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Phase Changes and Heating Curves

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  • 0:05 Phase Change
  • 1:41 Freezing and Melting
  • 2:43 Sublimation and Deposition
  • 3:53 Heating Curve
  • 4:34 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

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.

Phase Change

The four phases in chemistry
Chemistry Phases

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.

State Change Process Example
solid --> liquid melting ice --> water
solid --> gas sublimating dry ice --> CO2 gas
liquid --> solid freezing water --> ice
liquid --> gas vaporization water --> water vapor
gas --> liquid condensation water vapor --> water
gas --> solid deposition 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.
Deposition Frost

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.

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