Hardness of Water

Soft water forma a lather easily when mixed with soap, but hard water does not

• Hardness in water is caused by dissolved magnesium ions and calcium ions
• These can get into the water when it comes into contact with limestone and other rocks that contain calcium compounds
• One way of this happening is when rainwater flows over these rocks on its way to a reservoir

Permanent hardness is caused by dissolved calcium sulfate. Unlike temporary hardness, it is not removed by boiling the water

Temporary hardness is caused by dissolved calcium hydrogencarbonate

• Rainwater is naturally acidic because it contains dissolved carbon dioxide from the air
• It reacts with calcium carbonate in rocks to form calcium hydrogencarbonate, which is soluble
• calcium carbonate + water + carbon dioxide --> calcium hydrogencarbonate
• Temporary hardness is removed by boiling the water
• When this happens, the soluble calcium hydrogencarbonate decomposes to form calcium carbonate, water and carbon dioxide
• calcium hydrogencarbonate --> calcium carbonate + water + carbon dioxide
• The insoluble calcium carbonate forms a layer of limescale
• This may coat the heating element in kettles and irons, making them less efficient
• Limescale is unslightly and it clogs up hot water pipes and boilers

Washing soda

• Sodium carbonate, also known as washing soda, can soften both temporary and permanent hard water
• It is soluble in water and adds a large amount of carbonate ions to the water
• These react with dissolved calcium ions forming a precipitate of calcium carbonate

Calcium ions come from the water and carbonate ions come from the washing soda

Ion exchange

• Ion exchange resins can soften both permanent and temporary hard water
• As hard water passes through the column, sodium ions come off the resin and go into the water, while calcium ions come out of the water and stick to the resin
• Dishwashing machines ccontain ion exchange resin to soften the water used to was the dishes. The resin needs recharging with dishwasher salt once it becomes full of calcium ions