AP Chemistry : Types of Phase Changes

Study concepts, example questions & explanations for AP Chemistry

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Types Of Phase Changes

Liquid water at room temperature is cooled to -75

Which of the following descriptions applies to the process outlined above?

I: A physical change occurred

II: A chemical change occurred

III: There was a change in entropy

Possible Answers:

I,II, and III

II and III

I only

II only

I and III

Correct answer:

I and III

Explanation:

A physical change is a process where the physical properties of a substance are changed.  Here water undergoes a phase change from the more disordered liquid state to the more ordered solid state, which is indeed a physical change.  A chemical change describes when a substance undergoes a change in the identity of its constituent molecules.  Here although the water has gone from the liquid phase to the solid phase it is still composed only of water molecules so a chemical change has not occured.  Entropy describes the degree to which a system is ordered.  Here we do have a change in entropy becasue the liquid water was more disordered than the solid water. 

Example Question #2 : Types Of Phase Changes

A large number of molecules begin moving very fast, rarely bumping into one another, and taking up the entire space available to them. As the temperature drops and the kinetic energy of the particles decreases, the particles move more slowly and run into each other more often. They eventually form a lattice structure, only slightly moving. What is this process called?

Possible Answers:

Freezing

Sublimination, then freezing

Condensation, then freezing

Decrease in temperature

Vaporization, then condensation

Correct answer:

Condensation, then freezing

Explanation:

This is a description of condensation from the gas phase to the liquid phase, then freezing from the liquid phase to the solid phase.

Example Question #3 : Types Of Phase Changes

A phase diagram is used to show the phase that a compound will be in depending upon the environmental temperature and pressure.

Carbon dioxide will sublimate from a solid to a gas at room temperature. What would be necessary in order to cause solid carbon dioxide to melt?

Possible Answers:

Carbon dioxide cannot be melted

Increase the temperature

Decrease the pressure

Decrease the temperature

Increase the pressure

Correct answer:

Increase the pressure

Explanation:

At a pressure of one atmosphere, carbon dioxide can only exist in the solid phase or the gaseous phase. Solid carbon dioxide will sublimate at standard conditions.

Raising the temperature will simply make the carbon dioxide sublimate at a faster rate. In order to allow the solid carbon dioxide to melt into the liquid phase, the carbon dioxide must be subjected to a greater amount of pressure. This will shift equilibrium away from the gaseous state and toward the liquid state.

Example Question #4 : Types Of Phase Changes

A solution of water is at 0 degrees Celcius and 3atm. The pressure is constantly decreased until it was well below 0.000001atm. What will happen to the solution? 



Fullsizerender

Possible Answers:

The ice will turn straight into water vapor

The liquid water will turn into ice, then into water vapor

The ice will turn into liquid, then into water vapor

The water starts off as a vapor, and will remain as a vapor

The liquid water will turn into water vapor

Correct answer:

The liquid water will turn into ice, then into water vapor

Explanation:

Whenever you see a problem like this, immediately draw a mental point on the phase diagram to establish your starting conditions. In our case, the conditions fall in the range of the liquid phase of the water. This allows us to eliminate some of our answer options.

Next, determine what change is occuring and on which axis the change is occuring. We are decreasing pressure, so you are going to need to move down on the y-axis towards 0. Draw an imaginary line from the initial point to where the final point should be. In our case, the final pressure is extremely small. This point is not on the y-axis, but we can use our inference/inductive skills to determine that we will pass through the solid phase (ice) as we approach the gas phase (water vapor). Even if you didn't know the water would become vaporized, based on the graph you knew that the liquid phase had to be followed by the solid phase, and only one answer choice has that option. 

Example Question #21 : Phase Changes

If the boiling point of Selenium (Se) is , in what phase must the element be at ?

Possible Answers:

Plasma

Liquid

Gas

Solid

Not enough information to determine

Correct answer:

Not enough information to determine

Explanation:

We know that the temperature is well below selenium's boiling point, but because we do not know its melting point, we cannot be certain if the temperature has dropped low enough for the element to be a solid.

Example Question #22 : Phase Changes

Dry ice at room temperature undergoes the following reaction:

  

What is the name for the type of phase change where a solid changes directly to a gas (without any phase change in between)?

Possible Answers:

Evaporation

Deposition

Sublimation

Vaporization

Freezing

Correct answer:

Sublimation

Explanation:

When matter changes from one physical state (solid, liquid, gas, or plasma) to a different physical state, a phase change is defined to have occurred.

When a solid changes to a gas, the phase change that occurred is defined as sublimation:

Solid  Gas

When the opposite occurs and a gas changes to a solid, the phase change is defined as deposition:

Gas  Solid

When a liquid changes to a gas, the phase change that occurred is defined as vaporization. It is worth noting that there are two types of vaporization: evaporation (when the phase change occurs below the boiling point for the substance) and boiling (when the phase change occurs at or above the substance's boiling point).

Liquid  Gas

When the opposite occurs and a gas changes to a liquid, the phase change is defined as condensation:

Gas  Liquid

When a liquid changes to a solid, the phase change that occurred is defined as freezing:

Liquid  Solid

When the opposite occurs and a solid changes to a liquid, the phase change is defined as melting:

Solid  Liquid

When a gas changes to a plasma, the phase change that occurred is defined as ionization:

Gas  Plasma

When the opposite occurs and a plasma changes to a gas, the phase change that occurred is defined as recombination.

Plasma  Gas

Some may find it easiest to remember the phase changes by memorizing them in pairs. For example freezing and melting both refer to phase changes between solids and liquids. Once that is memorized, the possible phase changes that you have to choose from is reduced from 8 to 2. Also, it may be helpful to think that all the phase changes involving a decrease in entropy (increase in order) deposition, condensation, freezing, and recombination sound like more orderly words than the phase changes involving an increase in entropy (decrease in order) sublimation, vaporization, melting and ionization.

Example Question #23 : Phase Changes

How does the boiling point of water at sea level compare to its boiling point at an elevation of 3000 feet?

Possible Answers:

It is impossible to determine without knowing the specific heat capacity of water

The boiling point of water at sea level will be smaller

The boiling point of water at sea level will be greater

The boiling point of water will be the same at both locations

Correct answer:

The boiling point of water at sea level will be greater

Explanation:

This question is asking us about the effect that elevation has on the boiling point of water. In order to answer this, we need to have a fundamental understanding of what boiling is, and how it can be affected by factors such as elevation.

Boiling is a type of phase transition in which a liquid is converted into a gas. During this transition, the boiling point is defined as the temperature at which the vapor pressure of the liquid is exactly equal to the atmospheric pressure. Therefore, if the atmospheric pressure is lowered, which happens at increased elevations, then it becomes easier for the liquid to evaporate into the gas phase. Consequently, less energy (and a lower temperature) is needed to push the liquid into the gas phase. As a result, the boiling point of the solution is lowered at increased elevations.

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: