Question

Describe what happens to the molecules in a sample of ice as the sample is slowly heated until it liquefies and then vaporizes.

Short answer

As a sample of ice is slowly heated, water molecules first gain energy and vibrate more, eventually breaking the hydrogen bonds that hold them in a fixed lattice structure when reaching the melting point, transitioning from solid to liquid. In the liquid phase, hydrogen bonds are constantly breaking and reforming, allowing molecules to slide past each other. Upon reaching the boiling point, hydrogen bonds break entirely, and the water molecules transition into the gaseous phase. In this stage, water molecules have high-energy motion and are minimally connected by hydrogen bonding, leading to a significant increase in volume. Overall, the molecules gain energy and experience increased motion throughout the process.

Step-by-step solution

1. Solid Phase (Ice)

The sample starts as ice, which is the solid phase of water. In this stage, the water molecules are closely packed together and held in place by hydrogen bonds in a crystal lattice structure. The molecules vibrate slightly, but their movement is restricted due to these bonds.

2. Melting (Solid to Liquid)

As the ice is slowly heated, the water molecules gain energy and start vibrating more. When the temperature reaches the melting point (0°C or 32°F), the added energy allows the molecules to overcome the hydrogen bonds that hold them together in the lattice structure. This results in the molecules moving more freely and transitioning into a liquid phase (water).

3. Liquid Phase (Water)

In the liquid phase, water molecules are no longer arranged in a fixed lattice structure. They are still held together by hydrogen bonds, but these bonds are constantly breaking and reforming, allowing the molecules to slide past each other and flow. The temperature continues to rise, causing the molecules to gain more kinetic energy and move even more rapidly.

4. Boiling (Liquid to Gas)

As the water is heated further, it reaches the boiling point (100°C or 212°F) and begins to vaporize. At this point, the water molecules have gained enough energy to break the hydrogen bonds entirely. This allows them to escape the liquid phase and transition into the gaseous phase (water vapor or steam).

5. Gaseous Phase (Water Vapor)

In the gaseous phase, water molecules are far apart and moving more independently, as hydrogen bonding is minimal. They have high-energy motion and spread out to fill the available space, leading to a significant increase in volume compared to the liquid phase. To sum up, the molecules in a sample of ice undergo several changes as they are heated. They gain energy, break their fixed lattice structure, change from a solid to a liquid and finally to a gas, and experience an increase in kinetic energy and motion throughout the process.

Key concepts

Hydrogen Bonding

Hydrogen bonds are a special type of attractive force between molecules, which play a crucial role in phase transitions for water.
These bonds form when a hydrogen atom, covalently bonded to an electronegative atom (like oxygen), interacts weakly with another electronegative atom.

In the solid phase of water, hydrogen bonds hold the water molecules tightly in place within a crystalline lattice structure. This results in ice having a fixed shape and a lower density than liquid water—one reason why ice floats.

As the ice is heated, these hydrogen bonds absorb energy and start to weaken. This weakening allows the rigid lattice to break apart, enabling the water molecules to move more freely in the liquid phase.

• Hydrogen bonds are dynamic. They break and reform as water transitions from solid to liquid.
• When transitioning to the gas phase, hydrogen bonds break completely, allowing water molecules to move independently.

Understanding hydrogen bonding is key to grasping the phase changes in water's structure and behavior.

Kinetic Energy

Kinetic energy is the energy that an object possesses due to its motion. In the context of molecular motion and phase transitions, it's a pivotal factor
determining when and how molecules change states from solid to liquid to gas.

When ice (solid water) is heated, the water molecules absorb thermal energy, which increases their kinetic energy.

• Higher kinetic energy means molecules vibrate more intensely.
• This increase in movement causes the molecules to eventually break free from the rigid hydrogen-bonded lattice.

Once in the liquid state, as temperature increases further, the kinetic energy of water molecules continues to rise.
Finally, boiling occurs when the kinetic energy is high enough to break all hydrogen bonds, allowing molecules to escape into the gaseous phase as water vapor.
Without sufficient kinetic energy, molecules cannot overcome the forces holding them together.

Molecular Motion

Molecular motion is a representation of how molecules move at different phases. It is intrinsically tied to the concepts of kinetic energy and phase changes.
At lower temperatures and in the solid state, molecular motion is limited to subtle vibrations within the crystal lattice of ice.

As heat is introduced, this motion intensifies. The water molecules begin to move, rotate, and eventually slide past one another once in the liquid state.

• In the liquid phase, molecular motion allows for the flow and fluidity of water as molecules explore more spatial configurations.
• Once the boiling point is reached, water molecules move rapidly and energetically enough to enter the gas phase.

This marks a shift where molecules are no longer constrained by intermolecular bonds and can move independently in all directions.
Understanding molecular motion helps to explain the physical properties and behaviors of substances at different states.