I tried making a salt water solution as described in the article to kil weeds on my driveway. The salt will not disolve, even when I have boiled the solution.
Salt Water to Kill Weeds
Certified GKH Gardening Expert
It sounds like the water is completely saturated with salt. Most should dissolve, all except what has no room to be dissolved. You can take a small taste of the water to make sure that it is, in fact, salty. Are you using regular table salt? Water can only hold so much, so what is left over will not dissolve. Alternatively, if there is way too much in there, then it will start precipitating out into a solid again.