How would you find the density of an irregular solid that is water soluble, rock salt for example?

Answer 1

Then, use a liquid that will not dissolve rock salt. For instance, any light colored cooking oil will do, so that you can read off the value of the before and after volumes when you drop your rock salt into a cylinder or any glass, graduated measuring device.

Weigh the sample first. Then as usual, density = mass/volume

Answer 2

density = mass / volume

find object with known volume, weigh it
fill object with known volume up with salt
weight object with salt
subtract initial weight

Answer 3

drop it into 100% alcohol.