Physical Measurements Page 1 of 4 Like Dissolves Like Introduction

Physical Measurements Page 1 of 4 Like Dissolves Like Introduction

Physical Measurements
“LIKE DISSOLVES LIKE” - AN INTRODUCTION TO SOLUBILITY
Introduction:
You probably have heard the phrase “birds of a feather flock together.” This principle is applied in
chemistry as well. A compound is more likely to dissolve (or mix completely) into another compound if it is “like”
that compound. In chemistry, we compare the polarities of the different molecules and their bonds to see if they are
like one another and will therefore dissolve in each other. Polar bonds occur when the electronegativities of the two
atoms in a chemical bond are different, producing a partial positive charge on one atom and a partial negative
charge on another atom. Just as the positive and negative poles of magnets are attracted to each other, the partial
positive and negative charges on adjoining molecules are attracted to each other, causing the molecules are attracted
to each other and dissolve (i.e. “polar substances dissolve polar substances”). If you have a compound with no
polar bonds (or very weakly polar bonds), the substance is said to be nonpolar. Nonpolar compounds are soluble in
other nonpolar compounds (i.e. “nonpolar substances dissolve in nonpolar substances”). Because nonpolar
compounds do not have partial positive and negative charges, however, nonpolar substances typically are not
attracted to polar substances (i.e. nonpolar substances are not soluble in polar substances).
Goals:
Safety:
•
•
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Explore the solubility of various organic substances.
Use their observations to make predictions of the polarities of the molecules.
Visualize the intermolecular forces at work in the molecules using diagrams.
•
•
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Wear eye protection at all times in the laboratory.
If you get a chemical on yourself, wash it off immediately with soap and water.
Do not dispose of the solvents in the trash. Pour into the approved waste container.
Prelab:
1. Draw a sketch of a water molecule. Which is more electronegative, oxygen or hydrogen? Label the partial
positive and negative charges in the molecule using the symbols “ + ” and “–“.
2.
Add a second water molecule, showing the intermolecular forces between the molecules using dotted lines.
3.
Now, draw a sketch of cyclohexane, C6H12. Given the relative electronegativities of carbon and hydrogen and
the arrangement of the C and H atoms, do you expect cyclohexane to be more polar or less polar than water?
Why?
4.
Using available resources, draw the structures of ethanol and glycerol below and circle the polar bonds.
5.
Predict the relative polarities of water, cyclohexane, ethanol, and glycerol.
6.
Define miscible and immiscible. How do these terms relate to solubility?
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Like Dissolves Like
Physical Measurements
Procedure and Observations
Part I
For this experiment, will mix the solvents (water, cyclohexane, ethanol, and glycerol) in every combination - you
should be able to do this with 6 test tubes. List what you will combine in each test tube in your data table in the
procedure section.
1.
Obtain a rack with ten test tubes. Label the test tubes 1-10.
2.
Referring to your data table, add about 1 mL (about 1 cm high) of each of the two listed solvents to each of the
first 6 numbered test tubes. Stir the contents thoroughly with your glass rod. Then stir them more. Record
whether the two solvents in each test tube appeared to be miscible, slightly soluble, or completely immiscible in
Data Table I.
Test Tube #
Solvent 1
Data Table I
Solvent 2
Observation after stirring
1.
2.
3.
4.
5.
6.
Part II
1. Following the procedure in step 2, test the solubility of acetone with water and cyclohexane. Record your
results as the data for test tubes 7-8 in Data Table II.
Data Table II
Test Tube #
Solvent 1
Solvent 2
Observation after stirring
1
2
2.
Pour contents of all test tubes into the approved waste container.
Postlab Questions:
1. Cyclohexane and water represent the extremes of nonpolar and polar solvents. Based on your observations,
order the remaining solvents from least polar to most polar. Justify your conclusions.
2.
least polar
most polar
cyclohexane
water
How does your least polar to most polar ranking in prelab question 5 compare with the experimental order
above?
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Like Dissolves Like
Physical Measurements
3.
Using available resources, draw the structure of acetone.
4.
Acetone is sometimes referred to as the “Universal Solvent”.
experimental results to justify your answer.
Is this an appropriate term?
Use your
VISCOSITY
Goals: The students will
1. explore the relative viscosity of three substances
2. visualize the intermolecular forces at work in the molecules using diagrams.
Introduction:
The liquids you use daily can be described as "thick" or "runny" (syrup, water, shampoo, etc). Scientists use
the term viscosity to describe thickness, rather than the terms "thick" or "runny". For a substance to flow, individual
molecules need to be able to glide past each other. Substances that "resist flow" are described as being viscous.
At a molecular level, a liquid’s viscosity is primarily determined by the strength of polar attractions between
molecules and the ability for the molecules to get tangled. Substances with strong polar interactions resist flow
more than substances with weak polar interactions. This is like sliding apart two strong magnets vs. sliding apart
two weak magnets. Similarly, it is harder to separate a tangled mess. This can be visualized by comparing
macaroni and spaghetti. Both foods have the same composition (pasta), but macaroni is a lot easier to "pour" than
spaghetti because the spaghetti forms one big knot. Similarly, molecules can have the same composition (many
substances, called hydrocarbons, have only C and H, so their polarities are similar), but small round hydrocarbons
(turpentine) don't get nearly as tangled as the shape of long hydrocarbon chains (many waxes and oils).
Scientists commonly determine a liquid’s viscosity by measuring the time it takes for the substance to flow a
certain distance through a long narrow tube. Another way to measure the viscosity is to measure the time it takes for
a dense object to travel a certain distance through the liquid. In this activity, you will use a similar method to rank
the relative viscosity of three different liquids.
Prelab
1. In what context(s) have you heard the terms viscosity or viscous.
2.
In this lab, you will be studying the viscosity of water, ethylene glycol, and glycerol. Using available
resources, draw the structures of these compounds below and circle the polar bonds.
3.
Hypothesize why these compounds might have different viscosities.
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Like Dissolves Like
Physical Measurements
Safety:
Wear eye protection at all times in the laboratory.
!
If you get a chemical on yourself, wash it off immediately with soap and water.
!
Do not dispose of the solvents in the trash. Pour into the approved waste container.
!
Procedure and Observations:
You will be studying the relative viscosities of water, ethylene glycol, and glycerol. Using the correctly labeled
pipets (and not your fingers), evaluate the relative textures of the three liquids in their bottles. Record the textures
in the table below. Cap the bottle when you are done.
Get the long tubes filled with each of the three liquids. Holding the stoppers in place, simultaneously turn the tubes
upside down and describe the relative speeds at which the ball bearings travel in the table below. Repeat if
necessary.
Compound
Texture (step 1)
Ball bearing speed (step 2)
Postlab Questions:
1. What does viscosity mean? Order the three solvents (water, ethylene glycol, and glycerol) from least viscous to
most viscous.
2.
How do your experimental rankings of viscosities compare with your predictions in the prelab?
3.
Referring back to your structures in the prelab, justify your experimental ranking of viscosities.
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