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Old Saturday, March 04, 2006
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Default HeavyWater,HardWater,SoftWater:

HEAVY-WATER:
(somehow included in CSS Exam 1983,1985,1992,1999,2001,2002)

As we all know that Normal water is made up of two hydrogen atoms and an oxygen atom covalently bonded together. This gives the famous H20 formula.

Heavy water has exactly the same structure, except the hydrogen atoms are isotopes of hydrogen called 'Deuterium'. Standard hydrogen has one proton in its nucleus (Relative Molecular Mass = 1), deuterium has one proton AND one neutron in it nucleus, (RMM = 2) hence it is 'heavier' than normal hydrogen. This leads to the water being heavier.



Mass of water = 18,
Mass of heavy water = 20.

Hence Heavy water is deuterium oxide, or D2O (or 2H2O) . Its physical and chemical properties are similar to those of light water, H2O, but the hydrogen atoms are of the heavy isotope deuterium

(Gilbert Newton Lewis isolated the first sample of pure heavy water in 1933.)

SEMI-HEAVY-WATER:

Semiheavy water, HDO, also exists whenever there is water with hydrogen-1 (or protium) and deuterium present in the mixture. This is because hydrogen atoms (hydrogen-1 and deuterium) are rapidly exchanged between water molecules. Water containing 50 % H and 50 % D actually contains about 50 % HDO and 25 % each of H2O and D2O, in dynamic equilibrium.

Heavy Water compared with Normal Water:

the deuterium atom weighs about twice as much as the hydrogen atom.An oxygen atom is about 16 times heavier than a hydrogen atom. So a water molecule is about 16+1+1=18 times heavier than a hydrogen atom, and a heavy water molecule is 16+2+2=20 times heavier than a hydrogen atom. In other words, heavy water is 20/18=1.11 times heavier than ordinary water.



Uses:
1) Nuclear magnetic resonance:
Deuterium oxide is used in nuclear magnetic resonance (NMR) spectroscopy when the solvent of interest is water and the nuclide of interest is hydrogen. This is because the signal from the water solvent would interfere with the signal from the molecule of interest. Deuterium has a different magnetic moment from hydrogen and therefore does not contribute to the NMR signal at the hydrogen resonance frequency.

2) Neutron Moderator:
Heavy water is used in certain types of nuclear reactors where it acts as a neutron moderator to slow down neutrons so that they can react with the uranium in the reactor. The CANDU reactor uses this design. Light water also acts as a moderator but because light water absorbs neutrons, reactors using light water must use enriched uranium rather than natural uranium, otherwise criticality is impossible. In effect to achieve criticality in a reactor, one must enrich either the moderator or the fuel.
Because heavy water reactors can use natural uranium, it is of concern in efforts to prevent nuclear proliferation. Heavy water production reactors can be designed to turn uranium into bomb-usable plutonium without requiring enrichment facilities.
PRODUCTION:
On Earth, semiheavy water, HDO, occurs naturally in regular water at a proportion of 1 part per 3200. It may be separated from regular water by distillation or electrolysis and also by various chemical exchange processes, all of which exploit a kinetic isotope effect. In short, the difference in mass between the two hydrogen isotopes translates into a difference in the zero-point energy and thus into a slight difference in the speed at which the reaction proceeds. Once HDO becomes a significant fraction of the water, heavy water will become more prevalent as water molecules trade hydrogen atoms very frequently. To produce pure heavy water by distillation or electrolysis requires a large cascade of stills or electrolysis chambers, and consumes large amounts of power, so the chemical methods are generally preferred. The most important chemical method is the Girdler Sulfide process.

PhysicalProperties:(compared to light water)


Melting point (°C) of D2O= 3.82
Melting point (°C) of H2O= 0.0

Boiling point (°C) of D2O= 101.72
Boiling point (°C) of H2O=100.0

Density (at 20°C, g/mL) of D2O=1.1056
Density (at 20°C, g/mL) of H2O= 0.9982

Temp. of maximum density (°C) of D2O=11.6
Temp. of maximum density (°C) of H2O= 4.0

Viscosity (at 20°C, centipoise) of D2O=1.25
Viscosity (at 20°C, centipoise) of H2O= 1.005

Surface tension (at 25°C, dyn•cm)of D2O= 71.93
Surface tension (at 25°C, dyn•cm)of H2O= 71.97

Heat of fusion (cal/mol) of D2O= 1,515
Heat of fusion (cal/mol) of H2O= 1,436

Heat of vaporization (cal/mol) of D2O=10,864
Heat of vaporization (cal/mol) of H2O= 10,515

NOTE:

Heavy water should not be confused with Hard water or tritiated water.


HARD-WATER & SOFT-WATER

Water containing soluble salts of Calcium & Magnesiumand sometimes iron. (Ca 2+ , Mg 2+ , and/or Fe 3+ ions).

Hard water reduces the cleansing power of soap and produces scale in hot water lines and appliances.
Hard water is any water containing an appreciable quantity of dissolved minerals.
Soft water is treated water in which the only cation (positively charged ion) is sodium. The minerals in water give it a characteristic taste.

Some natural mineral waters are highly sought for their flavor and the health benefits they may confer. Soft water, on the other hand, may taste salty and may not be suitable for drinking.


Soap is less effective in hard water because its reacts to form the calcium or magnesium salt of the organic acid of the soap. These salts are insoluble and form grayish soap scum, but no cleansing lather. Detergents, on the other hand, lather in both hard and soft water. Calcium and magnesium salts of the detergent's organic acids form, but these salts are soluble in water.

Hard water can be softened (have its minerals removed) by treating it with lime or by passing it over an ion exchange resin. The ion exchange resins are complex sodium salts. Water flows over the resin surface, dissolving the sodium. The calcium, magnesium, and other cations precipitate onto the resin surface. Sodium goes into the water, but the other cations stay with the resin. Very hard water will end up tasting saltier than water that had fewer dissolved minerals.
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Last edited by Eram Khan; Saturday, March 04, 2006 at 11:30 AM.
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