Could volume by displacement be used to determine the volume of a lump of rock salt? Explain would be nice

Yes it could, but you'd have to set up the process very carefully.

I see two major challenges right away:

1). Displacement of water would not be a wise method, since rock salt

is soluble (dissolves) in water. So as soon as you start lowering it into

your graduated cylinder full of water, its volume would immediately start

to decrease. If you lowered it slowly enough, you might even measure

a volume close to zero, and when you pulled the string back out of the

water, there might be nothing left on the end of it.

So you would have to choose some other fluid besides water ... one in

which rock salt doesn't dissolve. I don't know right now what that could

be. You'd have to shop around and find one.

2). Whatever fluid you did choose, it would also have to be less dense

than rock salt. If it's more dense, then the rock salt just floats in it, and

never goes all the way under. If that happens, then you have a tough

time measuring the total volume of the lump.

So the displacement method could perhaps be used, in principle, but

it would not be easy.