Learn the phases - gas, liquid, solid and plasma. Learn what a phase diagram is and what terms describe the movement between phases: melting, sublimating, freezing, vaporization, condensation, and deposition. Discover why heating curves have plateaus, what a phase transition is, and what happens during this transition.
The four phases in chemistry
I love to drink ice water. The colder and icier the water, the better it is to me. On a hot summer day, I make huge glasses of it and sip on them all day. The glass starts out with mostly ice and a little water. As the hot day progresses, the ice melts and the outside of the glass gets wet. The water in my glass is going through several different phases. From solid ice to liquid water as it melts, to vapor as it evaporates, and the wetness on the outside of the glass is from condensation as the water vapor condenses back on the cold surface of the glass.
In chemistry, phase refers to one of the four states that a substance can exist in; liquid, solid, gas, or plasma. A phase has uniform composition and properties. A phase change is when a substance changes from one phase to another. Every change has a different name. As you can see from the table, every state change has a process name.
|solid --> liquid
|| ice --> water
|solid --> gas
|| dry ice --> CO2 gas
|liquid --> solid
|| water --> ice
|liquid --> gas
|| water --> water vapor
|gas --> liquid
|| water vapor --> water
|gas --> solid
|| water vapor --> ice
Freezing and Melting
Freezing is when a liquid changes phase and becomes a solid, like when the water you put in the ice cube tray turns solid in the freezer. Freezing involves the loss of energy in the form of heat.
Liquid - energy --> solid
At freezing, not only is there a loss of energy, but there is also an increase in the order of the particles in the substance.
Melting is the opposite, when a solid changes phase and becomes a liquid. Melting is when the ice in your drink turns to liquid.
Solid + energy --> liquid
As a substance melts, it becomes less ordered.
At first when you add energy to a solid, the temperature doesn't increase. The reaction of Solid + energy <--> liquid is in equilibrium. The added energy causes the reaction equilibrium to shift to the right until the entire substance turns to liquid. Only then does the increased energy added increase the temperature of the substance.
Sublimation and Deposition
At certain temperature and pressure conditions, some substances can't exist as liquids. In these conditions, a substance in solid phase will move directly to a gas without going through the liquid phase. When a substance changes phase directly from a solid to a gas, it is called sublimation. An example of sublimation is when dry ice turns directly into gas.
Frost is an example of deposition.
When the opposite happens, the change of state from a gas to a solid without going through the liquid phase, it is called deposition. An example of deposition is frost on a window.
Vaporization and Condensation
The process of a substance in liquid phase going to a gas phase is called vaporization. During vaporization, molecules escape the surface tension at the surface of a liquid and enter a gas phase. Boiling is the changing of a liquid to a vapor or gas at boiling temperature. There is also evaporation, which is when particles escape the surface of a non-boiling liquid.
Condensation is when a substance in the gas phase moves to the liquid phase. This is what happens with water on the outside of your glass on a hot day.
A heating curve shows what happens to a substance as heat is applied. It is a plot of temperature vs. time. You'll notice that the curve increases, then plateaus, then increases, then plateaus, before a final increase. At the plateaus, melting or vaporization is taking place.
Example of a heating curve
While the substance is transitioning between a solid and a liquid, adding heat does not increase the temperature until the whole substance has changed phase. The same thing happens between liquid and vaporization. These plateaus are called phase transition.
In chemistry, phase refers to one of the four states that a substance can exist in; liquid, solid, gas, or plasma. A phase has uniform composition and properties. A phase change is when a substance changes from one phase to another.
Freezing is when a liquid changes phase and becomes a solid. Freezing involves the loss of energy in the form of heat. At freezing, not only is there a loss of energy, but there is also an increase in the order of the particles in the substance. Melting is the opposite, when a solid changes phase and becomes a liquid.
Sublimation is when a substance changes phase directly from a solid to a gas, and deposition is when a gas goes directly to a solid without going through the liquid phase. The process of a substance in liquid phase going to a gas phase is called vaporization. Condensation is when a substance in the gas phase moves to the liquid phase.
At the end of this lesson, you'll be able to:
- List the four states of matter
- Describe what happens during each of the six phase changes: freezing, melting, sublimation, deposition, vaporization, and condensation
- Explain what the plateaus on a heating curve mean