How to determine what an object is composed of?

Question

Heads up: This question has never been asked (here) before the way I will ask it here, so let's shed some light on it a bit.

Prelude and anecdote(can be skipped): The other day I was walking home, and I passed a stop sign. I looked at it and asked myself, "What kind of atoms make up this macro-sized metal-like object that visible light enables my eyes to see, which are also atoms I don't know?"

So I thought to myself ... I really should go to physics.stackexchange.com to ask this. They have so many great people there who volunteer their time and help without any immediate financial gain, and there is some very smart people there who are open-minded, and know that repetition is key to success.

The question:

So here I am ... my question is how do you know what materials are composed of?

I ask this because:

1.There are no "grab and go" ways to quickly check out what atoms something is made of. How would I, as a curious person that I am who's into science in all respects, determine what the world is around me?

2.The tools needed to do this don't come cheap ... how could I, by using cheaper tools or techniques, determine what kind of atoms, molecules/chemical compounds, determine which exact atoms make up the things visible light enables me to see around me?

I would like to look at something and go beyond just "It's plastic-like", but actually dig down to which monomer, polymer, or molecular structure it is, and how I can break it down to the individual atoms.

Please ... I ask this with all respect, all seriousness, and all desire to want to further know the materials, compounds, and structures of the objects around me.

Answer 1

There is no universal, cheap, easy way to do identify unknown materials. There are some easy methods that apply to some materials: if something is attracted to a magnet then it is ferromagnetic, and probably contains a substantial amount of iron, nickel, or cobalt. There are a few other rules of thumb, but the general problem is complex and is the reason analytical labs can make money. If there was a cheap and simple way of doing it, everybody would be doing it!

The easiest way to to find out what something is made of is probably to ask the person who made it.

Answer 2

Well, the fancy experimental way would be to use things like mass spectrometry or x-ray diffraction, or many other techniques.

Doing it without those machines though... maybe chemistry would be your best bet. If you had a suspicion of what the material might be, you could use known chemical reactions to see what it does and doesn't react with, what it produces, etc. Note: This can obviously be dangerous, so I'm not suggesting you do this.

The fact that most things (like a stop sign) are a composite material made of many different materials (the metal, paint, fluorescent additives) will make this much more difficult.

Answer 3

One way to figure out what something is made of figure out its density by dividing the object's mass and volume;then look at a density chart!

Answer 4

The simplest method is to follow Archemides and his principle:

Archimedes' principle aids in the experimental determination of density by providing a convenient and accurate method for determining the volume of an irregularly shaped object, like a rock.

We are in a fortunate position to have the densities of all elements with a click on the internet links. One can then exclude stuff and guess at combinations of metals, in your example. Archimedes was supposed to have determined that the gold of a crown was not pure gold.

Once I found a very heavy stone, ejected from an ancient volcano, and got its density to be five times that of water. Not uranium :(.