The leaf-like pattern on the glass pictured here is as beautiful as any work of art. It formed on a windowpane on a very cold winter night. Do you know how it formed? The answer is deposition.
What Is Deposition?
Deposition refers to the process in which a gas changes directly to a solid without going through the liquid state. For example, when warm moist air inside a house comes into contact with a freezing cold windowpane, water vapor in the air changes to tiny ice crystals. The ice crystals are deposited on the glass, often in beautiful patterns like the “leaves” on the window above. Be aware that deposition has a different meaning in Earth science than in chemistry. In Earth science, deposition refers to the dropping of sediments by wind or water, rather than to a change of state.
Examples of Deposition in Nature
Deposition as a change of state often occurs in nature. For example, when warm moist air comes into contact with very cold surfaces—such as the ground or objects on the ground—ice crystals are deposited on them. These ice crystals are commonly called frost. Look at the dead leaf and blades of grass in the Figure below. They are covered with frost. If you look closely, you can see the individual crystals of ice. You can watch a demonstration of frost forming on the side of a very cold can at the URL below. (Click on the mulitmedia choice “Ice on a Can.”). The ice in the can has been cooled to a very low temperature by adding salt to it. If you want to do the demonstration yourself, follow the procedure at the URL. http://www.middleschoolchemistry.com/lessonplans/chapter2/lesson4
Q: In places with very cold winters, why might frost be more likely to form on the ground in the fall than in the winter?
A: Frost forms when the air is warmer than the ground. This is more likely to be the case in the fall. In the winter, the air is likely to be as cold as the ground.
Deposition also occurs high above the ground when water vapor in the air changes to ice crystals. In the atmosphere, the ice crystals are deposited on tiny dust particles. These ice crystals form clouds, generally cirrus clouds, which are thin and wispy. You can see cirrus clouds in the Figure below.
Q: Cirrus clouds form only at altitudes of 6 kilometers or higher above sea level. Do you know why?
A: At this altitude, the atmosphere is always very cold. Unless the air is cold, water vapor will condense to form water droplets instead of ice crystals.
- In chemistry, deposition refers to the process in which a gas changes directly to a solid without going through the liquid state.
- Examples of deposition in nature include frost forming on the ground and cirrus clouds forming high in the atmosphere.
Watch the video at the following URL, and then answer the questions below.
- What is dry ice? What gas is used to make dry ice?
- In making dry ice with a fire extinguisher, why is it necessary to have a bag to collect the dry ice?
- How can dry ice be changed back to a gas?
- What is deposition, as defined in chemistry?
- Describe an example of deposition in nature.
- Tom, pictured in the Figure below, started to scrape frost off his windshield before driving to school on a cold morning. Then he decided to let the car’s defroster do the work for him. What conditions caused the windshield to become covered with frost, and how does the defroster clear the frost from the windshield?