Phase Change - Andrew Kline
Phase Change Module - ENGR 101
Notes for the Instructor
Lab 1: Demonstration: foam insulation in a can sprayed into a box. Shows how an
aerosol propellant moves a liquid which transitions to a solid as cross-linking occurs and
the solvent evaporates.
Brief safety orientation on use of materials. Student groups follow Activity Sheet and
perform experiments. As students perform experiments, they should be asked to think
about variations in experiments (different than the Activity Sheet) that could be
performed with the given equipment.
Lecture 1: Definitions of physical properties and related materials.
Assign to students: Each group will design two experiments using the same equipment as
the Lab 1 activities that can illustrate or be linked with the definitions and terms from
Lecture 1. Students should focus on what data to collect, how to analyze it, what
definitions or theories it is linked to. Data collected should allow students to form a
hypothesis, and then prove or disprove the hypothesis. Multiple data points need to be
collected so students are able to build a figure or table to explore their hypothesis. Each
student group submits a written memo at the start of Lecture 2 detailing their experiments
(100 points value).
Lecture 2: Power Point presentation by student groups on their 2 experiments.
Presenter randomly selected by instructor. Constructive input from instructor and peers.
Finish definitions lecture, as needed. Other discussions to prepare for Lab 2.
Lab 2: Student groups perform their experiments and collect data. Written report to be
submitted per guidelines on what to include in a scientific report. Report is graded with
300 points of value towards semester grade (Total semester points: 2000).
Lecture 1 Notes
General Physical Properties and Terminology
Melting point: transition temperature from a solid to a liquid
Boiling point: transition temperature from a liquid to a vapor
Freezing point: transition temperature from liquid to solid (opposite of melting point)
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Phase Change - Andrew Kline
Sublimation point: temperature and pressure where material converts from a solid
directly to a vapor, without going through the liquid state
Motion of molecules in a solid: immobile or frozen
Motion of molecules in a liquid: fluid and translational
Motion of molecules in a vapor: higher energy, much more rapid
Diffusion: translational motion of molecules in liquid or vapor states
Enthalpy of fusion: energy required to be input (or removed from the system,
depending on convention), to convert a liquid to a solid
Heat of vaporization: energy required to be input (or removed from the system,
depending on convention), to convert a liquid to a vapor or gas
Shear forces: particles slide past each other when force is applied (think flowable!)
Blunt force: consistent force applied evenly over a larger surface area (e.g. placing your
hand on a table top and pressing down is applying a blunt force).
Stress: σ = force / area
Strain: Є = ∆L / L
no change in translational or rotational component of motion
body not rigid, undergoes deformation (∆L) as force is applied
Surface tension: force per unit length in the plane of the surface. For a gas-liquid
interface, liquid surface molecules are attracted to the bulk liquid, rather than the gas.
For a glass of water viewed from the side, you may notice a curvature of the water at its
top – this is due to surface tension and the fact that the water molecules are being
repulsed by the sidewalls of the glass itself.
Viscosity: “thickness” of a liquid. Strong f(temperature). Molasses is more viscous in
Michigan in January than in July. Materials are generally less viscous as temperature
increases.
Flowable or pourable: particles or molecules slide past each other and have
translational motion.
Corn starch: particles have a flatter, plate-like structure, rather than a round particle
shape. Plate-like structure allows them to be flowable when shear forces are applied, but
they act as a solid when a blunt force is applied.
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Phase Change - Andrew Kline
Polymers
Monomer: small molecular unit
H3C—CH3
Polymer: larger unit or structure composed of many monomer units joined together
Draw funky structures on the board.
Cross-linking: monomer units joining together to form branched compounds, 3deimensional, highly complex. If enough cross-linking occurs, a liquid polymer
transitions to a solid (assuming any solvent is also removed). As amount of cross-linking
increase, viscosity increases, as does the material’s molecular weight.
Phase transition: point at which a polymer moves from liquid behavior to solid
behavior. Also known as the “gel point.” Think of it as when Jell-O in your refrigerator
goes from a liquid to a solid.
Rheological measurements: experimental technique to determine viscosity, phase
transitions, and gel points of materials, including polymers.
Reactions
Chemical Reaction: chemical change of your starting materials, or create a new
chemical species or product.
Physical Reaction: physical change, materials remain the same chemically on a
molecular level.
Acid / Base Chemistry
Sodium bicarbonate (baking soda): NaHCO3(s)
2 NaHCO3(s) Æ
Na2CO3(s) + CO2(g) + H2O(g)
Heating or baking sodium bicarbonate causes it to form a solid, and two gases, carbon
dioxide and water. A similar reaction can occur by mixing with an acid. Baking soda
itself is a base.
Acetic acid (vinegar): C2H4O2
H3C—C==O
|
O-H
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Phase Change - Andrew Kline
Carboxyl group:
NaHCO3(s)
R—C==O
|
O-H
O-H
|
+ H3C-C==O
Æ
H2CO3 (carbonic acid)
Carbonic acid decomposes to form CO2 and H2O.
The carboxyl group in acetic acid gives off (releases) its H ion (the H single-bonded to
the O in the O-H group single bonded to the C==O), which attacks the baking soda, and
cleaves off the carbonate group (HCO3-) and bonds to form carbonic acid. As the
carbonic acid decomposes, carbon dioxide and water are released to inflate the balloon.
The double bond in the carboxyl group draws (or tries to pull) electrons from the C-O-H
bond in the carboxyl group. As a result, the O-H bond is placed under stress, and is
weakened, which allows the H ion to cleave off more easily, and be able to react with the
baking soda. Therefore, O-H groups that are part of a carboxyl group will more easily
give up the H ion than an O-H group attached to a simple C molecule, such as in citric
acid.
Kinetics: the more hydrogen ion available to cleave the baking soda, the faster the CO2
and H2O are produced, and the faster the balloon inflates.
Baking soda is a base.
Baking powder is baking soda, plus an acid, and corn starch. When water is added, the
solid is dissolved, the base and acid mix and generate CO2 and H2O.
Alka-seltzer or denture cleaner: composed of baking soda and citric acid. When placed
in a vinegar solvent, does not work well to blow up the balloon, as the large amount of
acid does not release the base needed (by dissolving in water), and very little gas is
evolved. Need water to mix with alka-seltzer or denture cleaner in order to release the
base so it has a chance to react with the acid.
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Phase Change - Andrew Kline
Citric acid is lemon juice C6O7H8
Structure of citric acid:
O-H
|
C==O
|
H | H
O==C---C---C---C---C==O
| H | H
|
|
O-H
|
O-H
O-H
Citric acid is a weaker acid than acetic acid, thus slower reaction and less gas evolved
when mixed with baking soda. Also potential problems with steric hindrance (bond
angles) when H ion is cleaving off the carboxyl group, thus having less (fewer)
potentially free H ions in solution to react with baking soda.
Vinegar: 5% acetic acid
Concentration of citric acid in lemon juice was unknown. If lower than that of vinegar, it
would also result in a slower reaction and less gas evolved when mixed with baking soda.
Other Terms
Saturated solution: solid materials sitting in the bottom of a container of liquid, and
does not dissolve into the liquid when agitated or stirred.
Solubility: measure of the amount of solid (“the solute”) that can be dissolved in a liquid
solvent. Solubility generally increases as the temperature of the solution increases.
Crystallization: recovery of the solid from a solution, usually by cooling the solution
and filtering it, or removing all the solvent by drying, heating, or evaporation.
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Phase Change - Andrew Kline
Elements of a Scientific Report
Introduction, Purpose, or Objectives
Background
Methodology Used
Findings (includes data tables or figures)
Results and Discussion of Findings
Conclusions drawn from report
Recommendations for further work, additional modifications, etc.
References
Sample Calculations
Goal of scientific report: Focus on a clear purpose; solve a problem and prove or
disprove a hypothesis; and present a clear, concise solution or result from your work.
Other notes: Figures and tables need to have titles (tables at top, figures at bottom is
the usual convention), and a consecutive numbering system throughout your report
(Table 1 comes before Table 2, followed by Table 3). Figures and tables must be called
out in the discussion text and discussed as to what they mean. Tables should be called
out and discussed in the order they are numbered (call out and discuss Table 1, then
Table 2, etc.).
Left justify text to make it easiest to read.
For a memo format report, single space paragraphs, one blank line between paragraphs,
no indent of paragraphs.
Use specific and descriptive subheadings, not generic “Introduction,” etc.
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