Question

What happens on the molecular level when a liquid freezes? What effect does a nonvolatile solute have on this process? Comment on the purity of water obtained by melting an iceberg.

Short answer

Molecules slow and arrange in a lattice; solutes lower freezing points; iceberg water is usually pure.

Step-by-step solution

Understanding Freezing

When a liquid freezes, its molecules lose energy and move more slowly. As they slow down, the molecules become more orderly, arranging themselves into a fixed, stable structure known as a solid lattice. This process occurs at the freezing point of the liquid.

Effect of Nonvolatile Solute

A nonvolatile solute, when added to a liquid, causes the freezing point to decrease. This is known as freezing point depression. The solute particles disrupt the formation of the orderly lattice, requiring the process to occur at a lower temperature.

Purity of Melted Iceberg Water

Icebergs are primarily composed of freshwater as they are formed from compacted snow or frozen freshwater. When they melt, the water is generally pure because it does not contain the salts found in ocean water. However, some impurities can be present due to environmental pollutants.

Key concepts

Molecular Structure of Solids

When a liquid transforms into a solid, it undergoes a fascinating transition at the molecular level. In their liquid state, molecules are relatively free to move around. They have higher energy and can slide past each other, which is why liquids can flow and take the shape of their containers.
However, as a liquid cools down and reaches its freezing point, the molecules begin to lose energy. They slow down significantly and start to come closer together, arranging themselves in a fixed pattern. This organized structure is called a solid lattice.
The solid lattice is a stable, rigid formation, where molecules are held together by forces such as hydrogen bonds, ionic bonds, or covalent bonds, depending on the substance. This arrangement gives solids their definite shape and makes them incompressible compared to their liquid form.
The freezing point—the temperature at which this transformation occurs—depends on the unique molecular structure of each liquid. Thus, the nature of molecular interactions within a substance determines its freezing behavior.

Effect of Nonvolatile Solutes

Nonvolatile solutes play a crucial role in altering the freezing behavior of liquids. When a solute that does not easily evaporate is added to a liquid, it causes a phenomenon known as freezing point depression.
This is because the solute particles interfere with the normal formation of the solid lattice. The solute molecules get in the way of the liquid molecules trying to organize themselves into a stable, solid structure. As a result, the liquid's freezing point lowers, meaning the liquid needs to be colder than its normal freezing point to solidify.
Some everyday examples of freezing point depression include the use of salt to melt ice on roads and making homemade ice cream by lowering the freezing point of cream. This concept is also crucial in understanding why sea water, which is rich in nonvolatile solutes like salts, has a lower freezing point than freshwater.

Purity of Water

Water from melted icebergs is often considered pure due to its origin. Icebergs form from compacted snow, which is essentially frozen freshwater. This means that, compared to sea ice, icebergs do not contain the high concentrations of salts typically found in ocean water.
However, even though iceberg water is generally cleaner, it is not entirely free of impurities. During their long journey through the atmospheric and marine environments, icebergs can accumulate contaminants. These may include environmental pollutants, debris caught during the initial formation, or particles picked up as the iceberg drifts through the ocean.
While the melted water is relatively pure, it may need to undergo further purification to ensure it is safe for drinking. Understanding the sources and nature of potential impurities can aid in accurately evaluating the purity of melted iceberg water.