Hard water

Hard water

1. Water which does not lather readily with soap

2. Hard water + Soap→Scum

   • Calcium ions + Sodium stearate→Calcium stearate + Sodium ions

3. Caused by DIsSOLVED CALCIUM + MAGNESIUM ions

4. Hard water has NO EFFECT on detergents

5. Fair test to compare the hardness of 2 samples of water

   • Measure EQUAL VOLUMES of both water samples. Add 1cm3 of WATER TO EACH sample + SHAKE. CONTINUE TO ADD 1cm3 of soap until a PERMANENT LATHER FORMS, sample REQUIRED MOST SOAP is the HARDEST

6. How hard water forms

   • 1. When it is raining, water reacts with CO2 in the air to form carbonic acid
     • 2. Weak acidic sol then corrodes limestone rocks forming calcium hydrogencarbonate→soluble
   • Calcium carbonate + carbon dioxide + water→Calcium hydrogencarbonate
   • Explanation in terms of ions
     • Calcium ions in limestone react with hrdrogen carbonate ions in acid rain to form calcium hydrogencarbonate which is soluble + causes hardness in water

7. Temporary HW

   • Can be softened by boiling- Calcium hydrogencarbonate is unstable + will easily decompose on heating
   • Describe how THW can be removed by boiling
     • THW contains calcium ions + hydrogenc arbonate ions (in sol). When heated the hydrogen carb ions chemically decompose to form carb ions. Carb ions react with calcium ions in water to form calcium carb which percipitates out removing hardness.

8. Permanent HW

   • Can't be softened by boiling - Compounds do not compose on heating → calcium / magnesium ions remain in water + it stays hard
     • Removed by....
       • Adding washing soda
         • Removes BOTH sorts of hardness
           • In terms of precipitation how washing soda/hydrated sodium carb removes permanent + temporary hardness
             • Ca2+ (aq) + CO3 2- (aq)→ CaCO3 (S) calc ions in HW react with carb ions in washing soda producing calcium carb→insoluble + can be removed by filtering
       • An ion exchange column
         • Removes BOTH sorts of hardness
           • Calc + mag ions in HW are swapped with sodium ions in the exchange column. Sodium ions pass out through the column in the water do not cause hardness

9. Compound that causes hardness in water

   • Calcium chloride
   • Magnesium chloride
   • Calcium sulfate
   • Magnesium sulfate

10. Disadvantages

    • Amount of soap req to produce a lather makes it more expensive than soft water
    • Leaves a 'fur' in kettles
    • Limescale can cause blockages in pipes

11. Advantages

    • Pleasant taste
    • Industries like breweries benefit eg. tanning of leather
    • Good for bones + teeth
    • Less likely too dissolve heavy metals