Use liquid soap and vigorous shaking in a clear, stoppered vessel to compare suds height and cloudiness in water samples. Soft water makes abundant stable foam; hard water forms little foam and turns cloudy as calcium/magnesium ions react with soap.

1. Add 100 mL of a water sample to a clean test tube; mark the water line (for reference).
2. Add 1.0 mL of a liquid soap solution (≈20 drops if no pipette); stopper.
3. Shake vigorously for 10 seconds; set down for 10 seconds; observe foam height and clarity of the liquid below.
4. Repeat the test with DI water (soft-water control) and additional sources (e.g., other taps) under identical volumes and timing.
5. Interpret: abundant, long-lasting foam with clear water indicates soft water; scant foam with cloudy/scummy water indicates hardness proportional to the soap volume required.

To Test for Water Hardness, Soap Test - Simple Science and Maths:

Water Soap Test - Gordon Water Systems:

📄 Easy Ways to Test for Hard Water - Home Water 101: https://homewater101.com/easy-ways-test-hard-water

• Prepare known mixtures of DI water and tap water (e.g., 25%, 50%, 75%) to create a semi-quantitative calibration of “soap volume to persistent foam.”
• Compare soap vs a synthetic detergent to illustrate why detergents lather even in hard water.
• Collect samples from hot vs cold taps and rank their relative hardness.

• Wipe spills immediately—soapy floors are slippery.
• Label all bottles; do not taste any samples.

• Why must soap be used, not detergent? (Detergents lather in hard water; true soap reacts with Ca²⁺/Mg²⁺ to form insoluble scum, suppressing foam.)
• Why does hard water turn cloudy on shaking? (Formation of insoluble calcium/magnesium soaps.)
• How does the “soap volume to persistent foam” relate to hardness? (More soap required implies higher concentrations of Ca²⁺/Mg²⁺.)
• Why run a DI water control? (To show the maximum foam response and check your shaking/timing method.)
• How would boiling affect your results? (It can remove temporary hardness by precipitating carbonates, increasing foam in the boiled sample.)