Question

The salty ocean has a higher level of entropy compared to fresh water due to the dissolved ions it contains, which interact with and disperse water molecules. For ice to form in the ocean, this entropy must be reduced to allow the crystalline structure shown in Figure 2.15 to form. If you were to break off a piece of this ice and melt it, would the water taste fresh or salty? Explain your answer.

Short answer

The melted ocean ice would taste fresh, as the dissolved ions causing the saltiness of the ocean water were excluded from the ice crystalline structure during its formation. This is due to the reduction of entropy that is needed to form the ice, causing a more ordered structure, which does not accommodate the dissolved ions.

Step-by-step solution

Entropy is the measure of disorder in a system. In the salty ocean, the dissolved ions create high entropy as they interact with and disperse the water molecules, making it more disordered. In order to form ice, entropy needs to be reduced to make the water molecules more ordered and create a crystalline structure, as shown in Figure 2.15. #Step 2: Formation of ice in the ocean#

As the ocean water starts to freeze, the entropy must be reduced for water molecules to come together and form a crystalline structure. The crystalline lattice of ice does not have enough space to accommodate the dissolved ions. As a result, the dissolved ions are excluded from the ice crystalline structure and remain in the liquid phase where they increase the salinity of the surrounding water. #Step 3: Melting the ocean ice#

When the ice formed in the ocean is broken off and melted, the water comprises mainly of the water molecules that were part of the crystalline structure, which does not contain the dissolved ions that make the ocean water salty. #Step 4: Taste of the melted ocean ice#

Since the dissolved ions were excluded from the ice crystalline structure during its formation, the water resulting from melting the ocean ice would not contain those ions and therefore would taste fresh, not salty. In conclusion, the water resulting from melting a piece of ocean ice would taste fresh because the dissolved ions causing the saltiness of ocean water were excluded during the ice formation due to reduced entropy.

Key concepts

Dissolved Ions in Ocean

Understanding the complexity of the ocean's saltiness is pivotal when studying entropy in water systems. The ocean is like a vast cocktail of various substances, primarily made salty by the presence of dissolved ions such as sodium (Na+) and chloride (Cl-). These ions originate from the weathering of rocks on land and are carried to the ocean by rivers.

In an aqueous environment, these ions add to the entropy of the system because they move freely and interact with water molecules. This creates a highly disordered state compared to pure water, where only water molecules intermingle. The specifically increased entropy is due to the ions disrupting the hydrogen bonds between water molecules, preventing them from maintaining a structured arrangement.

Ions in the ocean are ever-important in biogeochemical cycles and marine life's metabolic processes, but they also play a crucial role in changing water's physical properties like boiling and freezing points.

Crystalline Structure of Ice

When we think of ice, it's not just frozen water; it's a masterpiece of molecular architecture. Ice forms a crystalline structure, which means that the water molecules are arranged in a repetitive, ordered pattern that extends in all three dimensions. At the microscopic level, each water molecule is linked to four others in a tetrahedral arrangement, resembling a honeycomb lattice.

This structured lattice is less dense than liquid water, which is why ice floats. During the freezing process, the kinetic energy of the water molecules decreases as the temperature drops, and they arrange themselves into this more ordered state, reducing the system's entropy.

Exclusion of Ions in Ice Formation

Interestingly, the crystalline structure of ice doesn't accommodate the dissolved ions found in seawater. Thus, when seawater begins to freeze, the ions are pushed out, leaving behind a framework of essentially fresh water molecules. This phenomenon is the secret behind why melted sea ice tastes fresh and not salty.

Phase Change of Water

Water is extraordinary because it can exist in three different states – solid, liquid, and gas – which are referred to as the phases of matter. A phase change of water happens when energy in the form of heat is either absorbed or released, allowing water to transition between these states.

When heat is absorbed, water molecules gain kinetic energy; ice melts to liquid water, and liquid water evaporates to water vapor. Conversely, when heat is released, water vapor condenses to liquid and liquid water freezes to ice. Each of these transitions involves a significant energy exchange and is connected to changes in entropy.

Entropy Changes during Phase Transitions

The phase change from liquid to solid (freezing) entails a decrease in entropy because the molecules are arranged in a structured, ordered lattice. Conversely, melting or evaporating water increases entropy as the molecules gain freedom of movement and are less ordered. Understanding these phase changes and the associated entropy modifications is essential in various scientific disciplines, encompassing meteorology, environmental science, and physical chemistry.