Question

Consider a sample of steam (water in the gaseous state) at \(150^{\circ} \mathrm{C}\). Describe what happens to the molecules in the sample as the sample is slowly cooled until it liquefies and then solidifies.

Short answer

In conclusion, as a sample of steam at \(150^{\circ} \mathrm{C}\) is slowly cooled, it undergoes two phase transitions. First, it transforms from a gas to a liquid through condensation at around \(100^{\circ} \mathrm{C}\), as water molecules lose kinetic energy and form weak, constantly changing hydrogen bonds. Second, it transforms from a liquid to a solid through solidification at around \(0^{\circ} \mathrm{C}\), as water molecules lose more kinetic energy and form stable hydrogen bonds in a crystalline lattice structure called ice.

Step-by-step solution

Identify the starting point

The sample of steam is in the gaseous state at \(150^{\circ} \mathrm{C}\), which is above the boiling point of water at standard pressure. At this temperature, water molecules have enough kinetic energy to be in the gaseous phase.

Describe the cooling process and molecular behavior during condensation

As the steam is slowly cooled, the kinetic energy of the water molecules decreases. When the temperature reaches the boiling point of water (around \(100^{\circ} \mathrm{C}\) at standard pressure), the steam starts to condense into water droplets. This process is known as condensation. During condensation, the water molecules start to lose some of their kinetic energy and move closer together and form weak bonds called hydrogen bonds. The liquid state is characterized by these hydrogen bonds constantly forming and breaking as the water molecules move and interact with each other.

Cooling the liquid to reach the freezing point

Continuing to cool the liquid water, the temperature will eventually reach the freezing point (around \(0^{\circ} \mathrm{C}\) at standard pressure). At this point, the kinetic energy of the water molecules is significantly lower than in the gaseous state, and the hydrogen bonds become more stable and stronger.

Describe the molecular behavior during solidification

As the temperature drops below the freezing point, the water molecules do not have enough kinetic energy to break the hydrogen bonds. The water molecules form a regular, crystalline lattice structure, known as ice. In this solid state, the water molecules vibrate in their fixed positions but do not have enough energy to move freely as they did in the liquid state. This process is called solidification or freezing. In conclusion, as a sample of steam is slowly cooled, it goes through a phase transition from a gas to a liquid through condensation, and then from a liquid to a solid through solidification. The water molecules lose kinetic energy and form more stable hydrogen bonds as the sample cools.

Key concepts

Condensation

Condensation is a phase transition where a substance changes from its gaseous state to a liquid state. It occurs when the steam, or gaseous water, is cooled down and the molecules lose some of their kinetic energy.
As a result, the motion of the molecules slows down, causing them to come closer together. This process typically happens around the boiling point of water, which is approximately 100°C under standard atmospheric pressure.

• Molecules lose kinetic energy during cooling.
• Brings molecules closer to form bonds.
• Occurs at the boiling point.

During condensation, water molecules begin to stick together, forming droplets. This transformation is a common everyday observation, such as when steam from a hot shower fogs up a bathroom mirror.

Solidification

Solidification, also known as freezing, is the process by which a liquid turns into a solid. This occurs when the temperature drops to the freezing point of the substance—in the case of water, this is around 0°C.
As the water continues to cool down from a liquid stage, its molecules move even closer and their movement becomes limited.

• Temperature reaches around 0°C.
• Molecules form a fixed pattern.
• Energy is too low to allow free movement.

Solidification is the final transformation in our cycle, resulting from the continued loss of kinetic energy, which stabilizes the structure of the water molecules.

Hydrogen Bonds

Hydrogen bonds are a special type of dipole-dipole attraction occurring between molecules that have a significant difference in electronegativity. These are electromagnetic attractions between the positive pole of one water molecule and the negative pole of another.
In the context of water, these bonds play a crucial role during both condensation and solidification.

• Form when molecules lose kinetic energy.
• Aid in phase transitions.
• Provide structure to the water in liquid and solid states.

As water cools, hydrogen bonds cause molecules to initially cluster in the condensation stage, and then finally lock into a regular pattern in the solid state.

Kinetic Energy

Kinetic energy is the energy an object possesses due to its motion, which decreases as the temperature decreases. In gases, like steam, molecules move rapidly because they have high kinetic energy.
As the gas cools, this energy diminishes, causing molecules to slow down and come closer.

• High kinetic energy in gaseous state.
• Decrease leads to condensation.
• Lowest energy state leads to solidification.

Understanding kinetic energy is important to grasp how phase transitions occur. Essentially, it is the pivotal factor that determines how molecules move and interact with each other in different states.

Crystalline Lattice

A crystalline lattice is a highly ordered structure formed by molecules in a solid. In the case of water, when it becomes ice, the molecules arrange into a lattice pattern, which extends throughout the material.
This organization is enabled by hydrogen bonds that hold the molecules in fixed positions.

• Orderly arrangement of molecules.
• Formed during solidification due to low kinetic energy.
• Contributes to the rigidity and shape of ice.

The crystalline lattice gives ice its unique properties, such as its lower density compared to liquid water, which is why ice floats.