Problem 96 - General Chemistry

Question

Snow forms in the upper atmosphere in a cold air mass that is supersaturated with water vapor. When the snow later falls through a lower, warm air mass, rain forms. When this rain falls on a sunny spot, the drops evaporate. Describe all of the phase changes that have occurred.

Step-by-Step Solution

Answer

The phase changes are deposition (vapor to solid), melting (solid to liquid), and evaporation (liquid to gas).

Step 1: Understanding the Initial State

Beginning in the upper atmosphere, water vapor in a supersaturated state undergoes a phase change. In this instance, the water vapor transitions from a gaseous phase to a solid phase, forming snow. This process is known as deposition, where water vapor directly converts to ice crystals without becoming liquid first.

Step 2: Transition through Warmer Lower Air Mass

As the snow descends into a lower, warmer air mass, it undergoes a phase change from solid to liquid. This is the process of melting, where the snow transforms into rain due to the increase in temperature in the warmer air layer.

Step 3: Evaporation from the Surface

Once the rain falls on a sunny spot, it experiences further warming. This causes the liquid water to transform back into a vapor. This phase change, called evaporation, occurs as the liquid receives enough energy from the sun to turn into a gas bringing it back to the atmosphere.

Key Concepts

Deposition

Deposition is a fascinating phase change where a substance transitions directly from a gas to a solid, completely skipping the liquid phase. Imagine water vapor in the upper atmosphere. It is cold and supersaturated, meaning it contains more water than usual. When conditions are just right, this vapor undergoes deposition, forming ice crystals or snowflakes.

Deposition occurs under specific conditions, primarily when the air is saturated with moisture and temperatures are low. This is common in the upper reaches of the atmosphere. Here, water vapor loses energy rapidly, condensing straight into solid ice without passing through a liquid state.

• This process is crucial for snow formation.
• It typically happens at high altitudes where temperatures fall below freezing.
• Deposition is the opposite of sublimation, where a solid turns directly into a gas.

Understanding deposition helps explain how snow forms and falls to Earth's surface.

Melting

Melting describes the transition from a solid to a liquid. When snow falls into warmer air masses below, it absorbs heat. The temperature rise triggers melting, turning solid snow into liquid water, which we recognize as rain.

This phase change happens because molecules in the solid start vibrating faster as temperature increases. They eventually gain enough energy to break free from their fixed positions in the solid structure. As a result, the tightly bonded ice molecules become loose and flow freely, forming a liquid.

• Melting is essential for weather patterns, forming rain from snow.
• It requires temperatures above the freezing point (0°C or 32°F).
• The amount of heat needed to melt ice is called the 'heat of fusion.'

Melting is a fundamental process for many natural phenomena and a critical part of our water cycle.

Evaporation

Evaporation is the process by which a liquid turns into vapor or gas. This occurs when raindrops, warmed by sunlight, gain enough energy to transform into water vapor.

As rain hits a sunny surface, it absorbs heat. The added energy excites the water molecules, causing them to move rapidly and escape the liquid’s surface. These freed molecules then rise back into the atmosphere as vapor.

• Evaporation is vital for redistributing water in the environment.
• It plays a key role in the water cycle, returning moisture to the air.
• Factors like temperature, humidity, and wind speed affect evaporation rates.

Through evaporation, water from Earth's surface is continuously cycled back into the atmosphere, sustaining life and regulating climate.