Akul Posted December 7, 2013 Share Posted December 7, 2013 Any solid existing in the universe possesses melting point. But practically, only pure solids can achieve its molten state. I'll tell you why. Firstly, let us take an example of Iron rod. Consider a pure iron(or cast iron) rod. It will surely melt. This is because, the Iron molecules are confined intact with each other with a metallic bond. This metallic bond, in solids, is same as inter-molecular force of attraction. hence the temperature required to to break the metallic bond to melt it is same as the temperature required to break the inter-molecular force of attraction. Hence, the bond and the space molecules are broken at the same time. Let us take another example of a Polythene bag. It is chemically a polymer of Ethylene (CH2=CH2). The Melting point of ethylene is 104.0K. But due to the polymer bonds, the melting point of polyethylene is amplified to 280K. The whole substance is uniform, the melting point of each monomer is same and hence the net melting point is same. Now, let us take an example of wood. Wood is a mixture of Cellulose, Hemicellulose and Lignin majorly, and are composed of other different carbohydrates and proteins. The melting point of each compound is different and hence due to fractional distillation concept, the one that is more volatile will vaporize and then the less volatile one. But, due to difference in bond structure, this doesn't happen. Instead, when wood is heated, it converts into CO2 and H2O. This is because, wood is basically confined of three bonds:- van der Waal's force; hydrogen bond; and covalent bond. And due to heat, vdW bond breaks first, and then H2 bond and finally covalent bond. Wood can be melted only when all the three forces are equalized. This can be done only when the solid is compressed so that the "electronegativity difference-inter molecular force of attraction" between the atoms are equalized. This is practically not possible because, we require vacuum so that the combusting material does not react with oxygen or atmosphere air. Hypothetically, this is possible but not practically. Link to comment Share on other sites More sharing options...
John Cuthber Posted December 7, 2013 Share Posted December 7, 2013 "Any solid existing in the universe possesses melting point." No, for two reasons. Some things decompose before they melt. Melting temperatures are dependent on pressure, so they are not strictly points. Ice at the bottom of the ocean has a different melting point from ice on the surface. "Let us take another example of a Polythene bag. It is chemically a polymer of Ethylene (CH2=CH2). The Melting point of ethylene is 104.0K. But due to the polymer bonds, the melting point of polyethylene is amplified to 280K" Obviously wrong. 280 K is about 7C. Most polythene bags I have seen are warmer than that, and they are not molten. "The whole substance is uniform, the melting point of each monomer is same and hence the net melting point is same." No, polymers are mixtures of different components, that's why they usually have a rather poorly defined melting point (or, more realistically a melting range). "The melting point of each compound is different " Cellulose doesn't have a melting point, you can not melt it because it will decompose before it melts. "van der Waal's force; hydrogen bond; and covalent bond. And due to heat, vdW bond breaks first, and then H2 bond and finally covalent bond." No, the hydrogen bonds are the weakest, but there are lots of them so, if you try to heat cellulose until it melts the covalent bonds start to break before all of the hydrogen bonds are broken. That's why it decomposes. "Hypothetically, this is possible but not practically." No, not really. It'snot hypothetically possible either. "Any solid existing in the universe possesses melting point." No, for two reasons. Some things decompose before they melt. Melting temperatures are dependent on pressure, so they are not strictly points. Ice at the bottom of the ocean has a different melting point from ice on the surface. "Let us take another example of a Polythene bag. It is chemically a polymer of Ethylene (CH2=CH2). The Melting point of ethylene is 104.0K. But due to the polymer bonds, the melting point of polyethylene is amplified to 280K" Obviously wrong. 280 K is about 7C. Most polythene bags I have seen are warmer than that, and they are not molten. "The whole substance is uniform, the melting point of each monomer is same and hence the net melting point is same." No, polymers are mixtures of different components, that's why they usually have a rather poorly defined melting point (or, more realistically a melting range). "The melting point of each compound is different " Cellulose doesn't have a melting point, you can not melt it because it will decompose before it melts. "van der Waal's force; hydrogen bond; and covalent bond. And due to heat, vdW bond breaks first, and then H2 bond and finally covalent bond." No, the hydrogen bonds are weak, but there are lots of them so, if you try to heat cellulose until it melts the covalent bonds start to break before all of the hydrogen bonds are broken. That's why it decomposes. "Hypothetically, this is possible but not practically." No, not really. It's not hypothetically possible either. Link to comment Share on other sites More sharing options...
Akul Posted December 8, 2013 Author Share Posted December 8, 2013 sorry for the mistake!!- polythene covers have M.P of 380 K and not 280 K "The whole substance is uniform, the melting point of each monomer is same and hence the net melting point is same." What I meant was- polymers are usually or certainly homogenous in nature whereas , wood , for example, is a heterogenous mixture. I still have a doubt in : "hence the net melting point is same." because Nylon 66 is a polymer of 2 monomers adipic acid and another compound........ so can you explain the Melting procedure of nylon 66?? "No, for two reasons. Some things decompose before they melt. Melting temperatures are dependent on pressure, so they are not strictly points. Ice at the bottom of the ocean has a different melting point from ice on the surface."- exactly my point....... when you heat the wood in vacuum with high pressure applied, then the wood could melt rather than getting decomposed...... Link to comment Share on other sites More sharing options...
John Cuthber Posted December 8, 2013 Share Posted December 8, 2013 "exactly my point....... when you heat the wood in vacuum with high pressure applied, then the wood could melt rather than getting decomposed......" No, it doesn't. "What I meant was- polymers are usually or certainly homogenous in nature whereas , wood , for example, is a heterogenous mixture." Polymers are often, at least at some scale, heterogeneous. Wood is a mixture of polymers. Cellulose is a polymer, it's homogeneous or not depending on how you consider polymers of different molecular weights. Link to comment Share on other sites More sharing options...
BearOfNH Posted December 16, 2013 Share Posted December 16, 2013 This discussion is hard to follow because of the way the replies quote each other. In any event I sure as heck would like to see some melted wood. Link to comment Share on other sites More sharing options...
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