Melting points
The melting point of a compound can tell you quite a bit about its structure. The melting
point is the temperature at which the “units” (typically molecules or ions) in the
compound separate and begin to flow around each other (- giving what we call the liquid
state). The more energy needed to separate the units, the higher the melting point.
For example:
Ionic compounds tend to have very high melting points because in order to melt the
compound you have to supply enough energy to actually break ionic bonds. Typically
this occurs at very high temperatures, since ionic bonds are strong.
Melting points for ionic compounds range from ~500 – 3000 C
Molecular compounds tend to have lower melting points than ionic compounds; ranging
from very low (below room temperature) to ~300C (after this temperature compounds
often start to decompose). This is because when a molecular compounds are melted it is
the intermolecular forces (such as London forces, dipole-dipole forces or Hydrogen
bonds) that are broken, NOT the bonds that connect the atoms within the molecule itself.
Since intermolecular forces are usually weaker than covalent bonds or ionic bonds, then
the energy required to break them is smaller and so the melting temperature is lower.
Typically for molecular compounds the melting point increases
 as the molecular weight increases, (due to increases London forces),
 and as the strength of the intermolecular forces increases
So for two compounds with the same molecular weight, the most polar compound will
have the highest melting point.
There are also other factors that also affect the melting point such as how the “units” of
the compound pack together.
Boiling points
The boiling point of a compound can tell you quite a bit about its structure. The boiling
point is the temperature at which the “units” (typically molecules or ions) in the
compound completely separate and go off into the gas phase (- giving what we call the
vapor or gaseous state). The more energy needed to separate the units, the higher the
boiling point.
Boiling points for ionic compounds are not usually recorded since they are so high.
Molecular compounds tend to have lower boiling points than ionic compounds; ranging
from very low (below room temperature) to ~300C (after this temperature compounds
often start to decompose). This is because when a molecular compounds are vaporized it
is the intermolecular forces (such as London forces, dipole-dipole forces or Hydrogen
bonds) that are broken, NOT the bonds that connect the atoms within the molecule itself.
Since intermolecular forces are usually weaker than covalent or ionic bonds, then the
energy required to break them is smaller and so the boiling temperature is lower.
Typically for molecular compounds the boiling point increases
 as the molecular weight increases, (due to increases London forces),
 and as the strength of the intermolecular forces increases
So for two compounds with the same molecular weight, the most polar compound will
have the highest boiling point.
Mass spectrometry
Mass spectrometry is a technique that allows us to find the molecular weight of a
compound. The compound is vaporized and injected into a chamber where it loses an
electron (either by being hit with other high energy electrons, or high energy radiation).
The resulting ion may break up into smaller fragments or stay intact, but each fragment is
then accelerated by an electric field which passes through a strong magnet towards a
detector. The ions are deflected on the basis of their charge to mass ratio – Lighter ions
are deflected more than heavier ones. The detector is calibrated in mass units so that the
heaviest ion that the detector reads is usually the ionized molecule itself and this peak is
known as the molecular ion. The lighter fragments usually appear as smaller peaks at
lower molecular weight.
Elemental Analysis
Most simple compounds – particularly those containing carbon and hydrogen – can be
subjected to combustion analysis in which the compound is burned in air to produce
carbon dioxide and water.
CxHyO + z O2  x CO2 + y/2 H2O
From the mass of the initial unknown and the masses of carbon dioxide and water
produced, the % C, %H (and by difference the %O) can be calculated.
Typically results of combustion analysis are reported in this way.
From this % composition data, the empirical formula (the lowest mole ratio of the
elements in the compound) can be calculated.
Calculation strategy
Assume you have 100g compound (since the composition is given in %)
For example a compound with %C = 49, %H = 8., %O = 43.would contain
49 g C, 8.0 g H, and 43 g O.
Calculate the # moles of each element ( moles = mass x 1/molar mass)
Moles C = 49g C x 1mol C/12g C = 4.1 mol C
Moles H = 8.0g H x 1mol H/1.0g H = 8. mol H
Moles O = 43g O x 1mol O/16g O = 2.7 mol O
Fine the mol ratio of elements C:H:O by divide the number of moles of each element by
the smallest # moles
C(4.1/2.7):H(8.0/2.7):O(2.7/2.7) = C1.5H3.0O1
Multiply up until you get a whole number for each
C3H6O2
This is the empirical formula. The molecular formula cannot be found until you know
the molecular weight.
Solubility
The solubility of a compound can tell you a great deal about that compound’s molecular
structure. The general rule of thumb is “like dissolves like” so that polar or ionic
compounds tend to be soluble in water, and non polar compounds tend to be soluble in
non polar solvents like hydrocarbons.
If a compound has polar and non-polar parts the solubility will depend on the relative
amounts of each portion of the molecule.
For example low molecular weight alcohols with up to 3 carbons e.g. CH3CH2CH2OH
are completely soluble in water in all proportions, because the polar nature of the OH
group allows Hydrogen bonding to occur with water – which is a favorable interaction.
These compounds are also completely soluble in non-polar solvents also. However as
more CH2 groups are added (eg , CH3CH2CH2CH2OH) the solubility of the alcohol in
water decreases rapidly.
Some compounds have different solubilities in water depending upon the pH of the
solution. Typically weak acids or bases which often have
Lewis structures
Color
Maybe we should change Tests to
Analysis –
Mass spectrum
Elemental analysis
Properties
MP
BP
Solubility.