How To Determine If A Substance Have A Low Melting Point Or A High Melting Point?
Do you mean in the lab? In the work you just take a sample of the substance and monitor temperature until you see the substance melt.
If you want to estimate a bp from a compounds composition and structure you can do that as well. Size is proportional to melting point, the higher the MW the greater the bp. Also, the types of bonds present in the molecule determine mp as well. Ionic molecules tend to have higher mps. A molecule that is capable of H-bonding to itself will have a higher mp. For example, alcohols are hydrogen bond donors and acceptors and can undergo H-bonding and have higher mps than their corresponding alkanes. A corresponding aldehyde/ketone would have a lower mp than the alcohol, but a higher mp than the alkane. This is becaues aldehyde/ketones are only H-bond acceptors, not donors, so they can't H-bond to themselves. This is why they have lower mps than an alcohol of the same size. The aldehyde/ketone, however is polar, so it is capable of dipole interactions which makes its mp higher than that of the corresponding alkane.
Examples:
From left to right, the following molecules increase in mp….CH3CH2CH3, CH3CH2CHO, CH3CH2CH2OH, CH3CH2COOH
Notice all the molecules are roughly the same in size…so we are only evaluating bonding. The first is an alkane which only exhibits vander waals forces, so its mp is low. The second is a aldehyde (same bonding as a ketone) which is capable of vander waals and dipole interactions so its mp is a tiny higher. The 3rd is an alcohol which is capable of vander waals, dipole, and H-Bonding. The last is a carboxylic acid which is capable of vander waals, dipole, and H-bonding. The reason the acid’s mp is higher than the alcohol is because the acid has two H-bond accepting groups while the alcohol only has 1. Also, the acid has two polar groups while the alcohol only has 1. The stronger the intermolecular forces are, the higher the molecules mp will be. But remember, size is also important in mp. CH4 has a lower mp than CH3CH3 even though the only type of bonding present is vander waals.
Zac on November 30th, 2009
It depends on the type of substance it is:
In general, molecular compounds with network covalent bonding tend to have the highest melting points (diamond, etc)
Ionic compounds are generally next highest. The ones with the greater lattice energy have the higher melting points.(NaCl, etc)
Molecular compounds with intermolecular forces holding their molecules together have the lowest. The molecular compounds with hydrogen bonding (H2O, etc) have higher melting points than nonpolar molecular compounds (CH4, etc) which have only london dispersion forces.