Vanadyl sulfate - VOSO4·xH2O
Vanadyl sulfate is extremely hygroscopic and hard to get in the form of nice crystals. Even the reagent grade chemical (as shown in this sample) frequently is very wet and only an approximate formula VOSO4·xH2O can be given. The sample, shown here, has a value of x in excess of 5. The sample already was wet like this, when I received it.
Although the compound is very hygroscopic, it dissolves in water with remarkable difficulty. When some vanadyl sulfate is added to water, then it takes quite some time and a lot of shaking, before all of the solid has dissolved. The solution finally becomes clear and blue, much like a concentrated solution of copper sulfate.