As we are well aware of, changing sufficiently the temperature of a substance
can cause it to change from solid to liquid to gas and viceversa. Similarly,
state changes can also be achieved by suitably varying the pressure : a gas can
be liquified by exerting enough pressure onto it. This is what is done for
instance with the "liquid propane" (LP) combustible.
Propane, the third member of the hydrocarbon family, is a gas under normal
conditions. In order to store and transport it more efficiently, it is
pressurized until it becomes liquid, and contained under pressure into gas
bottles. Opening the tap, decreases the pressure at the outlet, and the propane
exits in the gas form.
Similarly, the boiling point of a liquid will depend on the pressure at the
liquid's surface : they say you cannot have good spaghetti in Denver...
The most familiar state changes are to/from solid and liquid, freezing
and melting and to/from liquid and gas, boiling and condensation.
A less familiar transition is from solid to gas directly, sublimation.
A mothball, left long enough to itself, will completely sublimate
into mothball smell.... Similarly, Carbon Dioxide, CO2, when frozen into
"dry ice", will directly change into gas when reheated.
The most important rule of state transition is that, when a substance reaches
its transition temperature, it will remain at that temperature until the whole
transition has taken place, no matter how much heat is added or removed from it.
If we put a mixture of ice and water on a flame, the mixture will remain at 0
degrees until all of the ice is melted, and only then its temperature will
begin to raise. Similarly the temperature of boiling water will remain at 100
degrees until all the water is boiled off.
The process of state transition involves a large amount of energy transfer.
It takes 80 calories to melt one gram of ice into water; if we remember that
the definition of calorie is the amount of thermal energy required to increase
the temperature of 1 gram of water by one degree, then we see that the same
amount of heat required to melt ice into water would also increase the water
temperature from 0 to 80 degrees !!!
Even more heat, i.e. 580 calories, are required to boil off one gram of water,
after it has reached 100 degrees.
Problem : we toss an ice cube (1 inch on the side) straight from the freezer
(-100) into a 100 ml glass of water at room temperature (200). What will
be the temperature of the mixture when all the ice is melted ?
And if we toss two ice cubes ?
Chemical Reactions and Energy
Chapt. 12, page 312-313
Within the atom, any given electron in a certain configuration has a certain
amount of potential energy, whose values depends on the relative location of the
electron with respect to the nucleus and the other electrons. This energy
originates from the fact of orbiting at a certain distance from the central
nucleus. In the same way, an object orbiting around a central mass has a
certain amount of gravitational potential energy. If this is not obvious,
think in the following terms : to bring an object from the surface of the earth
to a certain distance away, work must be done against the force of gravity.
By conservation of energy, this work results into the object acquiring an
equivalent amount of potential energy.
All chemical reactions involve the re-arrangement of outer shell electrons,
and any such re-arrangement results in a change of the energy for the electrons
involved. This change can occur in two directions, i.e. the global energy
corresponding to the configuration of the final products can be smaller or
larger than the initial energy.
In the first case, the extra energy will be released in the form of heat, and
the corresponding chemical reaction will be called exothermic. If
on the contrary the final configuration results into a larger energy, then
energy has to be supplied in orded for the reaction to take place, and we have
an endothermic reaction. (exo = towards the outside, endo = towards
the inside, think of exit and entrance).
Exothermic reactions can either occur spontaneously, or might need some sort
of initial energy input to be initiated, after what they will be
self-sustaining. In either case, if a large enough quantity of reactant is
present, the process can result into an explosion : the technology of
explosives is based on the understanding of exothermic reactions.
Endothermic reactions on the contrary need a continuous energy input in order
to proceed: cooking is a good example of endothermic reactions...
Properties of materials : Strength
The arrangement of electrons forming bonds among atoms determines not only the
chemical but also the physical properties of elements and compounds. Even in a
single element atoms can organize themselves to create materials of different
properties. The most striking example is Carbon, that is known to occur as
diamond, an extremely hard crystal, coal,
a not so hard poly-crystal, and graphite, a very soft amorphous
material. Moreover, a completely new form of elemental carbon was recently
discovered.
Mathematicians have known for a long time that one can form
a regular sphere-like three-dimensional structure with a combination of
hexagones and pentagones (take a good look at a soccer ball next time you come
across one...). This structure had been adopted by the architect Buckminster
Fuller to design the "geodesic domes" a strong and pleasing construction.
A breakthrough in chemistry occurred when it was realized that Carbon atoms
actually can arrange themselves to form the same structure (60 atoms are
needed to complete the full structure, therefore one has a C60 molecule).
In honour of the architect, this form of elemental Carbon was named
buckminsterfuller (also buckeyball, for short) and compounds based on it
buckminsterfullerenes. The study of the properties and the potential of
buckminsterfullerenes has just started, and the future might bring some
amazing new materials.
Coming back to more traditional substances, one important property is the
physical strength. It is a well known fact that different materials will
react differently to stress, and can be very resistent to some type
of force but very weak towards some other.
In order to make a classification, one distinguishes among :
- compressive strength, the ability to resist crushing. An egg,
generally considered a rather fragile object, has in reality a very good
compressive strength
- tensile strength, resistance to being pulled apart
- shear strength, resistance to twisting
Top quality materials (e.g. diamond) will exhibit all three forms of strengths;
this is usually the consequence of a symmetric atomic structure with equally
strong bonds in all directions. Other materials portraying particular strength
in a preferential direction can be reinforced by being assembled in alternate
layers, pointing in different directions (if you stack firewood, you are advised
to alternate layers with the logs pointing in two orthogonal directions).
Another technique for exploiting complementary properties of different materials
is to combine them together, e.g. as in reinforced concrete or in rubber tires
with embedded steel meshes.
Regardless of its strength, any material will always have a certain
elastic limit. Subjected to stresses below the elastic limit, an object will
be stretched or squeezed, but will then return to its original shape. The
deformation occurs for any material, even though some appear to be much more
elastic than others. Forces exceeding the elastic limit will cause either
permanent deformation or breakage.
