A common distinction is made between 'temporary' and 'permanent' hardness. There are also common types of hard water depending on the ion (eg. Mg or Ca) found in the water.
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. Boiling, which promotes the formation of carbonate from the bicarbonate, will precipitate calcium carbonate out of solution, leaving water that is less hard on cooling.
Upon heating, less CO2 is able to dissolve into the water. Since there is not enough CO2 around, the reaction cannot proceed from left to right, and therefore the CaCO3 will not "dissolve" as readily. Instead, the reaction is forced to go from right to left (i.e. products to reactants) to reestablish equilibrium, and solid CaCO3 is formed. Heating water will remove hardness as long as the 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
Permanent hardness is hardness (mineral content) 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 as the temperature rises. Despite the name this can be removed using a water softener, or ion exchange column.
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2 comments:
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