I've been doing some water softening experiments and have discovered something that has me scratching my head.
My assumption all along has been that if you add salt to water you will reach a point where no more salt can be dissolved and so any additional that you add will simply remain solid and thus not increase the TDS of the brine solution. Well it turns out that is completely wrong. Or at least my experiment is indicating as such. But I'm hoping that someone with more chemistry knowledge can explain why this is.
The experiment: I filled two containers with roughly equal parts of RO water. In container A I added rock salt until the undissolved salt was above the water line. In container B I added salt until there was a significant layer of undissolved crystals at the bottom, but nowhere near the top. I then left them for approximately 36 hours.
The hypothesis: TDS measurement would be the same in both solutions. After all, the water in both has dissolved all that it can given the starting TDS and temperature, right? Wrong.
The result: The TDS of liquid extracted from container A (packed full of salt) was over 3 times higher than container B (undissolved salt, but not packed).
So what gives?!