C3.2 Flashcards
What does water we drink contain?
Dissolved substances, usually calcium and magnesium
Why does water contain dissolved substances?
~ Water comes into contact with rocks
~ Compounds from rocks dissolve in the water
What is scum?
Insoluble salts
How do you identify whether water is hard or soft?
~ Shake the water with a soap solution
~ Soft water forms a lather
~ Hard water forms scum
What are disadvantages of hard water?
~ Increases cost as more soap is needed
~ Reduces efficiency of heating systems and kettles because temporary hard water produces scale
What are advantages of hard water?
~ Helps people to develop strong teeth and bones
~ Reduces the chance of developing heart disease
Why is silver added to water?
It discourages bacteria growth
What is an advantage and disadvantage of chlorine in water?
+ Kills bacteria
- Disinfectant taste
- People have no choice whether to consume it or not
What does drinking water have to have?
~ Low levels of dissolved salts
~ Low levels of microorganisms
What are the steps in making water safe to drink?
~ Screen (which removes large objects like leaves)
~ Sedimentation (which removes clumps of dirt)
~ Filtration (which uses sand to remove remaining insoluble solids)
~ Disinfection (chlorine is added to kill bacteria)
~ Ion exchange (removes hard ions)
~ Carbon and silver added (carbon decreases chlorine, silver discourages bacteria growth)
What does temporary hard water have?
HCO3 ions
What are advantages and disadvantages of fluorine in water?
+ Good for teeth and bones
+ Reduces chance of heart disease
- Has links with learning difficulties
- People have no choice whether to consume it or not
Why is carbon added to water?
It reduces the amount of chlorine and other chemicals e.g. traces of pesticide
What does permanent hard water have?
Sulphate ions and not many hydrogencarbonate ions, and is not softened by boiling
How is temporary hard water softened?
It contains hydrogencarbonate ions that decompose into carbonate ions when heated. These carbonate ions react with the magnesium/calcium ions to form precipitates of magnesium carbonate/calcium carbonate
How is hard water (permanent or temporary) softened?
~ Addjng sodium carbonate solution (washing soda)
~ or passing the water through an ion-exchange column
How is hard water softened by adding sodium carbonate solution (washing soda)?
The carbonate ions react with the calcium/magnesium ions to form calcium carbonate/magnesium carbonate which precipitate out of solution as they’re insoluble
How is hard water softened by passing it through an ion exchange column?
~ Column has resin that contain hydrogen/sodium ions
~ As the water passes through, the calcium and magnesium ions are replaced by the hydrogen/sodium ions from the resin
Why does the resin need to be replaced in an ion exchange column?
Because they will run out of hydrogen/sodium ions to exchange with the magnesium and calcium
How can any water be distilled to produce pure water?
~ By boiling the water to produce steam and then condensing the steam by cooling it to produce pure liquid water
~ This process uses lots of energy so is expensive
