Question

When two ice cubes are pressed over each other, they unite to form one cube. Which of the following forces is responsible to hold them together? (a) Van der Waals forces (b) Covalent attraction (c) Ionic interaction (d) Hydrogen bond formation

Short answer

(d) Hydrogen bond formation

Step-by-step solution

Understanding the Phenomenon

When two ice cubes are pressed together, they come into contact, which allows water molecules at their surfaces to interact. Ice is primarily kept together by hydrogen bonds between water molecules.

Analyzing the Options

Let's determine which force holds ice together: - (a) Van der Waals forces: generally weaker and not responsible for bonding in ice. - (b) Covalent attraction: these are strong bonds but do not form between water molecules in ice. - (c) Ionic interaction: typically occur in ionic compounds, not in molecular ice. - (d) Hydrogen bond formation: the primary attractive force between water molecules in ice, providing structure by bonding slightly positively charged hydrogen atoms to nearby oxygen atoms.

Choosing the Correct Option

Among the given options, hydrogen bonds are responsible for holding the ice cubes together when pressed because they form between water molecules and create the solid structure of ice.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including attraction or repulsion. These are not as strong as covalent or ionic bonds, which are intramolecular. Intermolecular forces have a significant influence on the physical properties of substances, such as boiling and melting points. In the context of ice, these forces are crucial because they determine how water molecules bond to each other in solid form.

There are several types of intermolecular forces:

• Van der Waals Forces: These are weak, short-range forces that occur between uncharged molecules. They arise from transient dipoles and are generally insignificant in ice.
• Dipole-Dipole Interactions: These occur between polar molecules. Water, being polar, experiences these interactions but to a lesser extent in solid form than hydrogen bonds.
• Hydrogen Bonds: Stronger than Van der Waals forces, these occur when hydrogen is directly bonded to a more electronegative atom, like oxygen. They are the dominant intermolecular force in ice.

Understanding intermolecular forces helps explain why ice, a seemingly fragile substance, can maintain its solid form at colder temperatures.

Hydrogen Bonds

Hydrogen bonds are a specific type of dipole-dipole interaction. They occur when a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen or nitrogen, interacts with another electronegative atom. Despite being weaker than covalent bonds, hydrogen bonds are quite strong relative to other intermolecular forces.

In the case of ice, hydrogen bonds play a crucial role in its structure:

• Formation: In ice, each water molecule can form up to four hydrogen bonds with surrounding molecules, creating an extensive network.
• Structure: This leads to a unique open hexagonal lattice structure. This structure is responsible for ice's lower density compared to liquid water.
• Unity: When two ice cubes are pressed together, the flexible hydrogen bonds allow the lattice to merge, leading to the cubes sticking together.

This network of hydrogen bonds provides stability and structure, allowing ice to retain its form under various conditions.

Properties of Water

Water is a remarkable substance with unique properties largely due to its ability to form hydrogen bonds. These properties are essential for a myriad of natural processes and have several daily applications.

• High Specific Heat: The presence of hydrogen bonds means water requires more energy to change temperature, which stabilizes climates and environments.
• High Heat of Vaporization: Water's ability to absorb considerable heat without changing to gas is due to hydrogen bonds needing to break, explaining why sweating cools effectively.
• Density Anomaly: Unlike most substances, water is less dense as a solid than as a liquid. This is why ice floats, as the hydrogen-bonded lattice in ice is more spacious.
• Cohesion and Adhesion: Water molecules tend to stick to each other (cohesion) and to other surfaces (adhesion), important for processes like capillary action in plants.

These properties make water not only vital to life but also fascinating in its physical characteristics, facilitating its diverse roles in nature.