What is hard water?
What are the differences between temporary and permanent hard water?
How can hard water be converted to soft water?
How to produce deionized water?
Water is the most important resource. Without water life is not possible.
From a chemical point of view, water, H2O, is a pure compound,
but in reality, you seldom drink, see, touch or use pure water. Water from
various sources contains dissolved gases, minerals, organic and inorganic
substances. This photograph of Guilin shows the beauty of natural water.
The rain curved an interesting landscape out of the lime stones in the area.
Natural waters are often important parts of wonders of the world.
The total water system surrounding the planet Earth is called the hydrosphere.
It includes freshwater systems, oceans, atmosphere vapour, and biological waters.
The Arctic, Atlantic, Indian, and Pacific oceans cover 71% of the
Earth surface, and contain 97% of all water. Less than 1% is
fresh water, and 2-3 % is ice caps and glaciers. The Antarctic
Ice Sheet is almost the size of North America continent.
These waters dominate our weather and climate, directly and indirectly
affecting our daily lives. They cover 3.35x108 km2.
The four oceans have a total volume of 1.35x109 km3.
The sunlight dims by 1/10 for every 75 m in the ocean, and humans
barely see light below 500 m. The temperature of almost all of the
deep ocean is 4°C (277 K).
The average ocean depth is 4 km, and the deepest point at the Mariana
Trench is 10,912 m (35,802 ft), which compares to the height
of 8.8 km for Mount Everest.
A Smithsonian Institution Traveling Exhibition
is on view at Museum of Science Boston, Massachussetts October 10,
1998 to January 3, 1999. This web site takes you to the sight and sound of
deep oceans almost just as good as the exhibits.
The distribution of water and other materials on Earth is given in
Many fact sheets are available in this link.
Hydrospheric processes are steps by which water cycles on the planet
Earth. These processes include sublimation of ice, evaporation of liquid,
transportation of moisture by air, rain, snow, river, lake, and ocean
currents. All these processes are related to the physical and chemical
properties of water, and many government agencies are set up to
study and record phenomena related to them. The study of these processes
is called hydrology
Among the planets, Earth is the only one in which there are solid, liquid
and gaseous waters. These conditions are just right for life, for which water
is a vital part. Water is the most abundant substance in the biosphere
is an important part of the water system. When vapor is cooled, clouds and
rain develop. Some of the rain percolate through the soil and into the
underlying rocks. The water in the rocks is groundwater,
which moves slowly.
A body of rock, which contains appreciable quantities of water, is called an
aquifier. Below the water table, the aquifier is filled
(or saturated) with water. Above the water table is the unsaturated zone.
Some regions have two or more water tables. These zones are usually separated
by water-impermeable material such as boulder and clay.
Groundwater can be brought to the surface by drilling below the water table,
and pumped out. The amount of water that can be pumped out depends on the
structure of the aquifer. Little water is stored in tight granite layers,
but large quantities of water are stored in limestone aquifier layers.
In some areas, there are under ground rivers.
Canada fresh water resources
Canada has 20% of the world's fresh water supply. But 60% of that water
flows north into the Arctic Ocean, away from the vast majority of Canadians.
Environment Canada provides
useful information regarding water resources, management, and related
Fresh water is a commodity. As such, it is a focal point of
trade between Canada
and the United States.
Common ions present in natural water
|Ions in sea water
Hydrology is also the study of how solids and solute interact in, and with,
water. In this link, the compositions of seawater, composition of the
atmosphere, compositions of rain and snow, and compositions of river waters
and lake waters are given in details.
A simplified table of the major ions present in seawater from the above
link is given here. The composition does vary, depending on region, depth,
latitude, and water temperature. Waters at the river mouths contain
less salt. If the ions are utilized by living organism, its contents
vary according to the populations of organisms.
Dust particles and ions present in the air are nucleation center of water drops.
Thus, waters from rain and snow also contain such ions:
Ca2+, Mg2+, Na+, K+,
NH4+. These cations are balanced by anions,
HCO3-, SO4-, NO2-,
Cl-, and NO3-. The pH of rain is between
5.5 and 5.6.
Rain and snow waters eventually become river or lake waters.
When the rain or snow waters fall, they interact with vegetation, top soil,
bed rock, river bed and lake bed, dissolving whatever is soluble.
Bacteria, algae, and water insects also thrive. Solubilities of inorganic
salts are governed by the kinetics and equilibria of dissolution.
The most common ions in lake and river waters are the same as those present
in rainwater, but at higher concentrations. The pH of these waters
depends on the river bed and lake bed. Natural waters contain dissolved
minerals. Waters containing Ca2+ and Mg2+ ions
are usually called hard water.
Minerals usually dissolve in natural water bodies such as lakes, rivers,
springs, and underground waterways (ground waters). These photographs
show crystals of typical
calcite, present abundantly in the Earth Crust.
Calcite is the
major mineral in limestones, marbles, and sea shells.
Calcium carbonate, CaCO3, is one of the most common inorganic
compounds in the Earth crust. It is the ingradient for both
and aragonite. These two minerals have different crystal structures
and appearance. This photograph shows crystals of typical
Calcium-carbonate minerals dissolve in water, with a solubility product
as shown below.
CaCO3 = Ca2+ + CO32-,
Ksp = 5x10-9
From the solubility product, we can (see example 1) evaluate the molar
solubility to be 7.1x10-5 M or 7.1 mg/L (7.1 ppm of
CaCO3 in water). The solubility increases as the pH decrease
(increase acidity). This is compounded when the water is saturated with
carbon dioxide, CO2. Saturated CO2 solution contains
carbonic acid, which help the dissolution due to the reaction:
H2O + CO2 = H2CO3
CaCO3 + H2CO3 = Ca2+ + 2 HCO3-
Because of these reactions, some natural waters contain more than 300 ppm calcium carbonates or
The carbon dioxide in natural water creates an interesting phenomenon.
Rainwater is saturated with CO2, and it dissolves limestones.
When CO2 is lost due to temperature changes or escaping from
water drops, the reverse reaction takes place. The solid formed, however,
may be a less stable phase called aragonite, which has the same chemical
formula as, but a different crystal structure than that of calcite.
The photograph here shows
aragonite and calcite. A common progression from aragonite due to
evaporative enrichment is seen this photograph, where a nodular mass of
calcite popcorn is tipped by sprays of aragonite needles.
The rain dissolves calcium carbonate by the two reactions shown above.
The water carries the ions with it, sips through the crack of the rocks.
When it reached the ceiling of a cave, the drop dangles there for a
long time before fallen. During this time, the carbon dioxide escapes
and the pH of the water increases. Calcium carbonate crystals begin
to appear. Calcite, aragonite, stalactite, and stalagmite are four
common solids found in the formation of caves.
Natural waters contain metal ions. Water containing calcium, magnesium
and their counter anions are called hard waters. Hard waters
need to be treated for the following applications.
- Heat transfer carrier in boilers and in cooling systems
- Solvents and reagents in industrial chemical applications
- Domestic water for washing and cleaning
Temporary and permanent hard water
Due to the reversibility of the reaction,
CaCO3(s) + H2CO3 = Ca2+ + 2 HCO3-
water containing Ca2+, Mg2+ and CO32-
ions is called temporary hard water, because the hardness can be
removed by boiling. Boiling drives the reverse reaction, causing deposit
in pipes and scales in boilers. The deposits lower the efficiency of
heat transfer in boilers, and diminish flow rates of water in pipes.
Thus, temporary hard water has to be softened before it enters the boiler,
hot-water tank, or a cooling system. The amount of metal ions that can
be removed by boiling is called temporary hardness
After boiling, metal ions remain due to presence of chloride ions, sulfate
ions, nitrate ions, and a rather high solubility of MgCO3.
Amount of metal ions that can not be removed by boiling is called
permanent hardness. Total hardness is the sum of temperary hardness
and permanent hardness. Hardness is often expressed as equivalence of
amount of calcium ions in the solution.
Thus, water conditioning is an important topic. The value
of water treatment market has been estimated to be worth $30 billion.
Lime-soda softening is the removal of temporary hardness by adding a
calculated amount of hydrated lime, Ca(OH)2:
Ca2+ + 2 HCO3- + Ca(OH)2(s)
® 2 CaCO3(s) + 2 H2O
Adding more lime causes the pH of water to increase, and as a result,
magnesium ions are removed by the reaction:
Mg2+ + Ca(OH)2(s)
® Mg(OH)2(s) + Ca2+
The extra calcium ions can be removed by the addition of sodium carbonate.
2 Na+ + CO32-
Ca2+ + CO32- +
In this treatment, the amount of Ca(OH)3 required is equivalent
to the temporary hardness plus the magnesium hardness. The amount of
sodium carbonate required is equivalent to the permanent hardness.
Thus, lime-soda softening is effective if both the temporary and
total hardness have been determined. The sodium ion will remain in
the water after the treatment. The pH of the water is also rather high
depending on the amount of lime and sodium carbonates used.
Addition of complexing reagent to form soluble complexes with
Ca2+ and Mg2+ prevents the formation of solid.
One of the complexing agents is sodium triphosphate Na3PO4,
which is marketed as Calgon, etc. The phosphate is the complexing
agent. Other complexing agents such as Na2H2EDTA
can also be used, but the complexing agent EDTA4- forms
strong complexes with transition metals. This causes corrosion
problem, unless the pipes of the system are made of stainless steel.
Today, most water softeners are using zeolites and employing ion exchange
technique to soften hard water.
Zeolites are a group of hydrated crystalline
aluminosilicates found in certain volcanic rocks.
The tetrahedrally coordinated aluminum and silicon atoms form
AlO4 and SiO4 tetrahedral groups.
They interconnect to each other sharing oxygen atoms forming cage-type
structures as shown on the right. This diagram and the next structural
diagram are taken from
an introduction to zeolites
There are many kinds of zeolites, some newly synthesized.
Whatever kind, the crystal structure of zeolites contains large cages.
The cages are connected to each other forming a framework with many
cavities and channels. Both positive and negative ions can be trapped
in these cavities and channels as shown below.
For each oxygen that is not shared in the AlO4 and
SiO4 tetrahedral groups, a negative charge is
left on the group. These negative charges are balanced by trapping
alkali metal and alkaline earth metal ions. When more cations
are trapped, hydroxide and chloride ions will remain in the
cavities and channels of the zeolites.
To prepare a zeolite for water treatment, they are soaked in concentrated
NaCl solution. The cavities trap as many sodium ions as they can accommodate.
After the treatment, the zeolite is designated as Na-zeolite.
Then the salt solution is drained, and the zeolite is washed with water
to eliminate the extra salt. When hard water flow through them, calcium and
magnesium ions will be trapped by the Na-zeolite. For every Ca2+
or Mg2+ trapped, two Na+ ions are released.
The treated water contains a rather high concentration of Na+
ions, but low concentrations of Mg2+ and Ca2+.
Thus, zeolite ion exchange convert hard water into soft water.
Pure water by ion exchange
ion exchange resins for water treatment.
Anderson has its
own brand, but the type of resin is not specified in their web site.
markets Strongly Acidic Styrene Type Cation Exchange Resin series,
Strongly Basic Styrene Type Anion Exchange Resin series, Macroporous
Weakly Basic Styrene Type Anion Exchange Resin series, Macroporous Weakly
Acidic Acrylic Acid Type Cation Exchange Resin series, and more than 30
types Ion Exchange Resin. All ion exchange resins are almost spherical
beads as shown in the picture (from DongDa).
In most cases, the resins are polystyrene with functional -SO3H
groups attached to the polymer chain for cation exchange resin, and with
functional group -N(CH3)3+ attached to
the chain for anion exchange resin.
To prepare the resin for making pure or deionized water, the cation
resin is regenerated with HCl so that the groups are really -SO3H.
The anion resin is regenerated with NaOH, so that the functional groups are
-N(CH3)3(OH). When water containing any metal ion
M+ and anion A- passes through the ion exchange
resins in two stages, the following reactions take place,
M+ + -SO3H ®
H+ + -SO3M
A- + -N(CH3)3(OH) ®
OH- + -N(CH3)3A
H+ + OH- = H2O
Thus, ion exchange provides pure water to meet laboratory requirement.
Reverse osmosis water filter system
This method can also be used to prepare water for domestic and laboratory
applications. This method has been discussed in
Magnetic Water Treatment
Problems of temporally hard water deposit plugging pipes and boilers
have been addressed above.
The following is a list of companies selling magnetic devices for
magnetic water treatment. All devices are based on results of
some ressearch indicating that when water runs through a magnetic field,
the calcium carbonate will precipitate as aragonite rather than the usual
calcite. For example,
has published an article in IEEE Transactions on Magnetics,
(Vol. Mag-21, No. 5, September 1985, pages 2059-2061). and stated the
The crystallization mode of the water's mineral content was found to
change from a dendritic, substrate-bound solidification habit to the
form of separate disc-shaped crystals after the water had moved through
a number of magnetic fields. The former scarcity of crystallization
nucleii in the water had been turned into an abundance of nucleation
centers in the water. The reduction of the number of the substrate-bound
crystals has been used as a quantitative measure of the magnetic effect.
Many companies have made various devices for magnetic conditioning of water,
and they claim that their devices will clean up the pipes and boilers
at little or no cost. I have yet to test one of these devices for its
claim, but my preliminary tests shows that permanent magnet has little
effect on the calcium carbonate deposit of temporary hard water.
The cleaning effect they have claimed is probably much overstated.
In any case, their web sites are well made, and they are listed here
for you to scrutinize. Listing their web sites is not a recommandation
of their devices. Any independent testing result is welcome.
water treatment products under the trade name
Water King. There are many
animated pages. Prices L110-2500.
Scale Watch The cost is $399 each.
This site gives the following documents: Scale Introduction | The Science |
How It Works | Benefits | Versus Magnets.
various models depending on pipe size. (400 for 1/2 pipe $125;
800 for 1" pipe $200; 848 for 2" pipe $200; 800 for 8" pipe, $480. See also
GMX Magnetic Water Conditioning
Prices are different. Model RV800 unit is mounted on plastic hose.
The device is also sold by
Magnetic Conditioning System
uses permanent magnets, $600 for the 512 Unit.
Magnetic Magic The Economical Hard Water Solution
Offers 3 units for $180.
Magnetic Water Conditioning Newspaper/Magazine Articles
offers a good description of the chemistry and physics of the methods.
It offers a lot of information on various types of AQUA units, including
a distributor list.
neodymium magnetic clamps. The two part sets simply clamp on the water pipes.
3 sets / $199
ADTenterprises Sales Mineral Magnets
for water pipe treatment, $50
HealthMagnets sales Mineral Magnets for $70
Ionstick (of Toronto) offers
non-chemical electrostatic water treatment device for water treatment, no
offers 3600 gauss magnet at $120.
KAT's Magnetic Water Conditioning System
starts at $180.
Western's MWC page
gives a fantastic water page, sells Western Enterprise's products.
has various types of water treatment devices, including magnet rings for
water conditioning, various sizes for tubes of diameters.
Neo-V2 Water Magnet
sells 3,600 gauss claim for $50.
ELECTROCHEMICAL WATER CONDITIONING
has many incorrect and vague statements. It offers magnetic, galvanic,
and catalytic conditioners.
Electrical Conditional Prevents Lime Scale product application for W512
Sigma Water System
Dipolar Water Conditioner, very similar to the previous document.
Fundamentals of Water Treatment
Non-chemical Water Conditioners
Wagner-Magnete.de - aqua-tec
Equipment Standards -
Harmony or Discord Stories regarding the European water treatment.
National Weather Service of U.S.
From the solubility product shown for the dissolution of calcium carbonate,
CaCO3 = Ca2+ + CO32-,
Ksp = 5x10-9
Evaluate the molar solubility of Ca2+ in saturated solution.
From the definition of solubility product, we have
[Ca2+] [CO32-] = 5x10-9
[Ca2+] = [CO32-] = 7.1x10-5 M
The concentration of 7.1x10-5 M is equivalent to
7.1 mg/L (7.1 ppm of CaCO3 in water).
There may be other ions present in the system and other equilibria conditions
in addition to the equilibrium mentioned here. Problems are more complexed
in the real world.
Confidence Building Questions
High Field Magnet Laboratory
provides high magent field for material research, but not water scale remover.
is a non-toxic, non-corrosive, non-flammable, non-injurious tar oil base
solvent, heavily fortified with wetting and penetrating agent. It is used
for the rapid removal of water scale, lime and rust deposits found on
the water side of any equipment that is water cooled, water heated or
water operated, in any manner. An Material Safety Data Sheet (MSDS)
Rydlyme is a liquid water scale
solvent, almost the same as MarSolve.