Water Softening
Difficulty: Easy | Time: 20 minutes | Visual Impact: Medium
Historical Context
Hard water - water containing dissolved calcium and magnesium ions - has been a domestic nuisance for centuries. The minerals come from rainwater percolating through limestone and chalk (calcium carbonate and magnesium carbonate dissolve slowly in the slightly acidic groundwater). Hard water prevents soap from lathering, leaves scale in kettles and pipes, and deposits the familiar white ring around bathtubs.
The connection to sodium carbonate (washing soda) was understood empirically long before the chemistry was clear. Washing soda was added to laundry water throughout the 19th century to improve soap performance. The systematic chemistry was worked out in the 1880s as the new field of physical chemistry began quantifying ion behavior in solution.
Modern water softeners use ion exchange resins (replacing Ca²⁺ and Mg²⁺ with Na⁺) rather than precipitation, but the sodium carbonate method remains a simple and instructive demonstration of precipitation chemistry and why “hard water” earns its name.
Materials
- Calcium chloride - 1g dissolved in 500mL water (artificial hard water)
- Sodium carbonate - 2g
- Liquid soap or soap flakes - 1 teaspoon
- 4 clear jars or bottles with lids
- Filter paper or coffee filter (optional, to clarify softened water)
- pH paper or indicator (optional)
Procedure
Prepare two water samples:
- Label two jars “Hard Water” and two jars “Soft Water.”
- Pour 200mL of calcium chloride solution (hard water) into each jar.
- To the “Soft Water” jars, add 1g of sodium carbonate and stir well.
- A white precipitate (calcium carbonate) should form. Let it settle, or filter to remove it.
- Add equal amounts of liquid soap (about 10 drops) to one hard water jar and one soft water jar.
- Cap both jars and shake vigorously for 10 seconds.
- Compare the lather: hard water produces little foam with a scummy residue; soft water produces abundant foam.
Bonus - scale demonstration:
- Boil some hard water in a small saucepan and observe the white scale (calcium carbonate) deposited as the water evaporates. This is limescale.
Reactions
\[\ce{Ca^{2+}(aq) + CO3^{2-}(aq) -> CaCO3(s) v}\]
\[\ce{Mg^{2+}(aq) + CO3^{2-}(aq) -> MgCO3(s) v}\]
With soap (sodium stearate, RCOONa):
\[\ce{2 RCOO^-(aq) + Ca^{2+}(aq) -> (RCOO)2Ca(s)}\] (soap scum, insoluble)
The Science
Soap is the sodium salt of a fatty acid. In soft water, soap molecules dissolve normally and their long hydrophobic tails can surround grease and dirt particles (micelle formation) - this is cleaning action. In hard water, Ca²⁺ and Mg²⁺ ions react with the soap molecules, forming insoluble calcium and magnesium soaps (soap scum). The soap is consumed making scum rather than doing useful cleaning - hence the need to use much more soap in hard water areas.
Sodium carbonate softens water by precipitating Ca²⁺ and Mg²⁺ as their insoluble carbonates, removing them from solution before they can react with soap.