That's actually how it works. They drink it up and take it back to the colony and regurgitate it for the rest of the colony to eat. You do have an influx of ants at first as they discover it and send out the scent that brings other worker ants to gather it up. Spraying the ants you see kills them and never gets back to the colony, so you are only killing a small portion of what is actually there, while the colony is still growing, unseen.
When I first started using it, it must have been a HUGE colony because they kept coming for days and I kept having to put more out. Eventually, they faded away and I would only see a few scout ants here and there. Now, I put the bait out any time I find ants that have discovered a food source. I deal with the worker ants for a day or two and then we are back to a few scout ants again.
Ants are annoying because of the colony. They are constantly growing and spreading and recolonizing, so to get rid of the colony, you have to be persistent and patient to allow the ants to take the food back to the colony to poison the ants you never see.
Yes, it does harden. It's a sugar-based liquid. If any spills, wet a paper towel and let it sit on the spill to help soften it so you can wipe it up.