The borate part of decahydrate and pentahydrate are identical. The difference is in the amount of water of crystallization. A saturated solution of either one at a given temperature contains exactly the same number of Na2B4O7 molecules. Once you have a saturated solution and have decanted the clear liquid, there is no point in trying to recrystallize the contents, because you are then back at the beginnibg. You still are not certain which of the crystals are pentahydrate and which are decahydrate. You still have to weigh the crystals to make any use of them, whereas the saturated liquid has a known content of the decahydrate per unit of volume and can be measured out with a graduated cylinder.
Originally Posted by Murray Kelly
The terms "pentahydrate" and "decahydrate" have no meaning in solution. However much water of crystallization was taken into a borax solution becomes part ot the solvent water. All you have in solution is Na2B4O7 molecules and H2O molecules. The molecule that entered as the pentahydrate carried 10 molecules of water with it. The molecule that entered as the pentahydrate carried 5 molecules of water with it. Now we have 2 molecules of Na2B4O7 floating around in 15 more molecules of water than we had before. There is no way we can tell which of the borate ions came in which which of the water molecules.