Water Flashcards
What is meant by hard water?
Water that will not easily form a lather with soap
What is hard water caused by?
The presence of calcium ions ( Ca 2+) or Magnesium ions (Mg2+)
Explain with the aid of a balanced chemical equation why does hard water not form a lather with soap?/ How does hard water waste soap?
Soap is a salt of a fatty acid.
Sodium stearate (C17H35COONa)
the Stearate ion in soap reacts with the Ca+ / Mg+ ions in hard water forming calcium/magnesium stearate, an insoluble grey ‘scum’
The soap does not form a lather until all of the Ca/Mg ions are used up, soap is wasted
Ca2+ + 2C17H35COO – –>
(C17H35COO)2 Ca
Chemical equation for Sodium stearate
Sodium stearate (C17H35COONa)
Name the two types of hardness in water
tempoary hardness
permanent hardness
Compare Tempoary hardness and Permanent hardness
Tempoary Hardness
Hardness that can be removed by boiling the water
caused by The presence of calcium hydrogencarbonate (Ca(HCO3)2)
or magnesium hydrogencarbonate (Mg(HCO3)2)
Permanent hardness
Hardness that cannot be removed by boiling the water
caused by the presence of calcium sulfate (CaSO4) or magnesium sulfate (MgSo4)
How does tempoary hardness in water arise?
Rainwater is slightly acidic due to carbon dioxide from the air dissolving it, forming a weak carbonic acid
as this acidic rainwater flows over limestone rocks, it reacts with it, forming calcium hydrocarbonate
How does boiling remove tempoary hardness?
The soluble calcium hydrocarbonate breaks down into calcium carbonate, CO2 and water
Calcium carbonate is insoluble and precipitates out of solution, the water is softened
Whats the common name for Calcium Carbonate (CaCo3)
Limescale
Give two disadvantages of hard water
causes limescale which builds up on and damages machines that use hot water- kettles
Hard water wastes soap - soap wont form a lather until the ions causing the hardness have been used up
What is total hardness?
The sum of permanent hardness and tempoary hardness
How can the total hardness in a water sample be removed?
Distillation
washing soda tablets (they use sodium carbonate)
Using an ion-exchange resin
Outline how water can be softened by an ion-exchange resin, so it is suitable for use in the home
a cation exchange resin containing Na= ions is used
as the hard water passes through the resin, each Ca/Mg ion is replaced by two Na ions
the softened water passes through
Outline how water how water can be softened by an ion-exchange resin
so it is suitable for use as deionised water in the laboratory
A mixed bed resin is used
cation-exchange resion w/ H+ ions
anion exchange resin w/ OH- ions
as water passes through
each Ca/mg ions is replaced by two H+ ions from resin
any negatice ions are replaced by OH- ions
the H+ and OH- ions form water
Compare deionised water and distilled water
deionised water
contains ni ions, but can contai suspended solids, dissolved organic material and dissolved gasses
not as pure as distilled water
distillled water
contains no ions, no suspended solids, no dissolved organic material
most pure form of water
Name the stages to process in water treatment to provide water for households and factories
Screening
Flocculation
Sedimentation
filtration
Chlorination
Fluoridation
PH adjustment
explain the process of Screnning
water is passed through a wire mesh, removing twigs, plastic bags and debris
explain the procces of flocculation
Flocculation is the clumping together of small, suspended particles into larger particles ( also known as coagulation)
What is a flocculating agest/coagulant
A flocculating agest is a chemical added to water to clump together small, suspended particles into larger particle, aiding sedimentation
name an example of a flocculating agent/ coagulant
Aluminium sulfate Al2(SO4)3
in Flocculation, why must the amount of aluminium sulfate added be controlled
causes corrosiom of pipes
affects the taste of water
describe the process of Sedimentation
water is passed into settling tanks and usupended solids settle to bottom
clean water overflows into channel
90% of particles removed
describe the process of filtration
water from top of the settlng tanks is passed through neds of sand and gravel underneath
all suspended solids removed
describe the process of chlorination
chlorine is added in small quantites to kill harmful microrganisms and sterilise it
why must the amount of chlorine added in Chlorination be carefully controlled?
Too much chlorine added is toxic and gives the water an unpleasant taste and smell
describe the process of Fluoridation
small amounts of fluoride compounds are added to strenghten enamel and help prevent tooth decay.