States of Matter

Solid, Liquid or Gas??

  • The state of a pure substance (solid, liquid or gas) depends on the strength of the attractive forces between its particles.

In Solid State: The attractive forces are relatively strong and each particle remains bonded to its adjacent particles.

As the solid is heated up, the average kinetic energy (temperature) of its particles increases.

Eventually, the particles have enough kinetic energy (motion) to break the bonds with neighbouring particles.

At this particular temperature (its melting point), the particles transition into liquid form.

  • At the melting point, all added energy is used to disrupt the intermolecular forces between the solid particles. This involves a change in potential energy.
  • During this phase change (known as fusion), there will be no change in temperature until the entire sample has melted.

In Liquid State: The particles can move past each other but they are constantly breaking and forming bonds with neighbouring particles.

As the liquid is heated up, their kinetic energy increases until the particles have enough energy to break all the intermolecular bonds and remain free of other particles.

At this particular temperature (its boiling point), the particles transition into a gas.

  • At the boiling point, all added energy is used to disrupt all the intermolecular forces between the liquid particles. This involves a change in potential energy.
  • During this phase change (known as vaporization), there will be no change in temperature until the entire sample has transformed into a gas.

  • The particles in a solid are rigidly attached to one another and often form a symmetrical structure. In a liquid, the particles are always in contact but can readily move past one another. In a gas, the only contacts between particles are collisions. The particles remain apart from one another after the collisions.

Melting and Boiling Points

Metals / Ionic Compounds / Molecular Compounds
Substance / Melting Point (oC) / Boiling Point (oC) / Substance / Melting Point (oC) / Boiling Point (oC) / Substance / Melting Point (oC) / Boiling Point (oC)
Li(s) / +181 / +1342 / CsBr(s) / +636 / +1300 / H2(g) / -259 / -253
Sn(s) / +232 / +2602 / NaI(s) / +661 / +1304 / Cl2(g) / -102 / -34
Al(s) / +660 / +2519 / MgCl2(s) / +714 / +1412 / H2O(l) / 0 / +100
Ag(s) / +962 / +2162 / NaCl(s) / +801 / +1465 / C6H6O(l) / +41 / +182
Cu(s) / +1085 / +2562 / MgO(s) / +2825 / +3600 / C6H12O6(s) / +146 / decomposes
The Melting and Boiling Points of Diatomic Molecules
Substance / Number of electrons in a molecule / Melting Point (oC) / Boiling Point (oC)
H2(g) / 2 / -259 / -253
N2(g) / 14 / -210 / -196
O2(g) / 16 / -219 / -183
F2(g) / 18 / -220 / -188
Cl2(g) / 34 / -102 / -34
Br2(l) / 70 / -7.2 / +59
I2(s) / 106 / +114 / +184
  • All of these molecules are symmetrical, non-polar molecules with a linear shape.
  • Therefore, the only interactions between the molecules are LDFs (London forces)

The Melting and Boiling Points of Molecules of Similar Size
Substance / Number of electrons in a molecule / Melting Point (oC) / Boiling Point (oC)
H2O(l) / 10 / 0 / +100
H2S(g) / 18 / -86 / -60
CO2(g) / 22 / (sublimation point) -78
  • In these compounds that have similar numbers of electrons, the previous trend is not clearly observed.
  • Since carbon dioxide becomes a gas at temperatures lower than the boiling points of the other two, it must have the weakest intermolecular interactions. This can be explained as it is the only non-polar molecule (it has a linear shape AX2) in the list. Therefore, although it may have the strongest of the London (dispersion) forces, it does not also have the dipole-dipole interactions found in the other two molecules.
  • Water and hydrogen sulphide are both bent molecules (AX2E2) and are polar. Yet why does water (with fewer electrons and therefore weaker London forces) actually have much higher melting and boiling points?
  • Water molecules have stronger attractive forces between them because of the hydrogen bonding that takes place between water molecules. Hydrogen bonding is not evident between hydrogen sulphide molecules.
  • The following graph will help to illustrate the influence of hydrogen bonding.

(The dotted line shows what the boiling point of water would probably be if it had no hydrogen bonds.)

  • The hydrides of the Group 14 elements are all symmetrical molecules with identical bonded atoms. They are all tetrahedral (AX4) and non-polar. As a result, the only intermolecular forces present are London (dispersion) forces and the trend of increasingly larger molecules having increasing higher boiling points is observed with no exception.
  • The hydrides of the Group 15, 16, and 17 elements are all symmetrical molecules but they are all polar. Group 15 hydrides (AX3E) are all trigonal pyramidal, Group 16 hydrides (AX2E2) are all bent, and the Group 17 hydrides (AXE3) are all linear.
  • Furthermore, in the molecules of these hydrides in which the central atom is from period 2 (N, O and F), the dipole-dipole interaction is particularly strong (hydrogen bonding).

Chem 20States of Matter Assignment

  1. Predict which substances will have the higher boiling point in each of the following pairs. Explain your predictions using bonding theories.

a. NH2Cl or PH2F
b. Ne or Xe / c. NH4Cl or CH3Br
d. CH4 or C2H6 / e. CO2 or SO2
f. CH3OH or C2H5NH2
  1. Compare the bonding and molecular polarity of SeO3(s) and SeO2(s).
  1. List the following substances in order of increasing boiling points: C3H8, ZnO, C7H15OH, and C5H12. Give a reason for your answer.
  1. The alkanes are a family of non-polar molecular compounds containing carbon and hydrogen. The data below contains the melting points and boiling points for alkanes having 1 to 10 carbon atoms. Using graph paper (and a ruler!!|!), draw a graph representing this data. Your graph should be alabelled line graph that plots temperature (on the y-axis) vs. molecular electron count (on the x-axis).

Name / Formula / Number of Electrons per molecule / Melting Point (oC) / Boiling Point (oC)
methane / CH4 / -182 / -161
ethane / C2H6 / -183 / -89
propane / C3H8 / -188 / -42
butane / C4H10 / -138 / -1
pentane / C5H12 / -130 / +36
hexane / C6H14 / -95 / +69
heptane / C7H16 / -91 / +98
octane / C8H18 / -57 / +126
nonane / C9H20 / -53 / +151
decane / C10H22 / -30 / +174