Re: Can gold ever be heated enough to become a different substance?

Dear Emily:

Gold is a very interesting substance to humans because of its colour, lustre, and inertness. By inertness I mean that it is pretty resistant to reacting with other chemicals and substances.

Like all other matter, gold and the rest of the metals consist of atoms. Let's say you have a chunk of gold (ooh yeah) in the solid state sitting on your table. If you could 'see' the individual atoms, you'll realise that they are packed closely together, such that they vibrate constantly, but don't deviate much from a certain fixed position. All atoms are always in motion, because at temperatures other than the so called 'absolute zero' (-273.15 degrees Celsius, or 0 Kelvins), the atoms possess energy. And since they have energy they tend to move around (as you know, heat is due to the motion of atoms). Hence, for solids, they move around but not very much. In a liquids and gases, collectively known as fluids, they have much more freedom of motion. An individual atom can actually travel from one place to another, rather than simply vibrate around one fixed position. The difference between liquids and gases on the atomic scale can be thought of as this: in a liquid, the atoms and molecules have to jostle past other atoms and molecules as they move along; whereas in a gas, they mostly travel in empty space, and very occassionally bang into another atom or molecule.

Why is this so? Well, as you heat an object or a substance, what you're doing is that you're giving the atoms and molecules in it more energy, and hence they move more. In a solid, when heated, the atoms start vibrating more strongly. If you continue to heat it, there will come a point where the atoms have too much energy can cannot remain vibrating around a fixed position. They will then break free and go into the liquid phase, where they have much more room for movement. This is the process of melting. Now when your liquid is heated even more to a sufficient temperature, then the atoms now need much more space to move, and cannot remain jostling around. Hence, it converts to the gas phase, where there is much more room and the particles contain much more energy. This is the process of boiling. Almost all matter goes through these changes of state, or 'phase transition' when you add or remove heat energy from it.

What determines the melting and boiling points? Well, the atoms may be moving around and jostling with their friends, but between atoms and molecules, there may be so-called inter-molecular forces or inter-atomic forces, that hold the block of substance together. As you know, molecules are held together by chemical bonds, which involve the full or partial transfer of electrons from one atom to another. Sometimes, the electrons are spread unevenly throughout the bond and the molecule, and this causes what is known as a 'dipole moment'. In other words, your molecule is acting like a slightly charged rod (remember your physics lessons?) and attracting other molecules too. Hence, when you are heating a substance and changing its state, what you're doing is that you're giving an individual molecule enough energy to overcome these inter-molecular forces, and break free of its influences.

Consider the case of gold. Gold atoms in a block of gold are bonded together by 'metallic bonding'. Basically, the nuclei of the atoms generally stay in the same place, and the valence electrons swarm around freely in the spaces in between, giving rise to a 'sea of electrons'. This is how they are bonded together. Depending on the metal, metallic bonds are generally quite strong, which account for the relatively high melting and boiling points of the metals.

Well, to answer your question directly, the solid gold, when heated, would only become liquid gold, then when heated some more, gaseous gold. This is, of course, assuming that there are no other substances around to react with your gold atoms. When they are heated up and become more energetic, it is more likely that they will have enough energy to form bonds with other substances, oxygen for instance. Hence, if you heat gold strongly enough in air, it will most probably be oxidised. Then in that case would it change into another substance -- gold oxide. In general, bear in mind that the individual atoms make up the character of gold. It is the number of protons in the nucleus of the atom that gives it its identity, that tells us whether it's gold or silver, or something else. When you heat the atoms, they only move around more, but their internal character do not change. On a normal scale, the amount of energy you are using to heat the atoms is not high enough to break apart the internal structure of the atom. There is a 'fourth state' of matter, known as plasma. Basically in plamsa, the gaseous atoms are heated to such an extent that the electrons break free from the nuclei, and instead of a neutral cloud of gas you have an 'ionized gas', where the electrons and protons swim about separately from each other. But this requires immensely high temperatures.

At even higher energies, much more than we can imagine, in the terawatt scale, individual atoms can be smashed together in what are known as 'atom smashers' or 'particle accelerators', which are used by scientists to study the structure of matter (incidentally by smashing it up into little bits). In this case, we are giving individual particles very high amounts of energy, and hence they can actually morph into very much different substances. But this is beyond he scope of this answer. You could find out more at websites like

http://www.cern.ch/ or http://www.fermilab.gov/

I hope this answer has been of use to you, and wish you all the best in your studies! If you have any more problems, don't hesitate to ask the Mad Science Network one more time =)

Regards,
Tan "Phase" Thiam Hock