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Experiment on the effect of water containing dissolved salts on soap solution

Answer Text:
Experiment: effect of water containing dissolved salts on soap solution
Procedure
- 2 `cm^3` of distilled water is put in a conical flask.
- Soap solution from a burette is added into the water and shaken until formation of lather is noted.
- If the soap fails to lather more soap solution is added from the burette till it lathers and the volume of the soap required for lathering
recorded.
- The procedure is repeated with each of the following: tap water, rain water, dilute solutions of`MgCl_2, NaCl, CaCl_2, (NO_3)_2, CaSO_4, MgSO_4, Mg(HCO_3)_2, Ca(HCO_3)_2, ZnSO_4 , NaHCO_3, and KNO_3`
- The procedure is repeated with each of the solutions when boiled.
Explanations
- Distilled water requires very little soap to produce lather because it lacks dissolved salts and hence termed soft water.
- Solutions containing `NaCl, ZnSO_4, KNO_3 and NaHCO_3` do not require a lot of soap to form lather Water containing `Ca^(2+) and`Mg^(2+) ions do not lather
easily (readily) with soap
Reason:
- These ions react with soap (sodium stearate) to form an insoluble salt (metal stearate) called (Mg and Ca stearate respectively); which is generally termed scum.
Equations:
With `Ca^(2+)`
`2C_17H_35COO^(-)Na^(+)(aq) +Ca^(2+)(aq)to (C_17H_35COO^(-))2Ca(s) + 2Na^(+)(aq)`
Sodium stearate calcium stearate
With `Mg^(2+)`
`2C_17H_35COO^(-)Na^(+)(aq) +Mg^(2+)(aq) to (C_17H_35COO^(-) )2Mg(s) + 2Na^(+)(aq)`
Sodium stearate Magnesium stearate
- Thus water with Mg and Ca is termed hard water and can only be made soft by removing these ions upon which the water will lather easily with water
- When`Ca(HCO_3)_2(aq)`and`Mg(HCO_3)_2(aq)` are boiled the amount of soap required for lathering decreases than before boiling
Reason
- Boiled decomposes the 2 salts into their respective carbonates which precipitates from the solution leaving soft water which leathers easily with
water
- The amount of soap solution used with solutions containing sulphates and chlorides of calcium and magnesium did not change significantly even after boiling
Reason
- The soluble sulphates and chlorides of Mg and Ca do not decompose upon boiling hence can not be precipitated out.