Hard Water Basics

Hard Water is water that has a high mineral content.  This consists of high levels of metal ions, mainly calcium (Ca) and magnesium (Mg) in the form of carbonates, but many include several other metals as well as bicarbonates and sulfates.  It is not generally dangerous.  The simplest way to determine if water is hard or soft is the lather/froth test.  If water is very soft soap will tend to lather up easily when agitated, whereas with hard water it will not.  Toothpaste will also not froth well in hard water.  More exact methods of hardness detection use a wet titration method to determine it.
Total water hardness (including both calcium and magnesium) is reported in mg/L of CaCO3.  Water hardness usually measures the total concentration of Ca and Mg, the two most prevalent divalent metal ions, although in some geographical locations iron, aluminum, and manganese may also be present at elevated levels.  Calcium usually enters the water supply from either CaCO3, as limestone or chalk from mineral deposits of CaSO4.  The predominant source of magnesium is dolomite, CaMg(CO3)2.
A Common distinction is made between temporary and permanent hardness. There are also common types of hard water depending on the ion (Mg or Ca) found in the water.

Temporary hardness is hardness that can be removed by boiling or by the addition of hydrated lime (calcium hydroxide).  It is caused by a combination of calcium ions and bicarbonate ions in the water.  Boiling, which promotes the formation of carbonate from the bicarbonate will precipitate calcium carbonate from the solution, leaving water that is less hard upon cooling.  Upon heating, less CO2 is able to dissolve into the water.  Since there is not enough CO2 around, the reaction cannot proceed and therefore the CaCO3 will not dissolve as readily.  Instead, the reaction is forced to reestablish equilibrium and solid CaCO3 that precipitates out is removed.  After cooling, if enough time passes the water will pick up CO2 from the air and the reaction will again proceed from left to right, allowing the CaCO3 to redissolve in the water.

Permanent hardness is hardness that cannot be removed by boiling.  It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble temperature rises as opposed to calcium carbonate which becomes less soluble as the temperature rises.  Despite its name, this can be removed through ion exchange or zeolite water softening. 

Hard water causes scaling, which is the precipitation of minerals to form a deposit called limescale.  Scale can clog pipes and can decrease the life of toilet flushing units.  It can coat the inside of tea and coffee pot, and clog and ruin water heaters. In industry hard water contributes to scaling in boilers, cooling towers and other industrial equipment.  A boiler whose tubes are coated with scale loses efficiency
very rapidly due to the insulating action of the scale.  In these industrial settings, water hardness must be constantly monitored and controlled to avoid costly breakdowns.  Hardness is controlled by addition of chemicals and by large-scale softening with zeolite resins or ion exchange resins. 

There are several different ways used to express the hardness of water.

These are:  
1)  mg/L calcium carbonate equivalent
2)  grains/gallon (gpg); 1 gpg equals 17.1 mg/L
3)  parts per million (ppm) which is basically the same as mg/L