Although it may not be common knowledge, there can actually be a difference between the type of hard water that is supplied to homes throughout the world - water can have a 'temporary' or 'permanent' hardness. There are also common types of hard water depending on the ion (eg. magnesium or calcium) found in the water, which can cause different types of hard water stains and limescale formation.
Temporary Hardness
Temporary hardness is hardness that can be removed by boiling or by the addition of lime (calcium hydroxide). It is caused by a combination of calcium ions and bicarbonate ions in the water. By boiling the water, it promotes the formation of carbonate from the bicarbonate and will precipitate calcium carbonate (the limescale) out of solution, leaving water that is less hard after it has cooled.
When it has been heated, less carbon dioxide is able to dissolve into the water. Since there is not enough carbon dioxide around, the reaction cannot take place, and therefore the calcium carbonate will not "dissolve" as readily. Instead, the reaction is forced to re-establish equilibrium, and the solid calcium carbonate is formed. Heating water will remove hardness as long as the limescale that precipitates out is removed. After cooling, if enough time passes the water will pick up carbon dioxide from the air and the reaction will again proceed, allowing the calcium carbonate to "redissolve" in the water.
Permanent Hardness
Permanent hardness is hardness (mineral content) that cannot be removed by boiling. When this is the case, it is usually caused by the presence of calcium and magnesium sulphates and/or chlorides in the water, which become more soluble as the temperature rises. Despite the name, the hardness of the water can be easily removed using a water softener, or ion exchange column.
Hard Water Testing
If you are on a municipal water system, and would like to find out the hardness of the water to your home, the water supplier can tell you the hardness level of the water they deliver. If you have a private water supply, you can have the water tested for hardness. There are several different scales used to describe the hardness of water in different contexts.
The precise mixture of minerals dissolved in the water, together with the water's acidity or alkalinity (pH) and temperature will determine the behaviour of the hardness, so single number on a scale does not give a full description. Descriptions of hardness correspond roughly with ranges of mineral concentrations:
Soft: 0 - 20 mg/L as calcium
Moderately soft: 20 - 40 mg/L as calcium
Slightly hard: 40 - 60 mg/L as calcium
Moderately hard: 60 - 80 mg/L as calcium
Hard: 80 - 120 mg/L as calcium
Very Hard: >120 mg/L as calcium
As a guide, 60% of the UK and 85% of the US population live in a hard water area, which can cause stubborn hard water stains on fixtures and fittings. Most water softener companies will be able to supply you with a free water testing kit. Once you've tested your water supply, the hardness of your water will be reported in grains per gallon, milligrams per liter (mg/l) or parts per million (ppm). One grain of hardness equals 17.1 mg/l or ppm of hardness.
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