Problem 10 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which one of the following statements about water is FALSE? [Main 2016] (a) There is extensive intramolecular hydrogen bonding in the condensed (b) Ice formed by heavy water sinks in normal water. (c) Water is oxidized to oxygen during photosynthesis. (d) Water can act both as an acid and as a base.

Step-by-Step Solution

Answer

(a) is false; water has intermolecular, not intramolecular, hydrogen bonding.

Step 1: Evaluate Option (a)

Option (a) states that there is extensive intramolecular hydrogen bonding in the condensed form of water. This is false because water shows extensive *intermolecular* hydrogen bonding, not intramolecular. Intermolecular hydrogen bonding is responsible for its liquid and ice structure properties.

Step 2: Evaluate Option (b)

Option (b) states that ice formed by heavy water sinks in normal water. This is true because heavy water (D2O) ice is denser than regular water, so it sinks instead of floating like ordinary ice.

Step 3: Evaluate Option (c)

Option (c) states that water is oxidized to oxygen during photosynthesis. This is true. During photosynthesis in plants, water molecules are split, releasing oxygen as a by-product.

Step 4: Evaluate Option (d)

Option (d) states that water can act both as an acid and as a base. This is true. Water is amphoteric, meaning it can donate protons (act as an acid) or accept protons (act as a base).

Key Concepts

Hydrogen Bonding

In chemistry, hydrogen bonding plays a crucial role in determining the physical properties of substances. A hydrogen bond is a weak bond between two molecules resulting from an electrostatic attraction. These occur specifically between a proton in one molecule and an electronegative atom in the other, like oxygen or nitrogen.
In water molecules, hydrogen bonds form due to the polar nature of water, where the oxygen atom is more electronegative than the hydrogen atoms. This gives rise to a partially negative charge on oxygen and a partially positive charge on hydrogen.

• Intermolecular hydrogen bonding occurs between different molecules of water, contributing to water's high boiling and melting points compared to similar-sized molecules.
• The unique structure of ice, which is less dense than liquid water, can be attributed to hydrogen bonds. Upon freezing, water molecules form a lattice that holds them further apart, which explains why ice floats.

Understanding hydrogen bonding is essential in the study of the physical properties of substances and how they interact in liquid and solid forms.

Photosynthesis

Photosynthesis is a biological process used by green plants, algae, and some bacteria to convert light energy into chemical energy stored in the form of glucose. During this process, water plays a vital role.
Photosynthesis can be summarized using the equation:\[ 6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2 \]The reaction indicates that carbon dioxide (CO_2) and water (H_2O) are converted into glucose (C_6H_{12}O_6) and oxygen (O_2) in the presence of light energy. Here, water is crucial for providing the electrons needed to form energy-rich molecules.

• Water molecules are split during the light-dependent reactions of photosynthesis, a process called photolysis, releasing oxygen as a by-product.
• Oxygen, released to the atmosphere, is essential for the survival of most living organisms that rely on it for respiration.

Photosynthesis illustrates the importance of water not only as a reactant but also as part of life-supporting systems on Earth.

Amphoteric Nature

The amphoteric nature of a substance refers to its ability to act both as an acid and a base. Water is a classic example of an amphoteric compound.
When water acts as an acid, it donates a proton (H^+) to form hydroxide ions (OH^−). Conversely, when it acts as a base, it accepts a proton to form hydronium ions (H_3O^+).

• This duality is exemplified in the autoionization of water: \[ 2H_2O \rightleftharpoons H_3O^+ + OH^- \]
• Amphoteric behavior is crucial in maintaining the balance of acid and base properties in various chemical reactions, especially in biological systems.

Understanding water's amphoteric nature helps explain its fundamental role in chemical reactions, buffering solutions, and supporting life, as it participates in various acid-base reactions.