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Shrinking
Liquid Experiment
 Masses
are additive but what about volumes? At first it seems reasonable
but mixing equal volumes of water and ethyl alcohol will result
in less than the total volume of the individual components.
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Chemical
Concepts:
•
Intermolecular forces
•
Polar molecules
•
Hydrogen bonding
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Materials:
- 50
mL, Ethyl alcohol, C2H5OH,
95% anhydrous
- 50
mL, distilled or deionized water
- 100
mL buret
- 100
mL beaker
- Green
food coloring
- Stirring
rod
- Alternately;
two 50 mL graduated cylinders and one 100 mL graduated cylinder
Caution:
This activity requires the use of hazardous substances
and has the potential for hazardous reactions. Carefully review
the Safety Precautions section and relevant Materials Safety Data
Sheets.
Procedure:
- Add about 60 mL of distilled or deionized water to the 100
mL beaker and stir in three drops of green food color.
- With the stopcock closed, pour the colored water into the
buret up to the 50 mL mark.
- Discard the remaining water and add about 60 mL of ethyl alcohol
to the 100 mL beaker.
- With the buret held at approximately a 45o angle;
slowly and carefully pour enough ethyl alcohol into the buret
to bring the combined volume up to the top graduation mark.
If this is done carefully, the water and alcohol will hardly
mix. The more dense water will remain at the bottom and the
less dense alcohol will float on top. If there is a small amount
of mixing at the boundary between the liquids, the demonstration
will not be effective. The green coloring in the water will
help to reveal when this step is done properly.
- Have students observe the total combined volume of the two
liquids, before mixing, bring the level up to the top mark.
- Sealing the top of the buret with your finger (or stopper)
invert the buret slowly allowing the liquids to mix. Invert
the buret several times to allow the two liquids to thoroughly
mix and the heat of solution to dissipate. The air bubble will
assist in the mixing process.
- Observe that the final volume of the liquid has fallen several
milliliters below the original level.
Alternate
Procedure:
If no buret is available the demonstration could be done using
two 50 mL graduated cylinders and one 100 mL graduated cylinder.
- Measure out, very carefully, 50 mL of distilled or deionized
water in one 50 mL graduated cylinder.
- With equal care, measure 50 mL of ethyl alcohol into the other
50 mL graduated cylinder.
- Pour both liquids into the 100 mL graduated cylinder and stir
with a stirring rod until all bubbles have come to the top.
- Observe that the final volume in the cylinder is less than
100 mL.
Discussion:
When 50 mL of water is added to 50 mL of water or when 50 mL of
alcohol is added to 50 mL of alcohol, the final volume will always
be 100 mL. In this demonstration, when the alcohol is added to
the water, intermolecular forces cause the two different molecules
to pack closer together. This is believed to be due to strong
hydrogen bonding.
Hydrogen bonding is an especially strong example of dipole-dipole
interaction. A dipole-dipole interaction is the attraction of
the positive end of one polar molecule for the negative end of
another polar molecule. In hydrogen bonding, a hydrogen atom serves
as a bridge between two highly electronegative atoms (nitrogen,
oxygen, or fluorine).
Safety
Precautions:
Ethyl alcohol is a dangerous fire risk; it is flammable. The addition
of denaturant (usually methyl alcohol) makes ethyl alcohol poisonous
by ingestion. Wear chemical splash goggles, chemical-resistant
gloves and chemical-resistant apron.
Disposal:
Consult a reliable reference manual for appropriate disposal methods.
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