Chemistry 115 Lab Notes —Hard Water
Hard Water
♦ Water that contains hardness ions: Fe3+(aq), Mg2+(aq), and Ca2+(aq)
♦ not a health risk, but a nuisance
− mineral buildup on fixtures, pipes, and water boilers
− poor soap and/or detergent performance
− “soap scum” (insoluble waxy, dirty gray residue) left behind on clothes, bathtubs etc.
Origin
♦ As ground water moves through soil and rock, it dissolves very small amounts of minerals and
holds them in solution
− hardness becomes greater as ion content increases
Measurement of water hardness
notice the similarity between ppm and percent by mass
mass of part
% by mass =
× 100%
mass of whole
♦ 1 mg dissolved in one liter of water is equivalent to 1ppm
ppm =
g CaCO3
× 1 × 10 6 ppm
g water
ppm
<17
17 to 60
61 to 120
121 to 180
Higher than 180
(
)
Classification
Soft
Slightly Soft
Moderately hard
Hard
Very Hard
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 1 of 5
Chemistry 115 Lab Notes —Hard Water
Anything we don't want attached to us (ie. dirt) can be stuck to us in one of two simple ways
1. it becomes trapped in microscopic crevices, for instance the dirt you get on a dry dusty road
2. or moisture makes it adhere, for instance the dirt you get from a muddy road
In both cases, a good hosing down with plain water with perhaps a little rubbing will do a
reasonable job of removing the dirt. Soap isn't really necessary yet.
♦ But if the dirt particles have a oily coating, or your skin is oily,
− the dirt particles will stick to your skin
− a spray of water will not dislodge it because water rolls off the oil
− To un-stick the oil-adhering-dirt, the sticky oil must be removed
How Does soap work?
Polar Molecules
Some molecules such as H2O are polar
♦ They have a “positive end” and a “negative
end”
Non-polar molecules
Molecules such as hydrocarbons or symmetrical
molecules are non-polar
♦ Symmetric charge distribution
“Like dissolves Like”
♦ Polar things interact with polar things and repel non-polar things
− Water (polar) will dissolve salt (polar) but not mix with oil (a hydrocarbon ⇒ non polar)
♦ Non-polar things interact with non-polar things and repel polar things
− Kerosene (non-polar) will mix with oil (non-polar) but not water (polar)
(Why do white bears like water? Because they are polar)
“Hydro”−water
Hydrophilic
“philic”− loving
♦ substances that interact with polar water are
water loving or hydrophilic (polar)
“Hydro”−water
Hydrophobic
“phobic”−hating
♦ hydrophobic substances are oils, grease and
anything non-polar
♦ includes salt, smaller alcohols, and anything
with polarity
Long chain hydrocarbons
♦ long chain hydrocarbons are non-polar covalent molecules
♦ adding polar functional groups like alcohol (−OH) or carboxyl (−COOH) causes one part of the
molecule to become polar
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 2 of 5
Chemistry 115 Lab Notes —Hard Water
Saponification Reaction
♦ soap is made by reacting a fatty acid with a base (alkali)
♦ this breaks the carboxyl bonds to form soap (free fatty acids) and glycerol
♦ addition of salt caused the soap to form a solid that excluded impurities, such as the alkali
Soap ⇒ natural in origin (fats and oils)
Detergent ⇒ from refined petroleum and coal
Note: you are making soap when you use Draino to
clean clogged sinks
Grease and fats + Draino (OH-) → soap!
Soaps
♦ A soap molecule has a polar hydrophilic ‘head’ and a non polar
hydrophobic ‘tail’
♦ The head will interact with the polar water molecules and the tail will
interact with the grease molecules
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 3 of 5
Chemistry 115 Lab Notes —Hard Water
When a soap is dissolved in water the soap molecules form micelles
♦ A micelle is an aggregate of molecules suspended in solution
− the hydrophilic ‘heads’ are in contact with the water and the
− hydrophobic ‘tails’ in the center of the micelle
♦ small particles of grease (non-polar) are attracted into the nonpolar
environment at the center of the micelle
Soap dissolves grease
♦ the non-polar part of soap dissolves the grease
♦ the other end associates with water and remains suspended in solution
♦ the micelles, along with the greasy dirt, is washed down the drain
Formation of soap scum
Soap contains salts such as sodium stearate and sodium palmitate
♦ The sodium and potassium salts of these compounds are water soluble, the calcium,
magnesium, and iron salts are not
♦ When soaps are placed in hard water that contains such ions, an insoluble, curdy solid forms,
“soap scum”
O
CH3(CH2)n
2+
C O-Na+ + Ca
a soluble soap
O
[CH (CH )
3
2 n
]
C O-
2
Ca2+ + 2 Na+
an insoluble soap
♦ This process removes soap ions from solution, and decreases the cleaning effectiveness of
soaps
Water softening methods
♦ means to remove or tie –up hardness ions (Ca2+, Mg2+, Fe3+)
1. Use washing soda to precipitate hardness ions as insoluble carbonates
2. Use of “water softeners”
− Replaces hardness ions with Na+ ions
3. Use of chelating agents such as EDTA
− EDTA wraps around and ties-up hardness ion, ion remains in solution but can no longer
interact with soap.
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 4 of 5
Chemistry 115 Lab Notes —Hard Water
Hard Water Lab
In this experiment, two different methods will be used to determine water hardness:
1. Titration with soap solution;
2. Titration with EDTA.
Calculations
Overview: Two steps to the calculations
Step 1: Calculate the mass of CaCO3 in sample
Step2: Determine the concentration of CaCO3 in ppm
Explanation
The soap and EDTA solutions used in the experiment are standard solutions. A standard solution is
one with an accurately known concentration. One can calculate the amount of CaCO3 in the water
sample from the reagent label and the volume of titrant used.
 mg CaCO 3 
milligrams of CaCO 3 in sample = 
 × ml titrant used
 mL titrant 
from bottle label
mL titrant used in exp
After the mass of CaCO3 has been determined, the hardness in ppm can then be calculated according to
 g CaCO 3 
 mg CaCO 3 
 1 × 10 6 = 

ppm = 
 g water 
 kg water 
Example: A student found that she needed 14.74 mL of a standardized EDTA solution to titrate 10.00
mL of her water sample to equivalence. Calculate the hardness of the water sample in ppm. The EDTA
solution was labeled as 1.00 mL = 0.114 mg CaCO3.
Solution:
Step 1:
0.114 mg CaCO 3
× 14.74 mL titrant used = 1.68 mg CaCO 3
mL titrant
 0.00168 g CaCO 3 
 1× 10 6 ppm = 168 ppm
Step 2: 
 10.00 g water 
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
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