C3 - Water
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What is hard water?
Water that contains dissolved compounds such as calcium and magnesium salts.
What does hard water cause?
Scum
How does scum form?
Soap reacts with dissolved salts.
Why isn’t scum a problem when washing clothes?
Modern detergents are soapless.
How is hard water formed?
When streams or rivers flow over rocks containing calcium or magnesium compounds, the compounds dissolve in the water.
How does limestone produce hard water?
• As rain falls, carbon dioxide dissolves in the water making the water in rivers slightly acidic. • CaCO3 reacts with the acidic solution forming calcium hydrogencarbonate. • Ca2+ ions get into the water making it hard.
What is the equation for forming hard water with limestone?
CaCO3 + H2O + CO2(aq) —-> Ca2+ + 2HCO3-
What is the equation for calcium reacting with soap?
sodium stearate + Ca2+ —-> calcium stearate precipitate + Na+ ions
Why is using hard water expensive?
More soap is needed since some is wasted reacting with the ions in the water.
How is limescale formed?
When water containing calcium ions is heated.
Give two advantages of hard water.
• Calcium ions help in the development of strong bones. - Hard water could help reduce heart disease.
What is soft water?
Water that does not contain ions that produce scum or scale.
What is temporary hard water?
Water that can have its hardness removed by heating.
What is permanent hard water?
Water that remains hard after boiling.
What types salts produce permanent hard water?
Sulfates
What is the equation for heating temporary hard water?
Ca2+ 2HCO- —-> CaCO3 + H2O + CO2
What are the two methods for softening water?
• Washing Soda - Ion Exchange System
What does washing soda contain?
Sodium Carbonate
How does washing soda soften water?
The soluble carbonate ions precipitate out calcium and magnesium ions.
How do ion-exchange columns work?
The sodium ions replace the magnesium or calcium ions when hard water passes through a resin.
How can ion-exchange systems be recharged?
Washing with sodium chloride solution.
What does chlorine do in water treatment?
Kill microbes
What does a water filter usually contain?
• Carbon • Ion exchange resin • Silver
What does the carbon in a water filter do?
• Reduces the levels of chlorine - Reduces amounts of pesticides and other impurities.