|The Challenger Accident|
Challenger, launched at 11:38 am EST|
after a freezing Florida night,
exploded 73 seconds after liftoff
due to the failure of an O-ring seal
on the right solid rocket booster
killing 7 passengers.
Redesign of the seal and modification
of the space shuttle took almost 3 years.
The replacement Endeavor resumed
flight Sept. 29, 1988.
A material undergoes transition under the influence of temperature and pressure, and these changes are physical in nature, because their molecules remain intact. During our school days, we were asked to distinguish physical and chemical changes. At that stage, we began to think in more details than what our senses have detected. Having the ability to distinguish physical properties from chemical properties is indeed a good beginning in the study of materials.
The Challenger disaster is due to a human failure to recognize the limitation of the property of o-ring material.
|Phase Transition at Constant Pressure|
Heat of vaporization
Heat of fussion
Phase transitions from solid to liquid, and from liquid to vapor absorb heat. The temperature of a system usually does not change as long as two phases are present. The (phase) transition temperature from solid to liquid is called the melting point whereas the temperature at which the vapor pressure of a liquid equals 1 atm (101.3 kPa) is called the boiling point. Thus, the measured boiling point depends on the atmosphere pressure. For compounds that decompose at high temperature, boiling point can be either specified at lower pressure or be replaced by the decomposition temperature. Thus, conditions as well as the value of boiling point listed in literature must be taken into account for application considerations. Boiling points of mixtures change with composition. The boiling points of some common mixtures are listed in handbooks, and boiling points can be used to assess the composition of a mixture or the purity of a compound.
However, a glassy material becomes soft in a wide range of temperatures. The temperature at which the material becomes soft (behave molten like) is called glassy temperature, but it may be a range of temperatures. Behavior of a substance as the temperature changes must be carefully considered in its applications. Behavior of a mixture as temperature rises is different from its components. There is no theoretical way to predict the behavior of a mixture from its components, even if its exact composition is known. Addition of one or more materials usually changes the melting or glassy temperature of a substance. Thus, we often employ a blend (mixture) of materials whose behavior is acceptable within the desirable range of temperatures. Antifreeze for automobile radiator and deicing liquid for airplanes are examples of this application.
The behavior of mixtures as temperature and pressure change often requires a phase diagram to represent, and there are several models of two-component systems. A phase diagram of a many-component system requires a lengthy study.
One-component phase diagrams for water and carbon dioxide are given here.
At pressure below 5.1 atm, solid and gas carbon dioxide coexist, but the vapor pressure depends on the temperature. The variation of vapor pressure is represented by a line, which separates the Dry ice phase from the CO2 Gas phase. The vapor pressure of dry ice at 194.6 K (-78.5°C) the pressure is 1 atm, and at 216.7 K (-56.4°C) the pressure is 5.11 atm. The line separates the conditions for the formation of solid and vapor. A similar curve borders between liquid and gas CO2, whereas a line separates dry ice from the liquid phase. At 216.7 K, vapor pressures of solid and liquid CO2 are the same, 5.11 atm. At, 5.11 atm and 216.7 K, all three phases coexist, and the condition is called the triple point.
Phase rules in soil science
Phase equilibria in one-component systems pdf file
Phase equilibrium: Pure substance Chapter 5 of a book
Phase and phase diagrams
Chemical energy Use FIND to search for "tin disease" after
|Linear thermal expansion|
coefficient per K at room
temperature of some substances
|Electric resistance of some|
|P (white)||109 Note the range of 1015|
The reciprocal of electric conductance is called electric resistance; thus, the higher the conductance, the lower the resistance. Electric resistance for some familiar materials are given in the table here. Note the large range of 1015 among these substances. Aluminium and copper are very good conductors, and their resistances are very low, in the order of 10-8, almost 100 times smaller than that of tungsten, W. Germanium, Ge, and silicon, Si, are typical semiconductors, whereas sulfur and phosphorous are insulation material.
Iron, cobalt and nickel are some ferromagnetic substances, there are some other alloys and oxides that behave this way. They possess a spontaneous magnetic moment. A magnetic field is present in these materials even in the absence of an external magnetic field. However, ferromagnetism is temperature dependent, and above the so called Curie temperatures of the substances, magnetism vanishes. The Curie temperature or Curie point of a substance is unique. The Curie points for Fe, Co, and Ni are 1043, 1400, and 630 K respectively.
Ferromagnetism are due to the presence of magnetic domains in the substance, and when these domains line up parallel to each other, they give a net magnetic field. If the domains line up antiparallel to each other at the Curie point, the substance is said to be antiferromagnetic. The magnetic susceptibility reaches a maximum at Curie temperature for antferromagnetic material. For example, FeO, MnO, CoO, NiO, FeF2, FeCl2, a-Mn, Cr2O3 etc. are some of the antiferromagnetic substances.
Ferromagnetic substances play important roles in recording tapes and disks for audio, video, and computer signals. Furthermore, ferromagnetic materials are used in permanent magnets, which are used in motors, antenna, and speakers. Recent development in strong magnets enables communication equipment and computers to be miniaturized.
|Refractive index of |
some common substances
Difference in refractive indexes of lights of different wavelengths can be separated using a prism. Refractive indexes for some familiar substances are given in a box. It should also be kept in mind that index of refraction changes with dissolved substance and concentration.
Washing and cleaning also involve solubility, and the formulation of an effective cleaning agent requires the knowledge of many substances. Substances can be classified according to polarity. Water, ammonia (NH3), and methanol (CH3OH) are polar, because their molecules have negative and positive sites, whereas methane (CH4), gasoline, and motor oil are non-polar. Regarding solubility, a rule of thumb reads like dissolves like, which means that polar solvents dissolve polar substances and non-polar solvents dissolve non-polar substances. An organic compound with a polar group attached to non-polar chain will bring water molecules to a non-polar surface, and hence it can be used as a detergent or wetting agent. This rule of thumb has potential for both domestic and industrial applications.
Surface tension results from intermolecular attraction, the higher of which, the higher surface tension. Energy required to stretch the surface by some defined unit is called surface tension, and whose unit is N.m/m2 (= N/m). Like viscosity, surface tension decrease with increase temperature. Surface tension causes the dew and raindrops to be round, and the idea to manufacture perfect spheres in zero gravity zone is making use of surface tension. Soap reduces surface tension of water and soapy water forms bubbles when air is blown into it.
Describe an application of a material based on any one of the physical properties.
Point out two physical properties that has not been mentioned here.
What substance has the highest dielectric constant?
What is a beam of polarized light?
Give a sketch of the molecular structure of an aminoacid.