Is melting of gold a physical property?

Question

I read that melting of gold is a physical property, but I wonder if this is true.

If we want to melt gold we will need to add heat, and once we add heat to gold to melt it, there must be a chemical change, not a physical change, right?

Answer 1

It is a phase change - gold exists as a solid crystal (fcc I believe off the top of my head), as a liquid, and as a vapor (gas). First order phase transitions exhibit a volume change, and an enthalpy of the phase change. In the case of gold, both the crystal and the liquid are metallic, with similar numbers of nearest neighbors.

All elements exhibit various phases. Since pure gold is, well, pure gold, I'm not sure how a phase change in gold could be considered a 'chemical change', particularly in the solid-to-liquid. The type of bonding does not change, and there are no 'chemical reactions' going on. This is all fairly basic thermodynamics.

Answer 2

Melting is a physical change, not a physical property. The distinction is subtle, but the way I think of it is:

A physical change is one that does not involve a change in any chemical bonds

Compare that to:

A physical property is one that can be measured without requiring a chemical change

Melting is the physical change, temperature and melting points are physical properties.

Simply adding heat to a substance doesn't necessarily mean that chemical bonds will be broken or formed. Metallic bonds are not usually considered to be chemical bonds, although it really depends on how you look at them.

Later you will learn that the distinction between physical and chemical bonds is fairly arbitrary, since they all have to do with changes in electron density.

For example, melting an ionic solid (like table salt) results in ionic bonds breaking, but this is still considered a physical change.

One way to identify a physical vs. chemical change when you don't know the chemical details is to imagine what would happen if you reversed the operation. If the system could go back to the way it was fairly easily, the chances are good that it is a physical change. This doesn't work for all situations, but it does work for the most common ones. The way I remember it is: "you can't unbake a cake," so "baking" is probably a chemical change. Going back to the melting example - if you cool molten metal down, you get back to where you started, and so that is probably a physical change.