Water is a good solvent and is, therefore, full of solubles. Hardness of water is the property, which reduces the cleaning effect of detergents and soaps. Hardness in water is expressed in terms of equivalent calcium carbonate measured in mg/l (ppm or mg dm -3 ). Hardness is not harmful to humans but is objectionable if calcium deposits build up on every thing. In industrial applications the hardness also causes fouling of heat exchangers and boiler tubes.
Hard water gives less lather with soap when compared to soft water. Presence of carbonates and sulphates of calcium and magnesium in water causes the hardness of water. Sometimes salts of chlorides and nitrates also cause hardness. The most common dissolved minerals are calcium bicarbonate, magnesium bicarbonate, calcium chloride, magnesium chloride, and magnesium sulfate. Bicarbonates of calcium and magnesium are soluble, and we refer to their presence as temporary hardness.When water with bicarbonates is heated, the bicarbonates will change to carbonates.Carbonates are not very soluble and will deposit out of solution, forming scale or precipitate. Scale on heat transfer surfaces decreases heat transfer. When scale accumulates in pipelines, it will eventually reduce the pipe diameter sufficiently to reduce the flow of water.
Carbonic acid is formed when carbon dioxide dissolves rainwater. Carbonic acid dissolves limestone and forms bicarbonates. The reactions are shown below:
CO 2 + H 2 O < —— > H 2 CO 3
Ca CO 3 + H 2 CO 3 < —— > Ca(HCO 3 ) 2
On boiling, this reaction is reversed, and the low-solubility calcium carbonate will precipitate. At the same time, reaction with sulfates and chlorides in the water will cause permanent hardness. This will cause a hard scale in pipelines or on equipment.To avoid this, the water is softened to remove the dissolved salts. If the source of water is near to saline water zone, the water may contain more chlorides.
Hard water gives less lather with soap when compared to soft water. Presence of carbonates and sulphates of calcium and magnesium in water causes the hardness of water. Sometimes salts of chlorides and nitrates also cause hardness. The most common dissolved minerals are calcium bicarbonate, magnesium bicarbonate, calcium chloride, magnesium chloride, and magnesium sulfate. Bicarbonates of calcium and magnesium are soluble, and we refer to their presence as temporary hardness.When water with bicarbonates is heated, the bicarbonates will change to carbonates.Carbonates are not very soluble and will deposit out of solution, forming scale or precipitate. Scale on heat transfer surfaces decreases heat transfer. When scale accumulates in pipelines, it will eventually reduce the pipe diameter sufficiently to reduce the flow of water.
Carbonic acid is formed when carbon dioxide dissolves rainwater. Carbonic acid dissolves limestone and forms bicarbonates. The reactions are shown below:
CO 2 + H 2 O < —— > H 2 CO 3
Ca CO 3 + H 2 CO 3 < —— > Ca(HCO 3 ) 2
On boiling, this reaction is reversed, and the low-solubility calcium carbonate will precipitate. At the same time, reaction with sulfates and chlorides in the water will cause permanent hardness. This will cause a hard scale in pipelines or on equipment.To avoid this, the water is softened to remove the dissolved salts. If the source of water is near to saline water zone, the water may contain more chlorides.
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.