what happen if its stored for a long time? is it evaporates? or become hard solid? cant it be diluted with some sort of alcohol?
It separates kind of like peanut butter that's been opened and left to sit too long. You can try to whip it back together, but it won't be the same.
Curious inventor has
a good writeup with pics on using solder paste.
Here's a good shot of what the paste looks like when it's fresh. It's basically tiny beads of solder in a flux pudding.
Gravity and time will pull the beads down and filter the flux up, and if left open the flux will dry out. That basically leaves you with a tube of dried tree sap and metal. No good!
Refrigeration slows down both processes and, like peanut butter, can make an open container last several months.
Hope that helps, even though I'm just restating what others have already covered.