Get premium membership and access revision papers with marking schemes, video lessons and live classes.

Processing. Please wait.

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.