Question

Give examples for (a) liquid-to-gas, \((b)\) solid-to-liquid, (c) solid-to-gas, and ( \(d\) ) gas-to-liquid mass transfer.

Short answer

Question: Provide an example of each of the following mass transfer processes: liquid-to-gas, solid-to-liquid, solid-to-gas, and gas-to-liquid. Answer: Liquid-to-gas: Evaporation of water; Solid-to-liquid: Melting of ice; Solid-to-gas: Sublimation of dry ice; Gas-to-liquid: Condensation of water vapor.

Step-by-step solution

Example of liquid-to-gas mass transfer

Evaporation of water is a common example of liquid-to-gas mass transfer. When water is heated, its molecules gain enough energy to break free from the liquid phase and enter the gaseous phase, becoming water vapor.

Example of solid-to-liquid mass transfer

Melting of ice is a typical example of solid-to-liquid mass transfer. When ice is exposed to heat, its molecules gain enough energy to overcome the bonds holding them together in the solid phase, and the ice transitions into the liquid phase, becoming water.

Example of solid-to-gas mass transfer

Sublimation of dry ice (solid carbon dioxide) is a well-known example of solid-to-gas mass transfer. When dry ice is exposed to room temperature, it transitions directly from the solid phase to the gaseous phase, bypassing the liquid phase, and becomes gaseous carbon dioxide.

Example of gas-to-liquid mass transfer

Condensation of water vapor is a familiar example of gas-to-liquid mass transfer. When water vapor in the air comes into contact with a colder surface, it loses energy and its molecules slow down, transitioning from the gaseous phase to the liquid phase and forming water droplets. This is the process by which dew forms on surfaces in the morning.

Key concepts

Understanding Evaporation

Evaporation is the process where liquid molecules transform into a gas. Imagine a puddle of water on a sunny day gradually shrinking, with no clear indication of where the water went. This is evaporation in action. The water molecules absorb energy from the sun, which gives them the kinetic energy to break the attractions holding them in the liquid state and escape into the air as water vapor.

In terms of mass transfer, evaporation signifies the movement of molecules from the liquid phase into the gas phase. This process doesn't require boiling; it occurs at temperatures below the boiling point, although the rate of evaporation increases as temperature rises. This concept is critical in many natural and industrial processes, such as the water cycle or cooling mechanisms in power plants.

Melting: Solid to Liquid Transition

Melting, the transformation from solid to liquid, is a familiar process we observe with ice. When solid ice gains heat, perhaps from the warmth of a room or direct sunlight, the energy disrupts the stable molecular arrangement of the ice. As the molecules vibrate more energetically, they break free from the solid structure and the ice transitions into water.

Specific heat energy, known as the heat of fusion, is needed to change the state of a substance without changing its temperature. The phenomenon of melting is crucial to understand because it regulates how materials respond to heat, influences natural phenomena like the melting of glaciers, and affects various applications ranging from metallurgy to culinary arts.

Sublimation: Direct Solid to Gas Phase Change

Sublimation might seem like a bit of magic — it's when a substance changes directly from a solid to a gas without ever becoming a liquid. Dry ice, the solid form of carbon dioxide, demonstrates this when it seems to vanish into thin air as it warms up.

This process occurs when the molecules in a solid gain sufficient energy to bypass the liquid phase altogether. Sublimation requires certain conditions: the substance’s vapor pressure must exceed the ambient pressure and its temperature must be below the triple point, the unique temperature and pressure at which all three phases can coexist. Sublimation is important in freeze-drying food and medications, a method that preserves material by removing water content without passing through liquid form.

Condensation: Gas to Liquid Change

Condensation is the opposite of evaporation, where gaseous molecules lose energy and transition to the liquid phase. This can be observed on grass as dew or on the outside of a cold beverage glass. The gas molecules cool down and slow, allowing intermolecular forces to pull them closer together, forming a liquid.

Condensation plays a vital role in the water cycle, as it contributes to cloud formation and precipitation. It's also the principle behind refrigeration and air conditioning, which rely on the removal of heat through the condensation of refrigerants. Understanding condensation is essential for designing systems that manage moisture, whether it's preventing window fogging or controlling industrial processes.