Video Transcript
Scientists mix 100 milliliters of hexane with 100 milliliters of water. They notice that hexane and water are not miscible. Why is this the case? (A) Because hexane molecules are heavy and water molecules are light. (B) Because hexane molecules and water molecules are both polar. (C) Because hexane molecules and water molecules are both nonpolar. (D) Because hexane molecules are nonpolar and water molecules are polar. (E) Because hexane molecules are large and water molecules are small.
In this question, we must explain why hexane and water are not miscible. Miscible means forming a homogeneous mixture when added together. If two liquids are miscible, we say they dissolve in each other. If we look at the diagram, we can see that the hexane and water have formed two
layers in the conical flask. This tells us that the two liquids are not miscible. They are immiscible.
Again, since miscibility is a word that describes two substances that can fully
dissolve in each other, from the diagram, we can see that this is not the case with
hexane and water. The relative polarity of substances determines whether they can dissolve in each
other or not. To understand what is meant by polarity, let us first define a term called
electronegativity. Electronegativity is the tendency of an atom to attract or pull shared electrons, or
electron density, in a bond towards itself. Elements are assigned unitless electronegativity values that indicate the relative
strength with which an atom of an element attracts bonding electrons. We will come back to how electronegativity influences polarity in a moment.
We can describe substances as being either polar or nonpolar. Polar substances are made of polar molecules, which have an asymmetrical or uneven
distribution of electron density. This asymmetrical distribution is caused by the presence of polar bonds within the
molecule, as well as the shape of the molecule. Polar bonds exist within certain molecules due to the difference in electronegativity
between bonded atoms. Nonpolar substances are composed of molecules, which have an even distribution of
electron density.
Let’s draw the structures of the two substances in this question to understand this
further. Water is a bent molecule. And it is composed of one highly electronegative oxygen atom whose electronegativity
value is 3.44 and two relatively less electronegative hydrogen atoms whose values
are 2.20. The electron density of the water molecule is asymmetrical and sits closer to the
highly electronegative oxygen atom since it has a higher electronegativity. This makes the oxygen end of the molecule slightly negatively charged, or 𝛿
negative. And the other end, or we could say pole, of the molecule, which contains the two
hydrogen atoms, is slightly positively charged, or 𝛿 positive.
So we can see that there are two poles or ends to this bent molecule, which are
different in their electron density. And hence, water is a polar molecule.
Hexane, on the other hand, is a hydrocarbon molecule containing only hydrogen and
carbon, with six carbons in a chain. Although carbon has a higher electronegativity value than hydrogen, 2.55 versus 2.20,
making individual bonds slightly polar, the entire structure of the hexane molecule
is nonpolar. Why is this? As a result of this chain structure, neither end of the molecule has a higher
electron density than the other. Electrons are evenly distributed across the hexane molecule. And so hexane is nonpolar. It is because of this difference in polarity that hexane and water do not dissolve in
each other.
Water will not mix with nonpolar substances, as the interactions between water
molecules are stronger than the interactions which would exist between water and
nonpolar molecules. This means it is energetically unfavorable for the two substances to mix. And hence, they are not miscible. So we can conclude that answer (D), because hexane molecules are nonpolar and water
molecules are polar, is the correct explanation as to why water and hexane are not
miscible.
Whether molecules are heavy or light does not determine whether they are miscible or
not, so answer (A) is not the explanation. Answer (B) states that both hexane and water are polar. This is not true. And answer (C) states that hexane and water are both nonpolar. This is also not true. Lastly, the sizes of molecules, whether large or small, does not necessarily
influence whether they will be miscible or not. So answer (E) is also an incorrect statement.
We have now verified that only statement (D) explains correctly why hexane and water
are immiscible. It is because hexane molecules are nonpolar and water molecules are polar.
