(POM) Phase Changes Lesson
Phase Changes
Matter is always changing states or phases. Meaning, matter can change from a solid to a liquid or a liquid to a gas and more combinations. In this lesson, we will look at how matter changes from one state (or phase) to another.
Changes of state are physical changes in matter. They are reversible changes that do not involve changes in matter's chemical makeup or chemical properties. Common changes of state include melting, freezing, sublimation, deposition, condensation, and vaporization. These changes are shown in the picture below.
Energy is always involved in changes of state. Matter either loses or absorbs energy when it changes from one state to another. For example, when matter changes from a liquid to a solid, it loses energy. The opposite happens when matter changes from a solid to a liquid. For a solid to change to a liquid, matter must absorb energy from its surroundings.
Let's think about the changes of state in terms of the kinetic theory of matter. As stated above, matter changes from a liquid to a solid when it loses energy. The particles of a liquid have energy and are free to flow around one another. When energy is removed or taken away from a liquid the particles have less energy and come closer together. Thus, they change from a liquid state to a solid state.
The opposite is true when matter changes from a solid to a liquid. The particles of a solid have little energy and are vibrating in place. When energy is added, the particles speed up and start moving faster. There are more collisions between the particles, which cause them to spread out and flow over one another. Thus, they change from a solid state to a liquid state.
Freezing
Think about how you would make ice cubes in a tray. First, you would fill the tray with water from a tap. Then you would place the tray in the freezer compartment of a refrigerator. The freezer is very cold. The warmer water in the tray loses heat to the colder air in the freezer. The water cools until its particles no longer have enough energy to slide past each other. Instead, they remain in fixed positions, locked in place by the forces of attraction between them. The liquid water has changed to solid ice.
The process in which a liquid changes to a solid is called freezing. The temperature at which a liquid changes to a solid is its freezing point. The freezing point of water is 0°C (32°F). Other types of matter may have higher or lower freezing points. For example, the freezing point of iron is 1535°C. The freezing point of oxygen is -219°C.
Melting
If you took ice cubes out of a freezer and left them in a warm room, the ice would absorb energy from the warmer air around it. The energy would allow the particles of frozen water to overcome some of the forces of attraction holding them together. They would be able to slip out of the fixed positions they held as ice. In this way, the solid ice would turn into liquid water.
The process in which a solid changes to a liquid is called melting. The melting point is the temperature at which a solid changes to a liquid. For a given type of matter, the melting point is the same as the freezing point.
Vaporization
If you fill a pot with cool tap water and place the pot on a hot stovetop, the water heats up. Heat energy travels from the stovetop to the pot, and the water absorbs the energy from the pot. If water gets hot enough, it starts to boil. Bubbles of water vapor form in boiling water. This happens as particles of liquid water gain enough energy to completely overcome the force of attraction between them and change to a gaseous state. The bubbles rise through the water and escape from the pot as steam.
The process in which a liquid boils and changes to a gas is called vaporization. The temperature at which a liquid boils is its boiling point. The boiling point of water is 100°C (212°F). Other types of matter may have higher or lower boiling points. For example, the boiling point of table salt is 1413°C. The boiling point of nitrogen is -196°C.
Evaporation
A liquid can also change to a gas without boiling. This process is called evaporation. It occurs when particles at the exposed surface of a liquid absorb just enough energy to pull away from the liquid and escape into the air. This happens faster at warmer temperatures.
Condensation
If you take a hot shower in a closed bathroom, the mirror is likely to "fog" up. The "fog" consists of tiny droplets of water that form on the cool surface of the mirror. Why does this happen? Some of the hot water from the shower evaporates, so the air in the bathroom contains a lot of water vapor. When the water vapor contacts cooler surfaces, such as the mirror, it cools and loses energy. The cooler water particles no longer have enough energy to overcome the forces of attraction between them. They come together and form droplets of liquid water.
The process in which a gas changes to a liquid is called condensation. Other examples of condensation are shown in the picture below. A gas condenses when it is cooled below its boiling point.
Examples of Condensation description Links to an external site.
Sublimation
Solids that change to gases generally first pass through the liquid state. However, sometimes solids change directly to gases and skip the liquid state. The reverse can also occur. Sometimes gases change directly to solids.
The process in which a solid changes directly to a gas is called sublimation. It occurs when the particles of a solid absorb enough energy to completely overcome the force of attraction between them. Dry ice (solid carbon dioxide, CO2) is an example of a solid that undergoes sublimation.
Deposition
The opposite of sublimation is deposition. This is the process in which a gas changes directly to a solid without going through the liquid state. It occurs when gas particles become very cold. For example, when water vapor in the air contacts a very cold windowpane, the water vapor may change to tiny ice crystals on the glass. The ice crystals are called frost. You can see an example in the picture below.
Summary
In order for matter to undergo changes in state, energy must either be added or removed from the system. Heat is a form of energy. The addition or removal of energy affects the movement of the particles. As heat is added, the particles speed up and spread out. Heat must be added for a solid to change to a liquid (melting), a liquid to a gas (vaporization and evaporation), and a solid to a gas (sublimation). As heat is removed, the particles slow down and come closer together. Heat must be removed for a gas to change to a liquid (condensation), a liquid to change to a solid (freezing), and a gas to change to a solid (deposition).
[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION