|In this experiment you will continue to add drops of soap to each tube
until the foaming upon shaking equals that in the first tube. The number of drops of soap
used is proportional to the hardness.
You will use distilled water as the reference to which foaming in other tubes will be compared. You will also measure the amount of soap needed to cause foaming in two water samples of different known hardness. Finally you will measure the hardness of an UNKNOWN water sample.
A classical formula for Soap is .................... C17H35COONa (Sodium Stearate)
Soap was originally made from animal fat and wood ashes. Animal fat is a triglyceride containing stearate groups and wood ash contains sodium and potassium carbonate. Boiling a mixture of lard and wood ash, adding salt and cooling precipitates out the soap. Glycerine is recovered as a byproduct hence the name "glyceride".
Back to the formula for Sodium Stearate. The left end of this is hydrophobic and dissolves in oils and grease. The right end is ionic and soluble in water (hydrophilic). The function of soap is to have the hydrophobic end dissolve in the grease or oil which holds dirt in clothing or on skin, and then the right end with its attraction for water emulsifies it so it can be washed away.
Hard water or acidity causes the precipitation of insoluble salts or the free acid.
It is these reactions which give rise to the "bathtub ring"
© R.W. Kluiber 1/17/2000