I had read about using liquid Castile soap to test water hardness. I was not looking for a numeric result here, but I was looking for an indication as to whether water softener cycle time should be shortened. Testing was not well controlled.
1. Glass on right... I put untreated well water (hardness in the 30s) into a clear plastic glass. I dropped a small drop Murphy's. It floated due to surface tension. I drizzled a bit more. There were very interesting and artistic patterns initially. The water was still swirling a bit from being filled. As it sat, the whole content became milky.
2. Middle glass. I put softened cold water from the tap tested to about 1 or 2 grains hardness with Hach 5B. I was disappointed to see some milky trails. The picture was taken about 20 hours later. A hovering ring had developed... it shows only as a line in the side photo, but it is a heavier version of the ring shown in the left glass.
3. I decided to check the validity of the testing. I put distilled water into another glass. I put in more soap than in the previous tests-- maybe 6 drops equivalent, although I just put it in as a bigger bit. It sank to the bottom, with just a little initial cloudiness trail. The next morning about 19 hours later, a sharp hovering ring was in the glass. From the top, the ring is pretty much circular and centered in the glass.
So as a test for hardness, this could be developed to be more useful. Perhaps a test where the soap is dropped in with a dropper would be more meaningful and repeatable. Maybe a 2:1 dilution with distilled water would be better, because the Murphy's is pretty viscous.
Maybe some student would want to develop this as science fair project.