Which one of the following statements about water is not true?
- (a)Hydrogen bonds are present in liquid water.
- (b)Water has a high boiling point.
- (c)Water has a high heat of fusion.
- (d)Water is a non-polar molecule.
Correct — D. Water is a polar molecule, not a non-polar one: its bent shape and the greater electronegativity of oxygen leave the oxygen end slightly negative and the hydrogen end slightly positive, giving a net dipole. That polarity is exactly what allows the hydrogen bonding responsible for water's high boiling point and high heat of fusion. So 'water is a non-polar molecule' is the untrue statement; the other three are correct.
- (a)Hydrogen bonds are present in liquid water. — True — liquid water is held together by an extensive network of hydrogen bonds between its polar molecules.
- (b)Water has a high boiling point. — True — hydrogen bonding gives water an unusually high boiling point (100 °C) for so light a molecule.
- (c)Water has a high heat of fusion. — True — water has a high latent heat of fusion (about 334 J per gram), again a consequence of hydrogen bonding.
Water is a bent, polar molecule: oxygen draws the bonding electrons toward itself, creating partial charges and a permanent dipole. These dipoles form hydrogen bonds between molecules, and that bonding explains water's anomalously high boiling point, high specific heat and high heat of fusion.
Three of the options are consequences of hydrogen bonding, which itself depends on water being polar. The odd one out denies that polarity, contradicting the very property that makes the other statements true.
- Water is a polar molecule — bent shape, oxygen slightly negative, hydrogens slightly positive.
- Polar water molecules form hydrogen bonds with one another.
- Hydrogen bonding gives water a high boiling point (100 °C) for its small molar mass.
- Water has a high heat of fusion (~334 J/g) and a high specific heat.
Water's polarity drives its hydrogen bonding, which in turn explains its high boiling point and heat of fusion — so 'non-polar' is the untrue statement.
- Calling water non-polar because the molecule as a whole is electrically neutral — it still has a permanent dipole.
- Overlooking that water's high boiling point and heat of fusion come from hydrogen bonding.
Asked as 'which statement about water is not true' — the false option typically denies water's polarity or its hydrogen bonding.
Assertion (A): To dilute sulphuric acid, acid is added to water and not water to acid. Reason (R): Specific heat of water is quite large.
- (a) Both A and R are true, and R is the correct explanation of A
- (b) Both A and R are true, but R is not a correct explanation of A
- (c) A is true, but R is false
- (d) A is false, but R is true
Answer(a) Both A and R are true, and R is the correct explanation of A
Same subject — water's anomalous thermal properties, which spring from its polarity and hydrogen bonding. UPSC uses water's high specific heat to absorb the heat of dilution safely; this NDA item tests that same polarity/hydrogen-bonding basis of water's behaviour.
- practice — not a real PYQ
The unusually high boiling point of water is mainly due to
- (a)its low molar mass
- (b)hydrogen bonding between molecules
- (c)its non-polar nature
- (d)covalent bonds within the molecule
Answer(b) hydrogen bonding between molecules — it takes extra energy to break these bonds.
- practice — not a real PYQ
The water molecule is
- (a)linear and non-polar
- (b)bent and polar
- (c)bent and non-polar
- (d)linear and polar
Answer(b) bent and polar — its bent shape gives it a net dipole moment.