The type of glue you use to join PVC pipes is called "solvent weld," and these two words tell you the reason why it's 99 percent impossible to separate pipes once they are glued together. This type of glue isn't a surface coating; it's a PVC solvent that actually melts plastic. When you spread this glue on a pair of pipes and join them, the melted plastic of both pipes hardens into a single piece. This makes trying to unglue a joint a bit like trying to unbake a cookie. When plumbers need to repair a PVC joint, which is common, they usually cut out the joint and install a new one. In very rare cases, you can pry pipes apart.
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Get It Before It Cures
Solvent weld glues dry fairly quickly. Depending on the pipe diameter, drying times can vary from a minute to five minutes. However, curing times are longer (about an hour) so if you try to separate the pipes before the glue has cured, there's a small chance you'll be successful, but you'll have to pull and twist with a lot of force to detach the two pieces. If you're gluing water pipes and you make a mistake, you can try to turn on the water and let the water pressure blow the pipes apart. The prospect of being able to pull pipes apart is a long shot, though, because even a few minutes after gluing, a joint can withstand normal water pressure without leaking or separating.
Cut Out the Joint
Once a properly glued joint has become permanent, the pieces cannot be separated. If you need to separate the joint, the only option is to cut it. You can use a hacksaw or a PVC cutter for this job, and once the fitting is gone, you can install a new one. To do this, you'll need to close the gap left by the missing fitting by gluing a short length of pipe (a nipple) to one of the pipes you wish to join. Because you need to install a coupling to do this (and the coupling needs space), it's best to make your cut three or four inches away from the joint when removing it. This leaves a wider and more manageable gap.
Tap the Joints Apart
You may be able to tap or chisel apart a joint that hasn't been properly assembled in the first place. Perhaps the installer used insufficient glue or failed to seat the pipes completely. Such a joint will leak, which is the probable reason you want to start over. To tap the pipes apart, set the edge of a chisel against the rim of the female part of the joint and tap the chisel sharply with a hammer. If you detect movement, you're on the right track, so keep tapping. If there's no movement, it's best to stop, as continued tapping on the chisel will damage the pipe. On the other hand, if you're just interested in extracting the male pipe, and you don't care what happens to the female one, keep going.
Heat softens plastic, so pointing a hair dryer or heat gun at a PVC joint may soften the plastic enough to allow you to pull them apart. Like tapping joints apart, this is a long shot -- a desperate measure for a desperate situation. Keep the heat on the joint while you pull steadily on the pipes to separate them. However, if you use enough heat to distort the pipe, you won't be able to reuse it.
Perhaps you want to extract a short piece of pipe from a fitting so you can reuse the fitting. There are two ways to supply enough heat to do this: You can put the pipe in boiling water or heat it with a candle flame. Once the plastic softens, you can try to extract the pipe with pliers.