Animated Solution for Chemistry - Chemical Bonding and Molecular Structure: Consider the following compounds in the liquid form : ${\text{O}}_2$, $\text{HF}$, ${\text{H}}_2\text{O}$, ${\text{NH}}_3$, ${\text{H}}_2{\text{O}}_2$, ${\text{CCl}}_4$, ${\text{CHCl}}_3$, ${\text{C}}_6{\text{H}}_6$, ${\text{C}}_6{\text{H}}_5\text{Cl}$. When a charged comb is brought near their flowing stream, how many of them show deflection as per the following figure?

Why Liquids Bend to a Comb's Will

Imagine you are standing in a lab, holding a statically charged comb near a thin, flowing stream of liquid. Suddenly, like magic, the stream bends towards the comb! This isn't magic; it's the beautiful interplay of macroscopic physics and microscopic molecular geometry. But why do some liquids bend while others fall straight down? The secret lies in the dipole moment.

When a molecule has a permanent dipole moment ($\mu eq 0$), it acts like a tiny magnet. The charged comb creates a non-uniform electric field. These tiny molecular 'magnets' align themselves with the field and experience a net attractive force towards the comb. If the molecule is non-polar ($\mu =0$), it ignores the comb and falls straight down. Our mission is to identify the polar molecules among the given suspects.

Let's first identify the molecules that will fall straight down. These are the non-polar molecules where symmetry reigns supreme.

Oxygen (${\text{O}}_2$) is a homonuclear diatomic molecule. Both oxygen atoms pull on the shared electrons with equal strength. There is no charge separation, meaning $\mu =0$.

Carbon tetrachloride (${\text{CCl}}_4$) features polar C-Cl bonds, but the molecule is perfectly symmetrical in a tetrahedral geometry. The four bond dipoles pull equally in opposite directions in 3D space, perfectly cancelling each other out. Thus, $\mu =0$.

Benzene (${\text{C}}_6{\text{H}}_6$) is a perfectly symmetrical planar hexagon. All the C-H bond dipoles cancel out across the center of the ring, resulting in a net dipole moment of zero.

Now, let's look at the molecules that will bend towards the comb. These are the polar molecules where symmetry is broken.

Hydrogen fluoride ($\text{HF}$) is a heteronuclear diatomic molecule. Fluorine is highly electronegative and hogs the electron cloud, creating a strong permanent dipole ($\mu eq 0$).

Water (${\text{H}}_2\text{O}$) and Ammonia (${\text{NH}}_3$) both have lone pairs on their central atoms. Water has a bent shape, and ammonia has a trigonal pyramidal shape. Because of these geometries, the individual bond dipoles do not cancel out. Both are highly polar.

Hydrogen peroxide (${\text{H}}_2{\text{O}}_2$) has a unique "open-book" structure. The two O-H bonds are not in the same plane, preventing the bond dipoles from cancelling. It is polar.

Chloroform (${\text{CHCl}}_3$) is tetrahedral like ${\text{CCl}}_4$, but one of the atoms is hydrogen. This breaks the perfect symmetry, meaning the dipoles no longer cancel. It is polar.

Chlorobenzene (${\text{C}}_6{\text{H}}_5\text{Cl}$) takes the symmetrical benzene ring and replaces one hydrogen with a chlorine atom. This introduces a strong C-Cl bond dipole that has nothing to cancel it out on the opposite side, making the molecule polar.

Counting our polar deflectors, we have: $\text{HF}$, ${\text{H}}_2\text{O}$, ${\text{NH}}_3$, ${\text{H}}_2{\text{O}}_2$, ${\text{CHCl}}_3$, and ${\text{C}}_6{\text{H}}_5\text{Cl}$.

That gives us a total of 6 molecules that will show deflection. This problem is a fantastic reminder of how the invisible, microscopic shapes of molecules dictate the macroscopic behaviors we can see with our own eyes!