The bond which is present between water molecules is

1. Fundamental Chemical Bonding: Ionic and Covalent (basic)

To understand the chemistry of life and the environment, we must first look at why atoms interact at all. In their natural state, most atoms are unstable because their outermost electron shells are incomplete. To achieve stability—often referred to as the 'Octet Rule'—atoms either transfer or share electrons with one another. This interaction creates a chemical bond, the fundamental force that holds matter together.

The first major type is the Ionic Bond. This occurs when one atom (usually a metal) completely gives up one or more electrons to another atom (usually a non-metal). This transfer creates ions: a positively charged cation and a negatively charged anion. Because opposite charges attract, these ions stick together in a rigid lattice. As noted in Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.58, ionic compounds typically have high melting and boiling points because it takes a massive amount of energy to break these strong electrostatic attractions. Furthermore, they conduct electricity when dissolved in water or melted, as the ions are free to move and carry a charge.

The second type is the Covalent Bond. Instead of a tug-of-war where one atom wins the electron, atoms here share pairs of electrons to fill their shells. This is very common in carbon-based compounds and gases like Oxygen (O₂) or Nitrogen (N₂). Unlike ionic compounds, covalent molecules are often held together by relatively weak forces between the molecules themselves. Consequently, they tend to have lower melting and boiling points Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. Because these bonds do not produce free-floating ions, covalent compounds are generally poor conductors of electricity.

To help you visualize the differences for your UPSC prep, here is a quick comparison table:

Feature	Ionic Bonding	Covalent Bonding
Mechanism	Complete transfer of electrons.	Sharing of electron pairs.
Participants	Typically Metal + Non-metal.	Typically Non-metal + Non-metal.
Electrical State	Forms charged ions.	Forms neutral molecules.
Conductivity	High (in solution or molten).	Low/Poor.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.58; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59

2. Electronegativity and Molecular Polarity (basic)

In our journey to understand how matter behaves, we must first look at the "tug-of-war" happening at the atomic level. Atoms bond together to reach a stable state, often mimicking the electron configuration of noble gases, which are naturally stable because their outer shells are full Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46. When atoms share electrons to achieve this stability, they form covalent bonds Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. However, this sharing is not always equal.

Electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself. Think of it as the "strength" of an atom in a tug-of-war. If two identical atoms (like two Hydrogen atoms in H₂) bond, they pull with equal strength, and the electrons sit right in the middle. This is a non-polar covalent bond. But when atoms of different elements bond, one is usually stronger than the other.

When there is a significant difference in electronegativity, we get Molecular Polarity. The more electronegative atom pulls the electron density closer, gaining a partial negative charge (δ-), while the other atom is left with a partial positive charge (δ+). This creates a "dipole"—a molecule with two poles, much like a magnet. A classic example is water (H₂O), where Oxygen is far more electronegative than Hydrogen, making the water molecule highly polar.

Bond Type	Electron Sharing	Example
Non-polar Covalent	Equal sharing; no charges.	H—H, O=O
Polar Covalent	Unequal sharing; partial charges (δ+, δ-).	H—Cl, O—H in H₂O

Understanding polarity is crucial because it dictates how molecules interact with one another. While the bonds within a molecule are strong, the intermolecular forces (the attraction between separate molecules) depend heavily on these partial charges Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. These forces determine whether a substance is a gas, liquid, or solid at room temperature and influence its boiling and melting points.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60

3. Intramolecular vs. Intermolecular Forces (intermediate)

To understand the behavior of matter, we must distinguish between the "glue" that holds an individual molecule together and the "attraction" that makes molecules stick to one another. These are Intramolecular and Intermolecular forces. Think of a molecule like a family: Intramolecular forces are the bonds between family members living in the same house, while Intermolecular forces are the social ties between different families in a neighborhood.

Intramolecular forces are the extremely strong chemical bonds that hold atoms together within a single molecule. For instance, in a water molecule (H₂O), the oxygen atom shares electrons with two hydrogen atoms through polar covalent bonds. These are the "strong bonds within the molecule" described in Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. Breaking these bonds requires massive energy and results in a chemical change—the molecule itself would cease to exist.

Intermolecular forces, on the other hand, are the attractive forces between separate molecules. These are much weaker than intramolecular bonds. In water, the most famous intermolecular force is the hydrogen bond, where the partial positive charge of a hydrogen atom in one molecule is attracted to the partial negative charge of an oxygen atom in a neighboring molecule. While van der Waals forces exist in all matter, hydrogen bonds are the dominant force that gives water its unique properties. It is these interparticle forces that determine the physical state of matter—whether something is a solid, liquid, or gas—based on how much thermal energy is present to overcome them Science, Class VIII, NCERT (Revised ed 2025), Particulate Nature of Matter, p.112.

Feature	Intramolecular Forces	Intermolecular Forces
Definition	Forces within a molecule (e.g., Covalent, Ionic).	Forces between different molecules.
Strength	Very High (Strong bonds).	Relatively Low (Weak forces).
Impact	Determines chemical identity.	Determines Melting and Boiling points.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, Class VIII, NCERT (Revised ed 2025), Particulate Nature of Matter, p.112

4. Anomalous Properties of Water (exam-level)

To understand why water behaves so differently from other liquids, we must look at its molecular architecture. While the atoms within a single water molecule (H₂O) are held together by polar covalent bonds, the magic happens between the molecules. Because Oxygen is highly electronegative, it pulls electrons away from Hydrogen, creating a 'dipole' (a molecule with a positive and negative end). This leads to Hydrogen Bonding—a strong electrostatic attraction between the positive hydrogen of one molecule and the negative oxygen of another. This 'intermolecular glue' is responsible for almost all of water's unique behaviors. One of the most critical properties is water's high specific heat. This means water can absorb a massive amount of heat energy before its own temperature rises significantly. As noted in Physical Geography by PMF IAS, Tropical Cyclones, p.369, this property explains why the Southern Hemisphere (which has more ocean surface) stays much cooler than the Northern Hemisphere. It takes far more time and energy to heat a kilogram of water than a kilogram of solid land, a concept essential for global climate regulation Physical Geography by PMF IAS, Ocean temperature and salinity, p.512. Perhaps the most 'anomalous' property is water's density behavior. Most substances contract and become denser as they get colder. Water follows this rule only until it hits 4°C. Below 4°C, water begins to expand and its density decreases. When it freezes at 0°C, the hydrogen bonds lock the molecules into a rigid, open hexagonal lattice. This makes ice less dense than liquid water, which is why ice floats. If water behaved 'normally,' lakes would freeze from the bottom up, destroying most aquatic life.

Sources: Physical Geography by PMF IAS, Tropical Cyclones, p.369; Physical Geography by PMF IAS, Ocean temperature and salinity, p.512

5. Biological and Environmental Significance of Water Bonds (exam-level)

To understand why water is the "elixir of life," we must look at how its atoms hold together. At the atomic level, a water molecule (H₂O) is formed when one oxygen atom shares electrons with two hydrogen atoms. This sharing of electron pairs creates covalent bonds Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. However, oxygen is more "electronegative" than hydrogen, meaning it pulls those shared electrons closer to itself. This creates a polar covalent bond, where the oxygen end becomes slightly negative (δ-) and the hydrogen ends become slightly positive (δ+).

While the covalent bond holds the individual molecule together, the Hydrogen Bond is the crucial intermolecular force that acts between different water molecules. Think of it as a magnetic attraction: the positive hydrogen of one water molecule is drawn to the negative oxygen of another. Although these bonds are weaker than covalent bonds, they are numerous and dynamic, forming a vast network that gives water its extraordinary properties.

Bond Type	Location	Primary Role
Covalent Bond	Within the molecule (O-H)	Stability of the H₂O molecule itself.
Hydrogen Bond	Between different H₂O molecules	Responsible for high boiling point, surface tension, and solvent properties.

From an environmental and biological perspective, these hydrogen bonds are transformative. They allow water to absorb significant heat before changing temperature, acting as a thermal buffer for the planet and our bodies. This network also creates high surface tension, allowing insects to walk on water and enabling capillary action, which helps water rise against gravity from the roots of a tree to its highest leaves. Furthermore, the ability of water to dissolve various substances makes it the perfect medium for complex biochemical reactions, such as the copying of DNA required for life Science, class X (NCERT 2025 ed.), How do Organisms Reproduce?, p.114.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, class X (NCERT 2025 ed.), How do Organisms Reproduce?, p.114

6. The Hydrogen Bond: Mechanism and Conditions (intermediate)

Concept: The Hydrogen Bond: Mechanism and Conditions

7. Solving the Original PYQ (exam-level)

You have just mastered the fundamentals of electronegativity and molecular polarity, and this question is the perfect test of how those building blocks interact. Crucially, the key to solving this lies in distinguishing between intramolecular forces (what holds a single molecule together) and intermolecular forces (what acts between separate molecules). While the oxygen and hydrogen atoms within a single water molecule are joined by polar covalent bonds, the question specifically asks for the bond between molecules. Because oxygen is far more electronegative than hydrogen, it creates a partial negative charge that attracts the partial positive charge of a neighboring molecule's hydrogen atom.

The correct answer is (C) hydrogen bond. This interaction is the defining characteristic of water, responsible for its high surface tension and boiling point. When navigating UPSC options, you must be wary of Option (B) covalent bond; this is a classic trap designed to catch students who confuse internal structure with external interactions. Similarly, (A) electrovalent bond (ionic) is incorrect because water is a molecular compound, not a lattice of ions. While (D) van der Waals bonds are technically present in all molecular systems, they are significantly weaker and are not the primary force described here. As noted in tims.itf.gov.ng - Covalent vs Ionic Bonds, it is this specific hydrogen bonding network that gives water its unique life-sustaining properties.