Mega Molecules, LLC
Name: ____________________
Date: ____________________
Hour: ____________________
Hands-On Science with Molecular Models
www.megamolecules.com
Molecular Modeling Activity for Solutions
Time required: one 50-minute period
Introduction
Imagine a tall glass of Kool-Aid on a warm summer day.
Kool-Aid is made by the process of dissolving. Water and Kool-Aid
powder were mixed to make the solution. We define a solution as a
homogeneous mixture of a solute, the Kool-Aid powder, in a
solvent like water. In this model-based activity, you will learn the
main concepts behind the process of dissolving.
Materials
The Solutions Kit
4 test tubes
water
sodium chloride (table salt)
ethanol
hexane
Procedure
Both solid-liquid solutions and liquid-liquid solutions are important in chemistry. A solid that
dissolves in a solvent is soluble while a solid that does not dissolve is said to be insoluble.
When the solvent and solute are both liquids, we say that the two liquids are miscible. If
the liquids do not dissolve in each other, they are said to be immiscible. Use the terms:
soluble, insoluble, miscible and immiscible to describe your observations.
1. Measure 20 drops of water into a clean test tube. Add 10-20 crystals of sodium
chloride to the water. Stopper and shake the tube. Record your observations
(soluble or insoluble) in Data Table 1.
2. Measure 20 drops of water into a clean test tube. Add 10 drops of ethanol to the
water. Stopper and shake the tube. Record your observations (miscible or immiscible)
in Data Table 1.
3. Measure 20 drops of water into a clean test tube. Add 10 drops of hexane to the
water. Stopper and shake the tube. Record your observations (miscible or immiscible)
in Data Table 1.
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Data Table 1
Mixture
water and sodium chloride
water and ethanol
water and hexane
Observations
Procedure
"Like dissolves like" is an expression chemists use when discussing the solubility of
substances. The "like dissolves like" rule, in general, means that nonpolar substances
dissolve in nonpolar solvents and polar substances dissolve in polar solvents. By
definition, polar molecules have a partially positive side and a partially negative side, or a
dipole. Water is a polar molecule.
4. Assemble the two molecular models for water, H2O, as shown in the picture below.
White hydrogen
atoms with magnets
The central atom in the water molecule is oxygen. It is
represented by the red atomic model. Oxygen is bonded to two
hydrogen atoms which are represented by white plastic atomic
models with magnetic centers. The oxygen atom has two
unbonded pairs of electrons (also called lone electron pairs).
The lone electron pairs are represented by neodymium magnets.
Water molecules in the liquid and solid states have a tendency to stick together. The
oxygen atom attracts the electrons within the covalent bonds leaving the hydrogen atoms
with a slightly positive charge. The lone electron pairs have a partial negative charge.
Water is a dipole (or polar molecule) because it has partial positive and partial negative
charges separated by a distance.
A hydrogen bond is formed when the slightly positive hydrogen atom of one water molecule
is attracted to a lone electron pair of another water molecule.
How can you form a hydrogen bond between two water molecules? _______________
_____________________________________________________________________
If you have ever played with magnets, you have noticed the attraction between magnets.
As the hydrogen atom of one water molecule approaches the lone electron pairs of another
water molecule, the attraction can be quite strong. Molecules are not magnets, but the
electrical attractions similarly increase as the water molecules approach each other. The
attraction between the molecules is called a hydrogen bond. Hydrogen bonds are much
weaker than covalent bonds; they break and easily form again.
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5. Look at the sodium chloride crystal. The purple model represents the sodium ion while
the yellow model represents the chloride ion. Move the hydrogen end of the water
molecule near to the yellow chloride ion.
Is the partial charge of the hydrogen end of the water molecule positive or negative?
_____________________________________________________________________
Since oppositely charged particles attract, what is the charge of the chloride ion?
_____________________________________________________________________
Work with the models to see how the water
molecules pull the ions out of the crystal.
Table salt (NaCl) is a crystal, held together by strong ionic bonds. An ionic bond is formed
from the electrostatic attraction between oppositely charged ions. In ions, electrons of one
atom are transferred to another atom. The atom that loses the electrons becomes a
positively charged ion, or a cation, while the one that gains electrons becomes a
negatively charged ion, or an anion.
When we try to dissolve an ionic compound by stirring it in water, the positive poles of the
water molecules are attracted to the anions, while the negative poles of other water
molecules are attracted to the cations, so the polar water molecules "pull" the ions out of a
crystal. As a result of these interactions, the ionic bonds eventually break and ions are
released into the water. When the salt is dissolved, every ion is surrounded by water
magnet
molecules. This process is called "hydration."
6. Assemble two molecular models for ethanol, C2H5OH. The
hydrogen atom that is attached to the red oxygen atom is polar and
has a magnet. The other five hydrogren atoms are nonpolar and do
not have magnets.
Look at your models for water and ethanol.
How are they alike? ____________________________________________________
How are they different? __________________________________________________
Will the two ethanol molecules attract? _____________________________________
Will the ethanol and water models attract? ___________________________________
Ethanol has a molecular structure that includes the polar OH group. The OH atoms on
the ethanol molecule are linked with a polar covalent bond. The shared pair of electrons in
the covalent bond gets "shifted" towards the oxygen atom, giving the oxygen a localized
negative charge. The hydrogen atom from which an electron was shifted, becomes
positively charged. Compared to ions, polar molecules carry much weaker localized
charges. The ethanol molecule shows an intermediate polarity because it has both a polar
OH group and a nonpolar CH3CH2 section.
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7. Build the two molecular models for hexane, C6H14. All
of the hydrogen atoms are nonpolar and therefore do not
have magnets. It may seem like there is no attraction
between the hexane molecular models. However, hexane
exists in the condensed state as a liquid at room
temperature. Therefore, attractions between the molecules,
although weak, must exist to cause them to cling together.
Look at your models for water and hexane.
How are they alike? _____________________________________________________
How are they different? __________________________________________________
Quite a different situation occurred when hexane and water were mixed. The layer of
hexane floats on top of the water. Hexane molecules have no OH group and are
nonpolar. Water molecules stick tightly together. Hexane molecules are not strongly
attracted to the water molecules and are immiscible in water. We call the hexane
molecules "hydrophobic" (water-fearing) because polar water molecules, being attracted to
each other, push the hydrophobic molecules away, usually to the surface.
You're the Chemist
Chemists use the "like dissolves like" rule: when two liquid molecules are alike in polarity,
they tend to be miscible and form a solution. The rule allows chemists to use the chemical
composition and molecular structure to predict the likelihood of two substances dissolving
in each other.
8. Look at your models for hexane and ethanol.
How are they alike? ____________________________________________________
How are they different? __________________________________________________
Make a prediction about the miscibility of ethanol and hexane. ___________________
_____________________________________________________________________
Procedure
9. Measure 20 drops of ethanol into a small, clean test tube. Add 10 drops of
hexane to the ethanol. Stopper and shake the tube. Record your observations
(miscible or immiscible) in Data Table 2.
Data Table 2
Mixture
Observations
Ethanol and hexane
Use what you have learned about the attractions between molecules to explain the results
obtained when ethanol and hexane were mixed. _________________________________
________________________________________________________________________
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Teacher's Key
1. Sodium chloride is soluble in water.
2. Water and ethanol are miscible.
3. Water and hexane are immiscible.
4. As the hydrogen atom of one water molecule approaches the lone electron pairs of
another water molecule, the attraction can be quite strong. The attraction between the
molecules is called a hydrogen bond.
5. The partial charge of the hydrogen end of the water molecule is positive.
The chloride ion is negatively charged.
6. The -OH ends are alike.
Ethanol has a CH3CH2
The two ethanol molecules will attract.
The ethanol and water models will attract.
7. Water and hexane molecules both have hydrogen and single bonds.
Answers may vary.
8. Hexane and ethanol are alike because they both have a hydrocarbon section of the
molecule. They are different because ethanol has the OH group.
9. Students should find that ethanol and hexane are miscible. The similarity of the
nonpolar hydrocarbon regions of the ethanol and hexane molecules allows the two
liquids to mix.
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