Problem 95 - General Chemistry

Question

If you leave your car parked outdoors in the winter, you may find frost on the windows in the morning. If you then start the car and let the heater warm the windows, after some minutes the windows will be dry. Describe all of the phase changes that have occurred.

Step-by-Step Solution

Answer

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

Step 1: Identify Initial Phase Change

Initially, the frost on the car windows is in the solid phase. Frost forms when water vapor in the air undergoes deposition directly onto the cold window surfaces, transitioning from a gas to a solid.

Step 2: Understand Warming Effect

Once you start the car and the heater begins to warm the windows, the heat energy flows into the frost, increasing its temperature gradually.

Step 3: Solid to Liquid Phase Change

As the frost absorbs heat, it reaches its melting point and undergoes a phase change from solid (frost) to liquid (water). This is called melting.

Step 4: Liquid to Gas Phase Change

Continuing to receive heat, the liquid water on the window eventually reaches the boiling point and begins the process called evaporation, where it transitions from a liquid to a gas (water vapor). This leaves the windows dry.

Key Concepts

Deposition

Deposition is a fascinating phase change that occurs when a substance transitions directly from a gas to a solid, bypassing the liquid phase altogether. This process is the reason frost forms on your car windows during cold winter nights. When the outdoor air is cold enough, water vapor in the air loses energy and turns directly into ice, creating frost. This happens because the surfaces of the windows become cooler than the surrounding air, allowing the water vapor to attach itself to the window as solid ice.
Understanding deposition helps us appreciate the conditions required for frost formation:

• Low temperatures: When temperatures drop significantly, more energy is removed from the water vapor.
• Surface condition: The surface must be cold enough to encourage the transition of water vapor directly to ice.

Melting

Melting is another key phase change that occurs when a solid turns into a liquid. When the heater in your car is turned on, it warms the windows, causing the frost to absorb heat. As the temperature of the frost rises and reaches the melting point, the ice particles gain enough energy to break free from their fixed positions.
This process results in a phase change from solid (frost) to liquid water, forming a thin layer of water on your windows. Key aspects of melting include:

• Melting point: This is the specific temperature at which solid water (ice) becomes liquid water.
• Heat absorption: Energy from the heater increases molecular motion, facilitating the melting process.

Evaporation

Evaporation is the transformation of a liquid into a gas. Once the frost on your car windows melts into water, the liquid begins to absorb more heat from the environment. As the temperature of the liquid water continues to increase, it reaches a point where molecules have enough energy to escape the liquid state and become water vapor.
This transition is known as evaporation, where the window eventually dries as the water molecules disperse into the surrounding air. The essential aspects of evaporation involve:

• Boiling point: Not necessarily reached in evaporation, but it marks when liquid rapidly turns to gas.
• Energy input: Continuous heat energy input increases molecular kinetic energy, causing evaporation.

Heat Transfer

Heat transfer plays a crucial role in the progression of phase changes of frost on windows. When you turn on the car heater, it's the transfer of heat energy that causes the changes from solid frost to water, and then to vapor. The movement of heat follows from a warmer object (the heater) to a cooler one (the frost-covered window), driving the changes.
Three major mechanisms contribute to heat transfer in this scenario:

• Conduction: Direct transfer of heat through contact between heated air and the window surface, warming the frost.
• Convection: Distribution of heat around the cabin, helping maintain a warm environment for evaporation.
• Radiation: Though less significant here, involves heat being radiated from the heater to the windows.

Understanding these processes helps in comprehending how energy transition facilitates every phase change.