Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne "Apparent Melting"


Name: Bernard Status: educator Grade: 6-8 Location: CA Country: USA Date: Spring 2012

Question:
This is about melting. I know you already answered "Wood will not melt." And also, "sugar does not melt, it decomposes." Is there a different name for this, then? Somewhere I saw "apparent melting."? What if wood was heated up in an inert gas environment, and placed under pressure. Could liquid wood then be produced? If it is true sugar does not melt (i.e. a physical change), then what are it is products if it "decomposes" (which is a chemical change)?

Replies:
Bernard,

The reason that some substances decompose rather then melt is because the energy required to break the chemical bonds within the molecule is less than the energy required to break the intermolecular forces holding the molecules in a solid state. Since the melting temperature can be a function of pressure (look at any phase diagrams), it is possible to increase or decrease the temperature of melting and as such it is possible to move the melting temperature above or below the decomposition temperature since decomposition temperature is not as strongly affected by pressure.

I could not find a phase diagram for sugar (just sugar-water mixtures) so I am not sure that there can be a pressure at which sugar melts without decomposition.

Wood is complicated, since what we call wood is a complex mixture of different substances (some of which are actually trapped liquid), this begs the question of at what point would we consider the wood as having melted - when 50% of all the different types of substances in wood have turned to liquid? 100%? What if some of the substances have already turned to vapor, would we still consider it wood?

Greg (Roberto Gregorius) Canisius College


Wood does not “melt” because the components (cellulose and lignin) of the wood are extremely high in molecular weight and all bonded together. In simple terms, it is all one big molecule. If heated in the presence of air (the oxygen) causes it to do a “slow burn”. In the absence of air (the oxygen) the carbon, hydrogen, and oxygen that are the components of wood, decompose producing carbon, water and carbon dioxide. It can never form “liquid wood”.

Common table sugar (sucrose) is different. It has a molecular weight of 342.3 and an approximate melting point of about 186C. As you mentioned that is referred to as its “apparent melting point” ( http://www.sciencedaily.com/releases/2011/07/110725123549.htm). But here is where things get “sticky”. Molten sugar is very viscous, so the usual methods of determining its melting temperature do not work. This behavior is not unique to sugar. The common chemical glycerin that you can find in any pharmacy has a melting temperature above 0 C., but almost nobody has ever seen solid glycerin because it is so viscous the molecules cannot form a crystal. It forms a glass. A “glass” is a high viscosity “liquid”. This also means that to measure the melting point special methods are required. I cannot find that the melting point of sucrose has ever been measured by the special methods required. One of the requirements is the exclusion of all air (i.e. oxygen).

Vince Calder


Click here to return to the Chemistry Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory