Question

The density of water is less in its solid state because (a) in solid state (ice), water molecules are arranged in highly ordered open cage like structure (b) more extensive hydrogen bonding is present in solid state ice (c) the water molecules are closest in solid state of water (d) water is a rigid crystalline, closely packed structure in its solid state.

Short answer

The correct answer is (a) because in solid state (ice), water molecules are arranged in a highly ordered open cage-like structure, making it less dense than liquid water.

Step-by-step solution

Understanding the Physical Properties of Water

To solve this problem, one must be familiar with the properties of water in its different states. In its liquid state, water has molecules that are close together but not in a fixed position, allowing them to flow past one another. In contrast, when water freezes, the molecules arrange themselves in a specific lattice structure that maximizes the number of hydrogen bonds between the molecules. This structure is more open and less dense than the arrangement of water molecules in the liquid state.

Analyzing the Options

Consider each of the provided options and how they relate to the known structural changes that water undergoes when transitioning from its liquid to solid state. Option (a) mentions an 'open cage like structure,' which corresponds to the reduced density of ice compared to liquid water. Options (b), (c), and (d) suggest increased density due to more extensive hydrogen bonding, closest molecules, or a rigid crystalline structure, respectively. However, these latter three options would lead to an increase in density rather than a decrease as seen in ice.

Eliminating Incorrect Options

Since the open lattice structure in ice leads to less density compared to liquid water, options that imply a more compact structure (c and d) or do not explicitly address the reduced density in ice due to the structure (b) are incorrect. The correct option will align with the expanded structure of ice.

Selecting the Correct Explanation

Given that the ice structure is an open lattice with wider spacing between molecules, the correct explanation is that which aligns with this structure. Therefore, option (a) is correct because it reflects that water molecules in the solid state are in an ordered open cage like structure, which causes ice to be less dense than liquid water.

Key concepts

Physical Properties of Water

Water is a substance with remarkable and unique physical properties, vital for the sustenance of life as we know it. At room temperature, water is a liquid, a state in which its molecules are constantly in motion, moving past each other. This motion contributes to water's fluidity and ability to conform to the shape of a container. However, there’s a twist when water transitions from a liquid to a solid; instead of becoming denser like most substances, it expands.

Understanding why this happens brings us to the concept of hydrogen bonding. In liquid water, each molecule can form up to four hydrogen bonds, which are transient due to the constant motion of the molecules. This allows for a relatively compact arrangement. Yet, as water cools down and eventually freezes, it reaches a point where these hydrogen bonds lead to a fixed and open lattice structure. It's this structure that makes ice less dense than liquid water, allowing it to float.

Another noteworthy trait is water's high specific heat capacity. This means it can absorb a lot of heat before it starts to get warmer. That's why it plays a critical role in Earth's climate and helps in regulating our body temperature. Moreover, the high latent heat of vaporization ensures that water absorbs significant energy before it transitions from liquid to gas, an essential aspect of the water cycle.

Hydrogen Bonding in Ice

Hydrogen bonding is the magnetic card that holds the deck of water molecules together in ice. It involves the attraction between the slightly positively charged hydrogen atom of one water molecule and the slightly negatively charged oxygen atom of another. But what makes ice special is how these hydrogen bonds organize themselves.

In ice, each water molecule forms four hydrogen bonds, creating a three-dimensional lattice that resembles an open cage-like structure. This solid-state lattice is highly ordered and maximizes the space between the molecules—contrary to what one might intuitively expect. As a result, when water freezes, the molecules are not packed tightly together; instead, they are held at a distance that makes ice less dense than water.

It's this peculiar behavior that explains the counterintuitive phenomenon of icebergs floating on the ocean and lakes freezing from the surface down, protecting aquatic life in the depths from the cold air above. Without hydrogen bonding, water would freeze from the bottom up, which would have profound impacts on ecosystems and the environment.

Solid State Lattice Structure

The solid-state lattice structure of ice is a fascinating arrangement that breaks the norm compared to most other substances. The ice lattice structure is a crystalline, spatial network where water molecules are arranged in repeating patterns called 'unit cells' that extend in all three dimensions.

Each molecule within this network is hydrogen-bonded to four others in a tetrahedral geometry, which is what gives ice its hexagonal crystal shape. This structure underlines the concept of 'less dense as a solid than as a liquid,' a characteristic you find in your frozen water tray, where ice cubes float to the top.

Here's the most interesting part: Although these bonds are rigid enough to keep the structure stable, they are also flexible enough to absorb energy without breaking. This flexibility is why ice can absorb a bit of heat (like when salt is spread on icy roads) and slightly rearrange its lattice without transitioning into a liquid state immediately. Therefore, this lattice not only gives ice its lesser density but also contributes to the unique melting properties that are crucial for environments experiencing seasonal change.