Consider the following compounds in the liquid form:
$$O_2, HF, H_2O, NH_3, H_2O_2, CCl_4, CHCl_3, C_6H_6, C_6H_5Cl$$.
When a charged comb is brought near their flowing stream, how many of them show deflection as per the following figure?

A thin jet of a liquid is deflected by a charged comb when the molecules of the liquid possess a permanent dipole moment. The electric field of the comb orients these dipoles, producing an attractive force that bends the stream. Non-polar molecules (zero permanent dipole) can only be polarised inductively; the induced dipole is too weak to give a visible deflection, so such liquids remain undeflected.

Therefore, we examine each liquid for the presence or absence of a permanent dipole.

Case 1: $$O_2$$ - linear and centrosymmetric, so the vector sum of bond moments is zero ⇒ no permanent dipole ⇒ No deflection.

Case 2: $$HF$$ - the H-F bond is polar and the molecule is heteronuclear (no cancellation) ⇒ permanent dipole present ⇒ Deflection.

Case 3: $$H_2O$$ - bent structure (bond angle ≈ $$104.5^{\circ}$$); the two O-H bond moments add vectorially ⇒ $$\mu \neq 0$$ ⇒ Deflection.

Case 4: $$NH_3$$ - trigonal-pyramidal; bond moments do not cancel (lone pair at the apex) ⇒ permanent dipole present ⇒ Deflection.

Case 5: $$H_2O_2$$ - $$HO-OH$$ skeleton is non-planar and unsymmetrical; the individual O-H bond moments give a net $$\mu \approx 2.1\,$$D ⇒ Deflection.

Case 6: $$CCl_4$$ - perfect tetrahedral symmetry; the four C-Cl bond moments cancel pairwise ⇒ $$\mu = 0$$ ⇒ No deflection.

Case 7: $$CHCl_3$$ (chloroform) - tetrahedral but one H is different from three Cl atoms, so cancellation is incomplete; net $$\mu \approx 1.1\,$$D ⇒ Deflection.

Case 8: $$C_6H_6$$ (benzene) - planar hexagon with equal C-C bonds and a fully delocalised $$\pi$$ cloud; overall $$\mu = 0$$ ⇒ No deflection.

Case 9: $$C_6H_5Cl$$ (chlorobenzene) - one Cl substituent destroys symmetry; the C-Cl bond imparts a net $$\mu \approx 1.7\,$$D ⇒ Deflection.

Counting all liquids that show deflection:

$$HF,\; H_2O,\; NH_3,\; H_2O_2,\; CHCl_3,\; C_6H_5Cl \; \Longrightarrow \; 6 \text{ liquids}$$

Hence, the number of liquids whose streams are deflected by a charged comb is 6.