Download Solids (revision)

Investigating the strength of the
forces within selected solids
Sugar
Sulfur
Salt
Ice
Wax
Silicon
Tin
Iodine
Graphite
Nine solids are place around the edge of a metal
plate which is placed on a heating element.
The plate is at room
temperature, and
even before the
heating element is
turned on the ice
begins to melt.
In less than a minute
the ice has melted.
The heating element is turned on and slowly
increases in temperature.
The wax starts to melt soon
after the heating begins.
It quickly turns to liquid and
then vaporises.
In the
background
the silicon is
unchanged.
Next the iodine begins to sublime, forming
faint clouds of purple vapour above the grey
crystals.
At 100 °C the sulfur is still
solid.
The liquid water begins to
boil.
The iodine continues to sublime.
The sulfur starts to melt at 119 °C.
The iodine crystals have
almost all vaporised now...
…and the sulfur is
completely melted.
The sugar starts to melt.
All that remains of the
iodine is a faint stain
caused by reaction with
the steel plate.
The strip of tin foil starts
to soften and cling to the
steel plate.
Meanwhile…
The molten sulfur is
getting darker…
…while the sugar is
beginning to caramelise
as the molecules
themselves start to
break down.
The sugar continues to
darken as it bubbles and
breaks down.
Eventually all that
remains is black carbon.
The tin softens, then melts and shrivels into a few drops
of molten metal.
All that remains of the sulfur is a
blackened spot where the iron in the
plate has reacted to form black FeS.
Meanwhile,
…the salt,
silicon
and graphite
remain unchanged.
Ice melts at 0 °C
Wax melts at 66 °C
Iodine sublimes as it warms. Under pressure, iodine melts
at 114 °C.
Water boils at 100 °C
Sulfur melts at 119 °C
Sugar melts at 186 °C and starts to decompose
Tin melts at 232 °C
Salt melts at 801 °C
Silicon melts at 1410 °C
Graphite melts at 3750 °C
Ice, wax, iodine, sulfur and sugar are all covalent molecular
substances. As these substances melt or boil it is
molecules which separate, not atoms.
Water in either the liquid and gaseous
phases is a molecular substance.
These between-molecule inter-molecular forces are
weak, which is why these substances melt easily.
Tin is a metal. Some metals have low melting points
(mercury melts at –39 °C), while others have much higher
melting points. The steel plate we used melts at around
1400 °C, while tungsten melts at 3410 °C. Metallic bonds
between metal atoms can be weak or strong, depending on
the metal.
Sodium chloride is an ionic substance. All ionic substances
have relatively high melting points because each ion is
held in place by others above, below and beside it in 3
dimensions.
Graphite and silicon are both covalent network
substances. They have very high melting points because
each atom is locked in place a number of strong
covalent bonds.