric pressure. It is also affected by surface area and amount of water.
Condensation
When the water vapors change into a liquid state, the process is called condensation. Water vapor condenses onto small airborne particles to form dew, fog, or clouds. Condensation occurs when either the water vapor bearing air cools or the amount of water vapors in the air reaches its saturation point.
Precipitation
Precipitation of water refers to the release of water from clouds in the form of rain, freezing rain, sleet, snow, or hail. It leads to the delivery of atmospheric water to the Earth. Precipitation is the process that occurs of water particles fall from the atmosphere and reach the ground. Precipitated water may fall into a water body or onto land. It is then dispersed in several ways. The water has ability to adhere to objects. It is carried away through the land into stream channels, or it may penetrate into the soil, or it may be intercepted by plants.
When rainfall is scanty and infrequent, a high percentage of water precipitation is returned to the atmosphere by evaporation. The precipitated water that appears in surface streams is called runoff.
Percolation
Percolation is the movement of water though the soil, and its layers, by gravity and capillary forces. The prime moving force of groundwater is gravity.
Transpiration
Transpiration is basically evaporation of water from plant leaves. In this process moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere.
Distribution of water
The most common liquid on our planet is water. It is vital to all life forms. Water is widely distributed on Earth as freshwater and salt water in the oceans (Table -1). A freshwater ecosystem is an aquatic ecosystem with a low salt concentration. It includes lakes, ponds, springs, rivers, streams and wetlands. These are of two types, the lentic or still water ecosystem and the lotic or the flowing water ecosystem.
Lakes and ponds belong to the lentic ecosystem. These bodies can be small or huge. Streams and rivers are bodies of flowing water. The water from the lotic ecosystem originates from springs, melted snow in the mountains, and even lakes.
Although the total amount of water on earth is enormous, only a small percentage is fresh water. Over 97 percent of the world’s total water supply is saline and found in oceans, of the remaining approximately 3 percentage fresh water. Over 68.7 percent of fresh water is locked up in ice and glaciers and another 30 percent of freshwater is in the ground. (Table-1)
The distribution of water globally is as shown in Table-1.
Table-1 Distribution of water
| Water source | Water volume, in
cubic miles |
Water volume, in cubic kilometers | Percent of freshwater | Percent of total water |
| Oceans, Seas, & Bays | 321,000,000 | 1,338,000,000 | — | 96.5 |
| Ice caps, Glaciers, & Permanent Snow | 5,773,000 | 24,064,000 | 68.6 | 1.74 |
| Groundwater | 5,614,000 | 23,400,000 | — | 1.7 |
| Fresh | 2,526,000 | 10,530,000 | 30.1 | 0.76 |
| Saline | 3,088,000 | 12,870,000 | — | 0.93 |
| Soil Moisture | 3,959 | 16,500 | 0.05 | 0.001 |
| Ground Ice & Permafrost | 71,970 | 300,000 | 0.86 | 0.022 |
| Lakes | 42,320 | 176,400 | — | 0.013 |
| Fresh | 21,830 | 91,000 | 0.26 | 0.007 |
| Saline | 20,490 | 85,400 | — | 0.007 |
| Atmosphere | 3,095 | 12,900 | 0.04 | 0.001 |
| Swamp Water | 2,752 | 11,470 | 0.03 | 0.0008 |
| Rivers | 509 | 2,120 | 0.006 | 0.0002 |
| Biological Water | 269 | 1,120 | 0.003 | 0.0001 |
Data in Table-1 are taken from reference, Igor Shiklomanov [“World fresh water resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Fresh Water Resources (Oxford University Press, New York)]
Chemical Composition of Water
Water has chemical formula H2O. Water is composed of one atom of oxygen with two atoms of hydrogen. Water is a polar molecule with oxygen having a partial negative charge and each hydrogen atom having partial positive charge. Water is a tasteless, odorless liquid and appears colorless in small quantities.
The composition of seawater, river water, lake water and groundwater is given in Tables 2,3 and 4. Seawater contains mainly Na+ and Cl- and river water contains mainly Ca2+ and HCO3-.
Table-2 Composition of seawater and river water (mmol kg-1)
| Element | Seawater (mmol kg-1) | River water (mmol kg-1) |
| Na | 468.0 | 0.26 |
| Mg | 53.1 | 0.17 |
| Ca | 10.3 | 0.38 |
| K | 10.2 | 0.07 |
| Sr | 0.09 | —– |
| Cl | 546.0 | 0.22 |
| SO4 | 28.2 | 0.11 |
| HCO3 | 2.39 | 0.96 |
| Br | 0.84 | —– |
Table -3 pH and concentration ranges of the ions ( mg L-1 ) in ground water
| Parameters / Ions | Ranges |
| Ca+2 | 200- 240 |
| Mg+2 | 58.3- 87.5 |
| Na+ | 28- 35 |
| K+ | 3.7- 4.7 |
| HCO3- | 150- 298 |
| Cl- | 188.9 – 212 |
| F- | < 0.5 |
| NO3- | 330- 400 |
| SO42- | 103- 202 |
| Cu | 0.01- 0.04 |
| Pb | < 0.1 |
| Zn | 0.60- 1.30 |
| Fe | 0.33- 1.12 |
| Mn | 0.01- 0.03 |
| pH | 7.36- 7.80 |
Table-4 Composition of Lake water ( mg L-1 )
| Calcium | 241 |
| Magnesium | 7200 |
| Sodium | 83,600 |
| Potassium | 4070 |
| Bicarbonate | 251 |
| Sulfate | 16,400 |
| Chloride | 140,000 |
| Silica | 48 |
| TDS | 254,000 |
| pH | 7.4 |
Unique Properties of Water
Water is one of God’s gift to mankind. It is the only abundant common pure compound, which is a liquid. It has many unusual chemical and physical properties. The Earth appears to be a majestic blue marble from top. This is due to the abundance of water on the earth surface. While water itself is not blue, water gives off blue light upon reflection.
In simplest terms, water makes up about 71% of the Earth’s surface, while the other 29% consists of continents and islands. To break the numbers down, 96.5% of all the Earth’s water is contained within the oceans as salt water, while the remaining 3.5% is freshwater lakes and frozen water locked up in glaciers and the polar ice caps. Of that fresh water, almost all of it takes the form of ice: 69% of it, to be exact. If you could melt all that ice, and the Earth’s surface was perfectly smooth, the sea levels would rise to an altitude of 2.7 km. Water is the clear sparkling fluid which covers three quarters of the earth’s surface. Not to mention water is better known as the basis of life. Water is the possessor of the world’s most recognizable chemical formula (H2O).
Water molecules are attracted to each other, and form hydrogen bonds. These weak interactions determine the most physical property of water and many of its chemical properties too.
A large part of the mass of most organisms is simply water. In human tissues, the percentage of water ranges from 20% in bones to 85% in brain cells. The water content is greater in embryonic and young cells and decreases on ageing. About 70% of our total body weight is water.
Water -The Universal Solvent
Water has the ability to dissolve most of the substances; some have high solubility whereas some are only sparingly soluble. It is, therefore, called a universal solvent. In rainwater, the substances and atmospheric gases present in air get dissolved.
The important physical properties of water are in Table-5.
Table -5 Properties of water
| Property | Value |
| Molar Mass | 18.015 |
| Molar Volume | 55.5 moles/liter |
| Boiling Point (BP) | 100oC at 1 atm |
| Freezing Point (FP) | 0oC at 1 atm |
| Triple Point | 273.16 K at 4.6 torr |
| Surface Tension | 73 Dynes at 20oC |
| Vapor Pressure | 0.0212 atm at 20oC |
| H of Vaporization | 40.63 kJ/Mol |
| H of Fusion | 6.013 kJ/Mol |
| Heat Capacity (CP) | 4.22 kJ/kg.K |
| Dielectric Constant | 78.54 at 25oC |
| Viscosity | 1.002 centipoise at 20oC |
| Density | 1 g/cc |
| Density Maxima | 4oC |
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References:
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- A. K. De(2014), Environmental Chemistry, New Age International Publishers, Delhi
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- http://ocean.stanford.edu/courses/bomc/chem/lecture_12.pdf
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