Animated Solution for Chemistry - s and p-Block Elements: Which one of the following statements about water is false?

This question is a beautiful journey through the fundamental properties of one of the most important molecules in the universe: water ($H_{2}O$). It tests your conceptual clarity across multiple domains of chemistry, from acid-base theories to intermolecular forces and redox reactions. Let's break down each statement to uncover the imposter.

Water as an Acid and a Base

Statement (a) claims that water can act as both an acid and a base. Is this true? Absolutely. According to the Brønsted-Lowry theory, an acid is a proton ($H^{+}$) donor, and a base is a proton acceptor. Water is uniquely equipped to play both roles, a property known as being amphoteric.

When water encounters a strong acid like hydrochloric acid ($HCl$), it acts as a base, accepting a proton to form the hydronium ion:

Conversely, when water reacts with a base like ammonia ($N{H}_3$), it acts as an acid, donating a proton to form the hydroxide ion:

Thus, statement (a) is a perfectly true fact about water.

Heavy Water Ice

Statement (c) takes us into the realm of isotopes. Normal water is made of protium (${}^{1}H$), the lightest isotope of hydrogen. Heavy water, however, is made of deuterium (${}^{2}H$ or $D$), which contains a neutron in its nucleus, making it twice as massive as protium. The chemical formula for heavy water is $D_{2}O$.

We all know that normal ice floats on normal liquid water because its open, hydrogen-bonded cage structure makes the solid phase less dense than the liquid phase. However, because deuterium is significantly heavier, the density of $D_{2}O$ ice is approximately $1.1{\text{ g/cm}}^3$.

Since the density of normal liquid $H_{2}O$ is about $1.0{\text{ g/cm}}^3$, the heavy water ice is denser than the surrounding liquid. Therefore, heavy water ice will sink in normal water. Statement (c) is true.

Water in Photosynthesis

Statement (d) states that water is oxidized to oxygen during photosynthesis. Let's look at the overall chemical equation for this life-sustaining process:

To determine if water is oxidized, we must look at the oxidation states. In a water molecule ($H_{2}O$), the oxygen atom has an oxidation state of $-2$. On the product side, the oxygen gas ($O_2$) is in its elemental form, meaning its oxidation state is $0$.

An increase in oxidation state (from $-2$ to $0$) corresponds to a loss of electrons, which is the very definition of oxidation. Therefore, water is indeed oxidized to oxygen gas during photosynthesis. Statement (d) is true.

Inter vs. Intra

Finally, we arrive at statement (b), which claims there is extensive intramolecular hydrogen bonding in the condensed phase of water. This is where the trap lies.

In chemistry, prefixes matter immensely: Intra- means within. Intramolecular forces are the forces that hold a single molecule together. In water, the bonds between the central oxygen atom and the two hydrogen atoms are strong covalent bonds, not hydrogen bonds. Inter- means between. Intermolecular forces are the attractions between different molecules.

The unique properties of water—its high boiling point, high surface tension, and high specific heat—are entirely due to the extensive network of hydrogen bonds that form between different water molecules. The slightly positive hydrogen atom of one water molecule is electrostatically attracted to the lone pairs on the slightly negative oxygen atom of a neighboring water molecule.

Because these bonds occur between separate molecules, they are strictly intermolecular hydrogen bonds. Water does not possess the geometry or the available atoms to form a hydrogen bond within its own single molecule.

Therefore, statement (b) is the false statement, making it the correct answer to our question. Always read carefully—a single prefix can change the entire physical reality of a molecule!