Melting and Boiling Points of Odd Materials

Question

In Chemistry, I was taught that there are three main states of matter: solid, liquid, and gas, and that heat and pressure determine that state. For some substances, the line is blurry between them.

Some materials don't seem intuitively to do this--nor have I been able to find data on them. For example, what is a reasonable estimate of a melting point for brick? What is the boiling point of paper? When will a carpet sublimate?

The common theme seems to be that these are all composite materials. Certainly all the elements have melting points (as applicable) and boiling points. Many compounds do too. However, something like cardboard is a mixture of fiber, glue, pigment, and possibly other things. Each of these might be made up of several compounds, with each compound having its own boiling point.

My suspicion is that for composite materials, individual compounds would exhibit properties roughly individually--so to melt wood, the water would boil off first, and then maybe it would start melting into a glucose-protein slag. Is this truly the right idea for what happens?

Answer 1

This is an interesting question, because there is no simple answer - many different things are going on. A quick answer is that many materials don't exhibit clear melting points because heating turns them into something else before they can melt. Here are a couple of examples:

Generally, pure substances have simple, well-defined melting and boiling points. But there are many exceptions. A key example is gypsum - CaSO4 . 2H2O - calcium sulfate dihydrate. When it's heated, it first dehydrates in two steps. Initially it loses 1.5 of the water molecules as vapor, then in a second phase it loses the last 1/2 water molecule as vapor. When this dehydration is complete, the remaining compound is not the compound that we started with - it is now CaSO4. And if heating is continued, much of the CaSO4 will decompose - producing CaO (solid) and SO3 (vapor). Eventually, some melting will be observed, but the melted material will actually be a mixture of CaO and CaSO4.

There's a very detailed description here: http://nvlpubs.nist.gov/nistpubs/jres/27/jresv27n2p191_A1b.pdf

For more complicated materials, similar processes occur, though not always as predictable. Your last question was about wood. As wood heats up it first dries out as you stated - i.e. the free water within the wood boils and leaves as vapor (this is not chemically bound water as in the gypsum). Then as heating continues, the cellulose and other complex organic molecules thermally decompose, producing a lot of light organic compounds such as methane, butane, propane, alcohols, etc. These are essentially small bits of the molecules containing carbon, hydrogen, and oxygen which break off and float away. If oxygen is present, these will burn to produce more heat to speed up the decomposition process, but even without oxygen this decomposition will occur.

Eventually, no more light hydrocarbons can be released, and we are left with carbon and various trace elements including potassium, calcium, magnesium, etc.. This is charcoal. Continued heating can eventually get to the point where these materials melt, but what remains is no longer wood - it's carbon and a few other things - so there is no point at which "wood" will melt.

Answer 2

This doesn't really make sense because the boiling points are "well defined" quantities. I can boil water, report the temperature when it boils and challenge you to reproduce it in your home town with "different" water. You can repeat the experiment and find that the water will boil at the same temperature (actually substances boil/freeze/etc over fairly narrow ranges not specific points, but that's another issue altogether).

In short, there is not a well-defined "brick", you can make it with straw or without straw, and paper can have acid or not have acid, etc...

Would you might be able to empirically reproduce is any result obtained from a homogeneous material composed of many parts where these portions are consistently kept in proportion to each other between samples. It is important that the material in this case at a minimum be homogeneous because at the small scales the material will look very different if it is not (so this would be like comparing apples to oranges). As in your example, cardboard is not homogeneous, so at small scale you would notice the various parts boiling (or burning) at different temperatures (the paper will surely act different than the glue). But what about the glue? Will glue A act boil at a different temp than glue B, absolutely! This is governed by the components.

On a side note, for gases we do know a lot about "composite gases" check out Raoult's Law for example.

Answer 3

I'm not sure I would focus on the the liquid to gas phase change. Sublimation, seems better behaved. I've heard that some alloys do not have well defined melting points. I don't know if something like that occurs in sublimation. But imagine a crystal lattice consisting of two fairly different substances. In this case it seems the material that sublimes at a lower temperature will induce the other to sublime at a much lower temperature. Since, after all the two substances would hold each other in the lattice. One starts to wonder what exactly is the cause for a solid to gas, phase transition to occur. Just a small change in temperature causes an abrupt change of state. Apparently, entropy of the system and its surroundings is maximized by this phenomenon. In other words sublimation at an abrupt point is natures best way to maximize entropy.

Answer 4

I think you are asking a Physics question in a Physics forum, so answers may be formulated along the lines of Physics principles. But the fact is that in your compound materials, there are going to be chemical reactions that happen prior to many melting or sublimation points.

The physics describe just fine what might happen to any particular molecule at a temperature/pressure point, but to transition from STP to that point, the molecule you started with may no longer be there.

It is a similar problem to demonstrating the boiling point of water, in a pot made of Cesium. Instead of watching some water boil, boom and everybody in the room is burned, poisoned and radioactive. With your everyday materials examples, the physics can't be studied in isolation because of the extenuating factors which in this case happen to be Chemistry factors.

By the way, scientists have the same general problem trying to demonstrate simple chemical properties in something more complex, like a living organism. Like, if you tried to inject baking soda and then vinegar into your arm to see if it would fizz some Hydrogen, you would likely run into a variety of operational problems before the experiment got underway very far.