Unit2Topic2 - Water Flashcards
What % of the earth is covered in water?
70%
Tests for presence of water
Anhydrous copper sulfate (white->blue)
Cobalt chloride paper (blue->pink)
Test for pure water
Melts at 0°C and boils at 100°C
What is hard water?
Hard water is water that doesn’t lather easily with soap
Which ions cause water to be hard?
Calcium and magnesium cations (Ca2+ and Mg2+)
What happens with ions in hard water areas?
The calcium ions combine with the stearate ions in soap to form insoluble calcium stearate (scum).
Formation of scum word equation
soap(sodium stearate) + hard water -> scum(calcium stearate) + sodium ions
Formation of scum symbol equation
2C17H35COONa (aq) + Ca2+ (aq) -> (C17H35COO)2Ca (s) + Na+ (aq)
Test to identify hard/soft water / comparison
Add 10cm3 of each water sample in separate conical flasks.
Add 1cm3 of soap solution from burette, put a bung in the conical flask and shake for 30 seconds.
Continue adding 1cm3 of soap solution and shake until a permanent lather lasting more than 30 seconds is formed.
Record volume of soap solution added.
The more soap required, the harder the water.
What is temporary hardness caused by?
These salts dissolved in water…
- calcium hydrogencarbonate
- magnesium hydrogencarbonate
How is temporary hardness formed?
Rainwater is naturally acidic (weak carbonic acid). The acid reacts with the limestone to form these soluble salts.
Temporary hard water word equation
Carbonic acid + calcium carbonate -> calcium hydrogencarbonate
Temporary hard water symbol equation
H2CO3 (aq) + CaCO3 (s) -> Ca(HCO3)2 (aq)
How is permanent hardness formed?
Permanent hardness is formed by rain water passing over gypsum rocks containing calcium compounds (eg calcium sulfate) which can dissolve into the water.
Features of a hard water area
Pot-holes
Caverns
Stalactites
Stalagmites
What happens when temporary hard water is boiled?
What is the symbol equation?
The calcium hydrogencarbonate decomposes and insoluble calcium carbonate, water and carbon dioxide are formed.
Ca(HCO3)2 (aq) -> CO2 (g) + H2O (l) + CaCO3 (s) (limescale)
same can be written with Mg2+ ions
Boiling the water causes the precipitation of solid calcium carbonate.
this removes the calcium/magnesium ions from the water, removing the hardness.
What is permanent hard water?
Hard water that cannot be softened by boiling
How is permanent hard water caused?
Caused by other types of calcium or magnesium salts in water (such as calcium chloride CaCl2 or magnesium sulfate MgSO4). These cannot be broken down by boiling.
Methods used to soften water
Washing soda
Ion exchange column
Washing soda
Hydrated sodium carbonate Na2CO3.xH2O
A precipitation reaction involves the carbonate ions from the washing soda reacting with the calcium ions from the hard water to make an insoluble substance (calcium carbonate)
*Ca2+ (aq) + ^CO32- (aq) -> CaCO3 (s)
*from calcium chloride
^from sodium carbonate
What is a precipitation reaction?
A precipitation reaction is when aqueous ions from 2 solutions form an insoluble solid.
Ion exchange column
Hard water contains calcium/magnesium ions
An ion exchange column is a glass tube filled with a resin with sodium ions attached
Calcium/magnesium ions become replaced by sodium ions as water flows through the column
The Ca/Mg ions have been removed from the water so it’s no longer hard
The sodium ions do not cause hardness in water
How is an ion exchange column regenerated?
Pouring sodium chloride solution (salt water) through it to replace Ca ions with Na ions
Advantages of hard water
- tastes better
- provide Ca ions for growth of bones and teeth
- better for tanning leather
- better for brewing beer
