*Hi Holly. I have this metal ball and have had it for awhile and no one can tell me what it is.It is 12129 grams and is 141.56mm across, silvery-white, and hard, acids don't attack it, polished a spot to mirror like and put it outside (about 7 years)and it has not tarnished or lost it's luster. Could you please try to help me out.*

* Thanks Owen *

Hi Owen,

I do get literally hundreds of emails a week, so sorry that it takes a while to get through them all.

Well, I can give you a simple guess, but that is only assuming the metal is pure, and not any strange alloy (variable mixture of two or more metals). This guess is based on density, which is a constant for pure elements (given every-day room temperature!)

Failing this, there are other chemical tests that could be performed, but this seems like the simplest thing to start off with.

To get density, you divide mass by volume. It is a measure of how much stuff there is in the area that the object takes up. Then you can look up standard densities of materials on the internet and if you find something with a similar number it *might* be the same.

Most densities of solids are given in grams per cubic centimeter so you will have to get it in those same units to compare it to what you can find in a table of metal's densities.

Oh, a cubic centimeter is the same thing as a cc, which is the same thing as a milliliter (mL). We chemists like to have many different names for the same things just to keep people confused.

OK, first you need the volume.

If you had something like a precisely calibrated liquid measure, like a graduated cylinder (most beakers and flasks are not accurate unless labeled as "volumetric") you could just drop the sphere in some water in the cylinder and see the volume of water the object displaces. This would be equal to the volume of the object.

But you don't have to do that.

When you say that it is "141.56 mm" across, is that the diameter, or width all the way across? You need that number. If you can figure out the diameter, divide that number by 2 to get the radius.

Something is really screwed up about your numbers, I am afraid. The mass, which you report as 12129 g, is the same as 12 kilograms and then some, so this thing is super heavy for its size. Unless you have part of a neutron star, I don't think these numbers are right.

If you have trouble measuring the diameter, you can also measure the circumference.

The circumference is the length all the way around the sphere.

Here is a way to get it, if you haven't already:

If you wrap a flexible thing like a string all the way around the sphere, mark where the ends meet, and then measure that length of string. We can call it "C"

Then you need the radius. For both a sphere and a circle the radius is C divided by 2pi

So, you need a calculator that has pi on it. Multiply pi by two and divide it into the circumference to get the radius.

But let's say that your "141.56 mm across" number is the diameter. You will see that something is wrong if I carry out this calculation.

So I get the radius is (dividing by 2)

70.78 mm

Hey, since we will need the density ultimately in cubic centimeters, let's just convert this right now into cm.

So, r = 7.078 cm. (A centimeter is just ten times larger than a mm, so move the decimal one place over.)

Now you need the volume (amount of space the material takes up, or what I jokingly call "bigness") of the sphere.

The volume is, for any sphere, 4/3 times pi times r times r times r =

(that is, cube r, or multiply r by itself three times, then multiply the answer by 4, then multiply the next answer by pi, then divide by 3)

So I get Volume = 150.5 cm cubed

Now we divide this into the mass, which is already nicely in grams

12129 g /150.5 cc = 80.6 g / cc

Wait, this is **way** too dense to be realistic! The densest element, osmium, is around 22.6 g / cc. (I used to say with a straight face that osmium, the densest element, was named after the Osmond family, which usually got a fascinating reaction out of my class when I used to teach in Salt Lake. I hastened to add that I was only joking but you never know who might be perpetuating that idea out there to this day.)

So, either there is a decimal missing from your mass (otherwise you are saying it is 12 kilograms plus ??? That is pretty heavy! ) or the measurement of the diameter is wrong. Check to be sure your units are correct, that what you call "mm" is really not cm, or that what you call grams isn't something else.

You can follow through the same calculations with the correct numbers, then look up values on the internet if you search for "metal densities tables"

for example

http://www.reade.com/Particle_Briefings/spec_gra.html

Hope that helps!

Holly

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