Hardness of water Flashcards
is water a good or bad solvent
an excellent solvent
what is the way in which water circulates around the earth?
the water cycle
Is rainwater pure water? explain your answer
It is fairly pure water but even rainwater contains dissolved gases (oxygen, nitrogen, carbon dioxide)
Since water has a great ability to dissolve substances, what can it dissolve found in the earth?
minerals found in the earth as it flows over the soil to form streams and rivers
what is hard water (simply)?
it is difficult to form a lather with soap.
Instead of forming a lather, the soap forms a grey precipitate - often referred to as scum
what does hard water produce when trying to form a lather with soap?
scum
what does hard water contain?
hard water contains definite quantities of Ca2+ or Mg2+ ions dissolved in the water
define hard water
hard water is water that will not easily form a lather with soap. Hardness is caused by the Ca2+ or Mg2+ ions dissolved in the water
What is one of the most common substances in soap?
sodium stearate
what is the chemical name for ‘scum’?
calcium stearate
what happens chemically when a lather is tried to be formed with hard water and soap?
When soap is added to hard water, the calcium or magnesium ions in the water react with the stearate ions to form a grey insoluble compound called calcium stearate (scum), which floats on the water
`Why is a lot of soap wasted when trying to form a lather with hard water?
because all of the calcium ions have to be precipitated in the scum first
why did the use of soap to wash clothes become less common with the introduction of modern detergents that are not affected by hard water?
as a lot of soap is wasted in trying to form a lather as a lather is only formed when all of the calcium ions have been precipitated in the scum
what are modern detergents made from? give examples of modern detergents
washing powders, washing-up liquids and shampoos do not contain soap - they are made from crude oil
What are detergents good at removing?
oil and grease
How is water hardness classified?
into temporary hardness or permanent hardness
define temporary hardness
temporary hardness is hardness that can be removed by boiling the water
why is rainwater slightly acidic?
because of the fact that it dissolves carbon dioxide as it falls through the air forming carbonic acid
H2O + CO2 —> H2CO3
How does temporary hardness arise?
limestone (calcium carbonate) is insoluble in water but does react with carbonic acid.
The acidic solution of carbonic acid reacts with the limestone in the soil to form calcium hydrogen carbonate
H2CO3 + CaCO3 —> Ca(HCO3)2 NB
What is water that has temporary hardness really?
a dilute solution of calcium hydrogencarbonate
In temporary hardness, what causes the hardness in the water?
the Ca2+ ions cause hardness. The HCO3^- ions have no effect on hardness
How does boiling remove the temporary hardness?
If water containing Ca2+ ions and HCO3^- ions is heated, this reaction happens:
(NB) Ca(HCO3)2 —> CaCO3 ↓ + CO2 + H2O
calcium hydrogencarbonate —> calcium carbonate ↓ + water + carbon dioxide
What effect does boiling temporary hard water have?
the calcium ions that were dissolved in the water are now precipitated as insoluble calcium carbonate (limestone)
What is the insoluble calcium carbonate also called?
fur or limescale
what is the test for the presence of carbonate ions (CO3^2-) and hydrogen carbonate (HCO3^-) in aqueous solution?
hydrochloric acid is added to the boiling tube of the sample (that was stoppered with a one-hole rubber stopper).
Effervescence (fizzing) is observed as the acid comes in contact with the carbonate.
Gas (CO2) is given off and is seen bubbling through the limewater in a test tube.
Limewater turns a milky white
What is the chemical reaction formula for the test for carbonate or hydrogen carbonate ions?
CaCO3 + 2HCl —> CaCl2 + H2O + CO2
How do you distinguish between carbonate and hydrogencarbonate ions?
magnesium sulfate (MgSO4) solution is added to the two solutions in separate test tubes.
A white precipitate forms in the carbonate sample.
No precipitate forms in the hydrogencarbonate sample as:
carbonate: Mg^2+ + CO2^2- —> MgCO3 ↓
hydrogencarbonate: Mg^2+ + 2HCO3^- —> Mg(HCO3)2
Where do the Mg2+ ions come from that are in water?
dolomite rock
define permanent hardness
permanent hardness is hardness which cannot be removed by boiling the water
WHat is permanent water hardness caused by?
By the presence of calcium sulfate (CaSO4) or magnesium sulfate (MgSO4) in the water
