C6 - Water Flashcards
What are the properties of soft and hard water? Why?
- Soft water contains low concentrations of calcium ions and magnesium ions, and easily forms a lather with soap.
- Hard water contains higher concentrations of these ions, which stops the soap forming a lather easily.
What causes hard water? Give the word and symbol equation.
Carbon dioxide in the air dissolves in the rain, producing a weak acid. Chalk and limestone consist mainly of calcium carbonate. Rain falling on these rocks reacts with the calcium carbonate to form calcium hydrogencarbonate.
Calcium carbonate + water + carbon dioxide -> calcium hydrogencarbonate
CaCO3 + H2O + CO2 → Ca(HCO3)2
What is the difference between calcium carbonate and calcium hydrogencarbonate?
Calcium carbonate is insoluble in water, but calcium hydrogencarbonate is not.
What are the two types of hardness and what are they caused by?
• permanent hardness – caused by dissolved calcium sulfate
• temporary hardness – caused by dissolved calcium hydrogencarbonate
What is one method to remove temp hardness that will not remove permanent hardness? Why? Give a word and symbol
equation.
When water with temporary hardness is boiled, the hardness is removed. This happens because calcium hydrogencarbonate decomposes when heated:
Calcium hydrogencacarbonate-> carbon dioxide+water+calcium carbonate
Ca(HCO3)2 → CaCO3 + H2O + CO2
What is limescale? How is it removed? Word equation.
- Limescale is calcium carbonate. It damages equipment that uses hot water and increases the amount of energy used in them.
- Limescale removers are weak acids, like citric acid.
- calcium carbonate + citric acid ->calcium citrate + water + carbon dioxide
How does washing soda remove water hardness? Give a word and symbol equation.
When washing soda is added to hard water, a precipitation reaction happens. For example:
calcium hydrogencarbonate +sodium carbonate -> calcium carbonate + sodium hydrogencarbonate
Ca(HCO3)2 + Na2CO3 → CaCO3 + 2NaHCO3
The hard water is softened because the calcium ions and magnesium ions are removed as a precipitate.
How does ion - exchange remove water hardness?
Ion-exchange columns contain ion-exchange resins. Hard water is softened as it passes through an ion-exchange column. As the hard water passes through, calcium ions and magnesium ions from the water attach to the resin and sodium ions leave it. The water is softened because the calcium ions and magnesium ions are removed. Eventually, all the sodium ions in the resin are replaced by calcium ions and magnesium ions. When this happens, the resin is regenerated by flushing sodium chloride solution through the column. Sodium ions are swapped back into the resin. Calcium ions and magnesium ions are pushed out and washed away in waste water.
