When two icecubes are pressed together, they join to form one cube. Which one of the following helps to hold them together?

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

At the heart of all chemistry lies a simple quest for stability. Most atoms are naturally unstable because their outermost electron shells are incomplete. To achieve a stable state—similar to the noble gases—atoms engage in chemical bonding to complete their outer shell, typically aiming for eight electrons, a concept known as the Octet Rule. As we see in chemical reactions, atoms don't just disappear or change into other elements; they simply rearrange by breaking and making these bonds to form new substances Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6.

Ionic Bonding occurs when there is a complete transfer of one or more electrons from one atom to another. This usually happens between a metal (which loses electrons to become a positively charged cation) and a non-metal (which gains electrons to become a negatively charged anion). These oppositely charged ions are held together by powerful electrostatic forces. Because these forces are so strong, ionic compounds typically have high melting and boiling points and can conduct electricity when dissolved in water or melted, as the ions are then free to move Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.58.

Covalent Bonding, on the other hand, is about partnership rather than transfer. Here, atoms share pairs of electrons to reach a stable configuration. This is common among non-metals, such as in a molecule of water (H₂O) or nitrogen (N₂). In nitrogen, for instance, each atom shares three electrons, creating a triple bond Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. Because covalent compounds consist of neutral molecules rather than charged ions, they are generally poor conductors of electricity and have lower melting and boiling points compared to ionic compounds Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

Feature	Ionic Bond	Covalent Bond
Mechanism	Complete transfer of electrons	Sharing of electron pairs
Structure	Giant lattice of charged ions	Neutral individual molecules
Conductivity	High (in molten/solution state)	Generally poor (insulators)
Melting Point	Very High	Relatively Low

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.58; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60

2. Intermolecular Forces and Van der Waals Interactions (basic)

To understand how matter behaves, we must distinguish between the intramolecular bonds that hold a single molecule together and the intermolecular forces that act between neighboring molecules. Think of it like a family: the covalent bond is the strong biological tie between siblings (atoms) inside a house (molecule), while intermolecular forces are the neighborly handshakes between different houses. As noted in Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60, while covalent bonds are very strong within the molecule, the forces between the molecules are relatively weak. This explains why carbon compounds like methane or ethanol have much lower melting and boiling points compared to ionic compounds like salt, where the attraction is much more powerful Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.
Among these attractions, Van der Waals interactions are the most universal. They occur when the electron clouds of neutral molecules shift slightly, creating temporary positive and negative poles that attract one another. While individually weak, they are responsible for the cohesion of many organic substances. A much stronger cousin of these forces is Hydrogen Bonding. This occurs when a hydrogen atom, already bonded to a highly electronegative atom like Oxygen, feels a strong attraction to the lone electrons of a neighboring molecule's Oxygen.
These forces are why two ice cubes can merge into one when pressed together. The pressure brings the water molecules close enough for the positively charged hydrogen atoms on the surface of one cube to form new hydrogen bonds with the negatively charged oxygen atoms of the other cube. This effectively "glues" the two separate crystalline lattices into a single block. These interactions are fundamental to why water is a liquid at room temperature and why DNA strands can unzip and zip back together.

Force Type	Relative Strength	Typical Example
Ionic/Covalent Bond	Very High	C-H bond in Methane
Hydrogen Bond	Moderate	H₂O molecules in Ice
Van der Waals	Low	Noble gases or Hydrocarbons

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59

3. Molecular Structure and Polarity of Water (intermediate)

To understand why water behaves so uniquely, we must first look at its molecular architecture. A single water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms. In this setup, oxygen shares one electron with each hydrogen atom, allowing the hydrogens to fill their K-shell and the oxygen to complete its outer octet Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59-60. However, this sharing is not equal. Oxygen is far more electronegative than hydrogen, meaning it has a much stronger 'pull' on the shared electrons. This creates a polar covalent bond, where the electrons spend more time near the oxygen atom than the hydrogen atoms. Because of this unequal sharing, the water molecule acts like a tiny magnet, or a dipole Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415. The oxygen side acquires a partial negative charge (δ-), while the hydrogen side carries a partial positive charge (δ+). This polarity is the 'secret sauce' behind water’s properties. It allows water molecules to interact dynamically with other substances, such as when hydrogen ions (H⁺) combine with water to form the hydronium ion (H₃O⁺) Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23. When multiple water molecules come together, the δ+ hydrogen of one molecule is attracted to the δ- oxygen of another. This specific electrostatic attraction is called a Hydrogen Bond. While much weaker than the covalent bonds holding the individual molecule together, hydrogen bonds are strong enough to give water its high surface tension and allow it to form a structured lattice in its solid state (ice).

Feature	Covalent Bond (Intramolecular)	Hydrogen Bond (Intermolecular)
Location	Inside the H₂O molecule (between H and O)	Between different H₂O molecules
Nature	Sharing of electron pairs	Electrostatic attraction between charges
Strength	Very Strong	Relatively Weak but cumulative

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59-60; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23; Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415

4. Anomalous Expansion of Water and Density of Ice (exam-level)

In the physical world, most substances follow a predictable rule: they expand when heated and contract when cooled. However, water is a remarkable exception. This unique behavior is known as the Anomalous Expansion of Water. While water contracts like a normal liquid as it cools down from room temperature, this contraction stops at 4 °C. At this specific temperature, water reaches its maximum density—meaning it is at its heaviest for a given volume Science, Class VIII. NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.148.

If you continue to cool water from 4 °C down to 0 °C, it does something counterintuitive: it begins to expand. As it turns into ice, the H₂O molecules arrange themselves into a rigid, tetrahedral crystalline lattice. This structure is held together by hydrogen bonds that force the molecules to maintain a specific distance from one other, creating open "cages" or gaps in the structure Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.121. Because these molecules now occupy more space than they did in the liquid state, the volume increases and the density decreases. This is why ice is lighter than liquid water and floats on the surface.

This anomaly is not just a scientific curiosity; it is a fundamental pillar of life on Earth. To visualize the relationship between temperature, volume, and density, consider the following:

Temperature Change	Volume Trend	Density Trend
10 °C down to 4 °C	Decreases (Normal)	Increases
At 4 °C	Minimum Volume	Maximum Density
4 °C down to 0 °C	Increases (Anomalous)	Decreases

When lakes or ponds freeze, the ice forms at the top because it is less dense. This floating layer of ice acts as an insulator, preventing the water below from freezing completely and allowing aquatic life to survive even in sub-zero temperatures. In geography, we often use the term "anomalous" to describe deviations from the expected norm, such as the anomalous warming of ocean waters during El Niño events Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.413.

Sources: Science, Class VIII. NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.148; Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.121; Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.413

5. Cohesive and Adhesive Forces: Surface Tension (intermediate)

Concept: Cohesive and Adhesive Forces: Surface Tension

6. The Concept of Hydrogen Bonding (exam-level)

To understand Hydrogen Bonding, we must first look at the concept of electronegativity. In certain molecules, atoms like Oxygen (O), Nitrogen (N), or Fluorine (F) have a much stronger 'pull' on shared electrons than Hydrogen (H) does. This creates a polar covalent bond where the electron density shifts away from the hydrogen, leaving it with a partial positive charge (δ+) and the other atom with a partial negative charge (δ-) Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60.

A Hydrogen Bond occurs when the δ+ hydrogen of one molecule is electrostatically attracted to the δ- atom (the 'lone pair' of electrons) of a neighboring molecule. It is essentially a strong 'intermolecular bridge.' While covalent bonds hold the atoms within a molecule together, hydrogen bonds act between molecules. Although they are much weaker than covalent bonds, they are significantly stronger than regular Van der Waals forces, which explains why substances with hydrogen bonding, like water or ethanol, have much higher boiling points than simple hydrocarbons like methane Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

Feature	Covalent Bond	Hydrogen Bond
Nature	Intramolecular (inside the molecule)	Intermolecular (between molecules)
Mechanism	Sharing of electron pairs	Electrostatic attraction of δ+ Hydrogen
Strength	Very Strong	Moderate (Weaker than covalent)

In the context of ice, hydrogen bonding is what gives it its unique structure. Water molecules arrange themselves in a tetrahedral lattice. When you press two ice cubes together, the molecules at the surface align their partial charges—the δ+ hydrogens of one cube attract the δ- oxygens of the other—re-establishing these bonds across the interface and fusing them into a single block. This directional nature of the bond is also why ice is less dense than liquid water, as the bonds force the molecules to stay a specific 'open' distance apart in the solid state.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60

7. Pressure and Phase Changes: Regelation (exam-level)

To understand why two ice cubes stick together when pressed, we must first look at a unique property of water. In most substances, increasing the ambient pressure raises the melting point, making it harder for a solid to turn into a liquid. However, water is a fascinating exception: an increase in pressure actually lowers the melting point of ice Physical Geography by PMF IAS, Earths Interior, p.56. This phenomenon is known as Regelation.

When you press two ice cubes against each other, the high pressure at the points of contact causes a microscopic layer of ice to melt into liquid water, even if the temperature is below 0 °C. Once the pressure is released, the melting point returns to its normal state, and that thin film of water refreezes almost instantly. This refreezing acts like a "glue," fusing the two cubes into a single block. This happens because solids like ice have specific interparticle forces of attraction that dictate their state Science Class VIII NCERT, Particulate Nature of Matter, p.103.

At the chemical level, the "glue" is actually the formation of Hydrogen Bonds. Water molecules (H₂O) are polar; the oxygen atom carries a partial negative charge and the hydrogen atoms carry a partial positive charge. In the solid state, these molecules are arranged in a tetrahedral lattice. By pressing the cubes together, you bring the surface molecules into such close proximity that the positively charged hydrogen atoms of one cube can attract the negatively charged oxygen atoms of the other. This electrostatic attraction establishes new hydrogen bonds across the interface, merging the two separate structures into one continuous crystalline lattice.

Sources: Physical Geography by PMF IAS, Earths Interior, p.56; Science Class VIII NCERT, Particulate Nature of Matter, p.103

8. Hydrogen Bond Formation in the Solid State (exam-level)

To understand why two blocks of ice merge when pressed together, we must first look at the polarity of the water molecule (H₂O). Within a single water molecule, the oxygen and hydrogen atoms are held together by covalent bonds, where they share electrons to achieve a stable configuration 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 dipole: the oxygen atom carries a partial negative charge (δ-), and the hydrogen atoms carry a partial positive charge (δ+).

In the solid state (ice), these polar molecules arrange themselves into a rigid tetrahedral crystalline lattice. This structure is maintained by hydrogen bonds—a special type of strong intermolecular attraction where the positively charged hydrogen of one molecule is attracted to the negatively charged oxygen of a neighboring molecule. In ice, these particles arrange themselves in a way that takes up more space than in liquid water, which is why ice has a lower density and floats Science, Class VIII, NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.148.

When you press two ice cubes together, you are performing a fascinating bit of molecular engineering. The pressure can cause a microscopic layer of ice at the surface to melt (a process known as regelation) or simply bring the surface molecules into extremely close proximity. This allows the molecules at the interface of the two cubes to "see" each other. The electrostatic attraction between the H-atoms of one cube and the O-atoms of the other leads to the formation of new hydrogen bonds across the boundary. Essentially, the two separate crystalline structures stitch themselves together into one continuous lattice, creating a single solid block.

While other intermolecular forces like Van der Waals forces are present, it is the specific strength and directional nature of hydrogen bonding that is primarily responsible for the cohesion and structural integrity of ice in the solid state.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, Class VIII, NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.148

9. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of molecular polarity and intermolecular forces, this question allows you to see those building blocks in action. You've learned that water is a polar molecule where oxygen exerts a stronger pull on electrons, creating a partial negative charge, while hydrogen remains partially positive. In its solid state, ice forms a specific crystalline lattice where these molecules are held at arm's length. When you press two cubes together, you are forcing the molecules at the surface interface into close proximity, allowing the electrostatic attraction between the hydrogen atoms of one cube and the oxygen atoms of the other to snap into place, resulting in Hydrogen bond formation.

To arrive at the correct answer, think like a scientist: the pressure you apply can cause a microscopic layer of ice to melt (a process often called regelation), but the act of "joining" into a single block requires the re-establishment of the solid lattice. As the pressure is released or the molecules align, the hydrogen bond acts as the primary "glue." While other forces are at play, the correct answer (A) is chosen because hydrogen bonding is the dominant and defining force that gives ice its structural integrity and unique properties, as detailed in NCERT Class 11 Chemistry.

UPSC frequently uses "distractor" options to test your depth of understanding. You might be tempted by Van der Waals forces or Dipole interaction; while these forces do exist between all polar molecules, they are significantly weaker than the specific, directional strength of a hydrogen bond. Covalent attraction is a classic trap—remember that covalent bonds hold the atoms inside a single water molecule together, whereas we are looking for the intermolecular force that joins two separate cubes. Distinguishing between these scales of bonding is essential for success in the General Science section.