Water Softening
Summary
Hard water’s calcium ions wreck soap lather and leave scum; adding washing soda precipitates1 those ions out, “softening” the water. You’ll be able to explain why calcium makes soap scum but sodium doesn’t, why softening works, and the difference between temporary and permanent hardness.
History
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.
Hazards & preparation
PPE: safety glasses. This is a low-hazard experiment.
- Sodium carbonate (washing soda) is mildly alkaline — rinse splashes off skin and keep it out of eyes.
Disposal: the softened water and calcium-carbonate precipitate are harmless — flush down the drain with water. See the Safety page.
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
- Label two jars “Hard Water” and two “Soft Water.”
- Pour 200mL of the calcium chloride solution (hard water) into each jar.
- To the “Soft Water” jars, add 1g sodium carbonate and stir well.
- A white precipitate (calcium carbonate) forms — let it settle, or filter it out.
- Add equal amounts of soap (about 10 drops) to one hard-water and one soft-water jar.
- Cap both and shake vigorously for 10 seconds.
- Compare the lather: hard water gives little foam and scummy residue; soft water gives abundant foam.
Bonus — scale: boil some hard water in a small pan and watch white scale (limescale) deposit as it evaporates.
What you should see
Adding washing soda to the hard water clouds it instantly with fine white particles. When you shake the two soaped jars, the difference is obvious: the soft water froths into a thick head of bubbles, while the hard water musters only a thin, greyish foam with curds of scum floating in it.
| Symptom | Likely cause | Fix |
|---|---|---|
| Both jars lather the same | Not enough washing soda, or water not actually hard | Add more sodium carbonate; use the CaCl₂ hard water |
| No precipitate on adding soda | Too little calcium | Use more calcium chloride in the hard water |
| Soft water still slightly scummy | Precipitate not removed | Let it settle fully or filter before adding soap |
The 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)
carbonate precipitates the hardness ions out of the water; otherwise those ions would react with soap to make insoluble scum instead of lather.
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.
Questions to Explore
Why does soap form scum with calcium but not sodium? Same carboxylate group — why soluble with Na⁺ and insoluble with Ca²⁺?
Hint / answer
Calcium is doubly charged, so it bridges two soap molecules into a large, tightly-held, water-repelling clump — insoluble scum. Sodium is singly charged and loosely held, so sodium soap stays dissolved and free to clean.
Why does soda soften rather than acidify? Carbonate is a base — does softening change the pH, and does it matter?
Hint / answer
Adding carbonate does make the water slightly alkaline, but its main job is to pull calcium out as solid carbonate. Mild alkalinity actually helps washing (it boosts soap and loosens grease), so it’s not a problem for cleaning.
Temporary vs. permanent hardness? Boiling removes some hardness but not all. Why does it work for bicarbonate but not sulfate?
Hint / answer
Boiling decomposes calcium bicarbonate into insoluble carbonate that drops out (temporary hardness gone). Calcium sulfate has no such heat-driven breakdown, so it stays dissolved no matter how long you boil — “permanent” hardness.
How does an ion-exchange softener differ? It swaps Ca²⁺/Mg²⁺ for Na⁺ with a resin. Advantage and trade-off?
Hint / answer
Ion exchange removes the hardness without leaving a precipitate to filter, giving continuously soft water. The trade-off is it adds sodium to the water (a concern for low-sodium diets) and the resin must be periodically regenerated with brine.
Why does hardness hit hot systems hardest? Scale forms in kettles and boilers. Why does heat deposit calcium carbonate?
Hint / answer
Heat drives CO₂ out of dissolved calcium bicarbonate, converting it to insoluble calcium carbonate that plates onto hot surfaces. Cold pipes stay below that threshold, so the calcium remains dissolved and no scale forms.
Going further
- Titrate the hardness. Quantify how hard your tap water really is with an EDTA titration in Hard Water Titration.
- Compare softeners. Soften with washing soda vs. a pinch of borax and compare the lather.
- Next in the Acid–Base track: map the whole pH scale by colour in The pH Landscape.
Footnotes
Precipitate — An insoluble solid that forms and separates out when two solutions are mixed.↩︎