Question

Explain what happens to water molecules when ice melts.

Short answer

Water molecules absorb heat, break hydrogen bonds, and move freely, transitioning from solid to liquid.

Step-by-step solution

Understanding the Change in State

When ice melts, it undergoes a phase change from a solid (ice) to a liquid (water). This process is called melting and occurs when the temperature rises above 0°C (32°F) for pure water at standard atmospheric pressure.

Increase in Molecular Energy

As the temperature increases, the water molecules in the ice absorb energy in the form of heat. This energy increases the vibration of the molecules, allowing them to overcome some of the forces holding them in a fixed position within the solid structure.

Breaking Hydrogen Bonds

In solid ice, water molecules are held together in a rigid structure known as a lattice, maintained by hydrogen bonds. As more heat is absorbed, these hydrogen bonds are broken, allowing the molecules to move more freely.

Transition to Liquid State

Once the hydrogen bonds weaken enough, the water molecules can start to move past one another, causing the solid structure to collapse into a liquid. In the liquid state, the molecules are still close together but have more freedom to flow around each other, which defines the liquid phase.

Equilibrium and Energy

At equilibrium, the water molecules continue to move, exchanging energy through collisions, but the average energy remains constant. The substance remains in the liquid phase as long as the temperature does not decrease back to the freezing point.

Key concepts

Water Molecules

Water molecules are the fundamental building blocks of water, consisting of two hydrogen atoms bonded to one oxygen atom. This gives water its unique chemical properties, such as its ability to form hydrogen bonds. When in the form of ice, water molecules are structured in a crystalline lattice. This lattice ensures each molecule is positioned at a fixed distance from its neighbors, making ice a stable, rigid form.

• Composed of H₂O, water's identity doesn't change during melting.
• Molecules vibrate constantly, even in solid form, due to thermal energy.
• In solid state, these vibrations are restricted to small areas.

As ice begins to melt, the vibrations increase in intensity with added heat, leading to greater movement of these water molecules.

Melting Process

The melting process involves the transition of a substance from a solid state to a liquid state. For water, this occurs when ice is heated above 0°C (32°F).

• This phase change requires energy to overcome the forces holding molecules in the solid state.
• Heat energy is absorbed, increasing kinetic energy within the molecular structure.

More energy results in more movement, allowing the molecules to break free from their rigid positions in the ice lattice.
Hence, melting is not just a change in state but also a change in the arrangement and movement dynamics of the molecules.

Hydrogen Bonds

Hydrogen bonds play a crucial role in keeping water molecules at fixed positions within ice. These are weak bonds but strong enough to maintain the structure of ice

• Each molecule forms hydrogen bonds with up to four neighboring molecules in ice.
• As the ice absorbs heat, these bonds begin to weaken.
• The breaking of these bonds is fundamental to the melting process.

As the bonds break, the structure loosens, leading to the free flow of molecules past each other in a liquid state.

Solid to Liquid Transition

The transition from solid ice to liquid water is characterized by the breakdown of structural rigidity.

• The molecules, initially part of a fixed lattice, gain freedom of motion.
• This freedom allows them to move past one another, signifying a thinned and less ordered structure.

In the liquid phase, water retains a closely linked network, but unlike solid ice, the molecules can slide, roll, and tumble past each other with ease.
This free movement characterizes the fluidity and adaptability of liquids, distinct from solid rigidity.