Question

At its melting point, ice is lighter than water because (1) \(\mathrm{H}_{2} \mathrm{O}\) molecules are more closely packed in solid state. (2) Ice crystals have hollow hexagonal arrangement of \(\mathrm{H}_{2} \mathrm{O}\) molecules. (3) On melting of ice, the \(\mathrm{H}_{2} \mathrm{O}\) molecules shrink in size. (4) Ice forms mostly heavy water on first melting.

Short answer

Option (2)

Step-by-step solution

Title - Understand the Question

Identify that the question is asking why ice is lighter (less dense) than water at its melting point.

Title - Review the Options

Carefully consider each of the provided options to determine which one explains why ice is less dense than water.

Title - Evaluate Option 1

Option (1) states that \(\text{H}_{2} \text{O}\) molecules are more closely packed in the solid state. This would make ice denser, not lighter, so this option is incorrect.

Title - Evaluate Option 2

Option (2) states that ice crystals have a hollow hexagonal arrangement of \(\text{H}_{2} \text{O}\) molecules. A hollow structure means less mass per unit volume, making ice lighter than water. This option appears correct.

Title - Evaluate Option 3

Option (3) states that on melting, \(\text{H}_{2} \text{O}\) molecules shrink in size, which would normally increase density. Hence, this option is incorrect.

Title - Evaluate Option 4

Option (4) states that ice forms mostly heavy water on first melting. The presence of heavy water would not explain why ice is lighter than water, making this option incorrect.

Title - Conclusion

Since the hollow hexagonal arrangement explains why ice is lighter than water, the correct answer is Option (2).

Key concepts

molecular structure of ice

The molecular structure of ice is fascinating because it directly impacts its physical properties, such as density. Each water molecule in ice consists of two hydrogen atoms bonded to one oxygen atom. These molecules form hydrogen bonds with neighboring water molecules.
This bonding creates a unique arrangement:

• Hydrogen bonds are relatively strong, stabilizing the structure.
• The oxygen atom of one water molecule is attracted to the hydrogen atom of another, forming a network.

In its solid state, this network creates an open, cage-like structure. This openness is why ice occupies more volume than liquid water, making ice less dense. As temperature decreases, this structure solidifies, maintaining the unique arrangements of its molecules.

hexagonal arrangement

Ice crystals have a characteristic hexagonal arrangement of water molecules. This geometric pattern is a result of the hydrogen bonds aligning in a specific way. Let’s break down the details:

The hexagonal arrangement:

• Forms a honeycomb-like structure with empty spaces within the hexagons.
• These empty spaces make the ice less dense compared to water.
• Because of these gaps, ice requires more room, hence floats on water.

These hollow spaces are crucial. If water molecules were to be closely packed without gaps, ice would be denser rather than lighter. This hexagonal arrangement ensures less mass in a given volume, explaining why ice floats.

physical properties of water

Water has unique physical properties that change with temperature. Let’s explore a few of these characteristics that are pertinent to understanding why water is denser than ice:

• Density: Water is most dense at 4°C. As it cools further and freezes, the molecules form the hexagonal structure, occupying more space.
• Expansion on Freezing: Most substances contract when they freeze, but water expands. This property explains why ice floats on water.
• Hydrogen Bonding: The hydrogen bonds in water are strong but more flexible in the liquid state, allowing closer packing of molecules.

When ice melts, the hydrogen bonds break, allowing water molecules to pack more closely together. This transition explains the increase in density when ice turns to water. These distinctive properties of water make it critical for supporting life and influencing various natural processes.