Phases

Question 1

Define:

a phase of matter

Answer 1

A phase is a physically distinct form of a substance that can be separated from another form. Generally “phase” is used interchangeably with “state of matter.”

Solid, liquid, and gas are the three phases of matter that the MCAT will test. Though plasmas are an additional state of matter, the MCAT does not explicitly test them.

Question 2

Which two state functions can be used to actively change the phase of a substance?

Answer 2

Temperature can be added or removed. In general, an increase in temperature drives the phase change solid⇒liquid⇒gas.

Pressure (volume) can be increased or decreased. In general, an increase in pressure (or decrease in volume) drives the phase change gas⇒liquid⇒solid.

Question 3

What name is associated with a phase change from

1. solid to liquid?
2. liquid to solid?

Answer 3

1. A change from solid to liquid is melting, or fusion.
2. A change from liquid to solid is freezing, or solidification.

Question 4

What name is associated with a phase change from

1. gas to liquid?
2. liquid to gas?

Answer 4

1. A change from gas to liquid is condensation.
2. A change from liquid to gas is boiling, or vaporization.

Question 5

What name is associated with a phase change from

1. gas to solid?
2. solid to gas?

Answer 5

1. A change from gas to solid is deposition.
2. A change from solid to gas is sublimation.

Question 6

What phase is the substance in regions A, B, and C in the diagram below?

Answer 6

A represents the region of the solid phase, while B is the liquid phase and C is the gas phase.

Question 7

What phase conversions are being shown with the arrows marked A and B?

Answer 7

A is fusion (or melting), while B is freezing (or solidification).

Question 8

What phase conversions are being shown with the arrows marked A and B?

Answer 8

A is vaporization (or boiling), while B is condensation.

Question 9

What phase conversions are being shown with the arrows marked A and B?

Answer 9

A is sublimation, while B is deposition.

Question 10

What points are the arrows marked A and B pointing to?

Answer 10

A is the triple point; B is the critical point.

Question 11

Explain the significance of the critical point.

Answer 11

The critical temperature and critical pressure combine to form the critical point.

• The critical temperature is the temperature above which a distinct liquid-to-gas vaporization can no longer be accurately determined.
• The critical pressure is the pressure above which a distinct gas-to-liquid condensation can no longer be accurately determined.

Question 12

Explain the significance of the triple point.

Answer 12

The triple point is the point at which a substance can exist in equilibrium in all three states (solid, liquid, and gas).

For example, water at its triple point exists as ice, liquid water, and steam - all at one temperature and pressure. A substance at its triple point can instantaneously interconvert between any phase.

Question 13

How does the phase diagram of water differ from the one below?

Answer 13

The solid/liquid boundary for water has a slight negative slope.

This occurs because water’s solid phase is less dense than its liquid phase.

Question 14

Different materials require different amounts of heat in order to transition into a new phase. Why is this?

Answer 14

This difference exists due to intermolecular forces.

The attraction of molecules to each other determines the temperature at which a substance will change phase, and subsequently dictates the amount of heat necessary to undergo that transition (termed the heat of vaporization or the heat of fusion, depending on the phase change).

Question 15

Name three types of intermolecular forces.

Answer 15

From strongest to weakest, the intermolecular forces are hydrogen bonds, dipole-dipole forces, and London dispersion forces.

Intermolecular forces are attractive forces between separate molecules, not actual bonds.

Question 16

Define:

dipole-dipole forces

Answer 16

Dipole-dipole forces occur when a molecule has a net dipole moment. The molecule’s dipole is attracted to the dipoles of other molecules in the same sample.

This attraction exists due to Coulombic forces; the positive end of the dipole is attracted to negative charges and vice versa.

Question 17

Define:

hydrogen bonds

Answer 17

Hydrogen bonds are an extreme form of dipole forces. They occur when a hydrogen atom is covalently bonded to O, N, or F.

The high electronegativity difference between these atoms and hydrogen creates strong dipoles, which result in the strongest dipole-dipole interactions between molecules.

Question 18

Define:

London dispersion forces

Answer 18

London dispersion forces are attractions between nonpolar molecules as a result of instantaneous dipoles.

While nonpolar molecules do not contain permanent dipoles, their electron clouds can reorganize and lead to partial polarization. The more valence electrons present in a molecule, the larger the instantaneous dipole can be.

Question 19

How do intermolecular forces affect the boiling point of a substance?

Answer 19

The stronger the intermolecular forces present, the higher the boiling point. In other words, the more tightly the molecules are attracted in the liquid phase, the more difficult the substance is to vaporize.

In the absence of other factors, hydrogen bonding correlates with the highest boiling points, followed by dipole-dipole forces. Nonpolar molecules, which only exhibit London dispersion forces, boil the most easily.

Question 20

List the following forces from weakest to strongest: dipole-dipole forces, hydrogen bonding, London dispersion forces.

Answer 20

1. London dispersion forces (weakest)
2. dipole-dipole forces
3. hydrogen bonding (strongest)

Question 21

Define:

heat of vaporization (H_vap)

Answer 21

The heat of vaporization (H_vap) represents the energy necessary to convert one mole of a substance from the liquid phase to the gas phase at constant pressure.

Note that this process takes place at constant temperature, and is represented on a heating curve by a plateau region. ΔH_vap is always positive, since vaporization is an endothermic process.

Question 22

Define:

heat of fusion

Answer 22

The heat of fusion represents the energy required to melt one mole of a substance from the solid phase to the liquid phase at constant pressure.

Note that this process also takes place at constant temperature, and is represented on a heating curve by a plateau region. ΔH_fus is always positive, since melting is an endothermic process.

Question 23

What processes are associated with the plateaus labeled A and B on the general heating curve below?

Answer 23

• At A, fusion is occurring.
  H_fusion can used to calculate the heat needed for this process.
• At B, vaporization is occurring.
  H_vap can be used to calculate the heat needed for this process.

Question 24

In the diagram below, why are the slopes zero for the plateaus labeled A and B?

Answer 24

A slope of zero means that no temperature change is occurring. At these regions, the substance is undergoing a phase change, so any heat input is dedicated to breaking bonds between molecules instead of increasing temperature.

For example, during the vaporization of water, heat is needed to break the hydrogen bonds between liquid water molecules and convert it to gaseous steam.

Question 25

In the diagram below, what equation determines the heat added in the regions where the temperature is rising?

Answer 25

q = mcΔT

Where:

q = heat required (J)
m = mass of the sample (g)
c = specific heat of the substance (J/g*K)
ΔT = temperature change (K)

Question 26

What is the value of the specific heat (c) of water?

Answer 26

The specific heat of water is 1 cal/g*K.

In SI units, c ≈ 4.18 J/g*K.

Question 27

In the diagram below, what equation can be used to calculate the heat required throughout the plateau regions?

Answer 27

q = mH_L

Where:

q = heat required (J)
m = mass (g)
H_L = latent heat of phase change (J/g)

This formula calculates the heat required to change a certain mass of a substance from one phase to another. For the first plateau, H_L represents the heat of fusion; for the second plateau, it represents the heat of vaporization.

Question 28

How much heat is required to increase the temperature of 20 grams of water by 5 degrees, starting at 20 ºC?

Answer 28

100 cal is required.

At 20 ºC, water is a liquid and has a specific heat of 1 cal/gºC.

Since temperature is changing within this temperature range but phase is not, use the formula q = mcΔT.

q = 20 (1) (5) = 100 cal

Question 29

Define:

colligative property

Answer 29

A colligative property of a mixture is one that depends only on the number of molecules of solute dissolved in the solvent, not the solute’s chemical nature.

Vapor pressure, boiling point, freezing point, and osmotic pressure are all colligative properties.

Question 30

What relationship exists between a liquid’s vapor pressure and its boiling point?

Answer 30

The boiling point is the temperature in which the vapor pressure above a liquid equals the surrounding atmospheric pressure.

To boil a liquid, either its vapor pressure must be increased (usually by heating), or the atmospheric pressure must be decreased.

Question 31

Define:

mole fraction

Answer 31

Mole fraction is the ratio of moles of a particular substance to the total moles of all substances present.

The formula for mole fraction is:

x_A = n_A / n_total

Where:

x_A = mole fraction of A
n_A = number of moles of A
n_total = total number of moles of all substances present

Question 32

Define:

Raoult’s Law

Answer 32

Raoult’s Law defines the pressure above a mixture of liquids.

The formula for Raoult’s Law is:

P_total = P⁰_Ax_A + P⁰_Bx_B + …

Where:

P_total = total pressure above the mixture
P⁰_A = vapor pressure of pure substance A
x_A = mole fraction of substance A in the mixture

Each term represents the partial pressure above the mixture of one of the components.

Question 33

What is the partial pressure of acetone above a 50/50 molar mixture of water and acetone at STP, if the pure vapor pressure of acetone at STP is 100 torr?

Answer 33

The partial pressure of acetone above the mixture is 50 torr.

Raoult’s Law: P_total = P⁰_Ax_A + P⁰_Bx_B + …

The partial pressure of a liquid above a mixture is equal to the vapor pressure of the pure liquid times its mole fraction in the mixture. In a 50/50 mixture of water and acetone, each substance will only contribute half of the vapor pressure of the pure compound; for that reason, acetone’s partial pressure will be half of its pure vapor pressure.

Question 34

Define:

non-ideal solution

Answer 34

A non-ideal solution is one that does not follow Raoult’s Law. Non-ideal solutions can have pressures either greater or less than those predicted by Raoult’s Law.

For example, a solution with large attractive forces between solute and solvent, such as salt water, will have a lower vapor pressure than that predicted by Raoult’s Law.

Question 35

Define:

van’t Hoff factor (i)

Answer 35

The van’t Hoff factor (i) is the number of particles that an ionic solute yields upon dissociation.

For example, NaCl dissociates into Na⁺ and Cl^- ions, a total of 2 per equivalent of NaCl. The van’t Hoff factor for NaCl is i = 2.

Question 36

What are the values of the van’t Hoff factor for:

1. KOH
2. C₆H₁₂O₆
3. H₂SO₄
4. CaCl₂

Answer 36

1. KOH: i = 2 (K⁺ and OH^-)
2. C₆H₁₂O₆: i = 1
3. H₂SO₄: i = 3 (2H⁺ and SO₄^2-)
4. CaCl₂: i = 3 (2Cl^- and Ca²⁺)

Note that glucose, like other covalent compounds, does not dissociate.

Question 37

Define:

molality (m)

Answer 37

Molality represents the number of moles of solute dissolved into one kilogram of solvent.

Molality uses the lowercase letter “m” in equations; be careful not to confuse this with mass. Also, note that the molality is measured per kilogram of solvent, not total solution.

Question 38

What equation can be used to find the boiling point elevation of a solvent after solute has been added?

Answer 38

ΔT_b = K_bmi

Where:

K_b= a solvent-specific constant (kg*K/mol)
m = the molality of the solution (mol/kg)
i = the van’t Hoff factor of the solute
ΔT_b = the temperature change of the solvent’s boiling point (K)

Question 39

Which will have a higher boiling point: a 1 m NaCl solution or a 1 m glucose solution?

Answer 39

The NaCl solution will have the higher boiling point.

Remember that ΔT_b = K_bmi. The i value for NaCl is 2, while the value for glucose is 1. The equimolar NaCl solution will thus have twice the boiling point elevation.

Question 40

When can the equation for boiling point elevation not be used?

Answer 40

The boiling point elevation equation cannot be used when:

• the mixture is volatile
• the solute does not fully dissolve or dissociate
• a significant density difference exists between the mixture components, causing one to sit on top of the other

Question 41

How does addition of a solute affect the freezing point of a liquid?

Answer 41

The freezing point of a liquid will be lowered, as the solute hinders proper crystallization of the solvent molecules.

Freezing point depression is a colligative property.

Question 42

What equation can be used to find the freezing point depression of a solvent after solute has been added?

Answer 42

ΔT_f = K_fmi

Where:

K_f = a solvent-specific constant (kg*K/mol)
m = the molality of the solution (mol/kg)
i = the van’t Hoff factor of the solvent
ΔT_f = the temperature change of the solvent’s freezing point (K)

Question 43

Which will have a lower freezing point, a 1 m CaCl₂ solution or a 2 m CaCl₂ solution?

Answer 43

The freezing point of the 2 m solution will be lower.

Based on the equation ΔT_f = K_fmi, the 2 m solution will have twice the drop in freezing point of the 1 m solution.

Question 44

A chemist wants to melt the ice on her driveway on a very cold day, and has equal moles of each of the following substances to add to the ice. Based on colligative properties, which would work the best?

1. H₃PO₄
2. MgCl₂
3. C₁₂H₂₂O₁₁

Answer 44

H₃PO₄ will melt the ice the most effectively.

Assuming equimolal addition, H₃PO₄ will yield the largest decrease in the water’s freezing point because it has the highest van’t Hoff factor.

1. The i value of H₃PO₄, or phosphoric acid, is 4.
2. The i value of MgCl₂, or magnesium chloride, is 3.
3. The i value of C₁₂H₂₂O₁₁, or sucrose, is 1.

Question 45

Which of the following aqueous solutions will have the lowest freezing point?

1. 1 m BrCl
2. 0.5 m Na₂SO₃
3. 1.5 m KCl

Answer 45

1.5 m KCl (aq) has the lowest freezing point.

Freezing point depression is dependent on molality and the number of solute atoms per kilogram of water.

1. BrCl = 1 m x 2 ions = 2
2. Na₂SO₃ = 0.5 m x 3 ions = 1.5
3. KCl = 1.5 m x 2 ions = 3

Question 46

How are hypertonic, isotonic, and hypotonic solutions defined?

Answer 46

A solution’s tonicity is defined relative to another solution, often referred to as the “standard.”

• A hypertonic solution has a higher solute concentration than the standard.
• An isotonic solution has the same solute concentration as the standard.
• A hypotonic solution has a lower solute concentration than the standard.

Question 47

Define:

osmotic pressure

Answer 47

Osmotic pressure is the pressure that results from a tonicity difference across a semipermeable membrane.

Water molecules will be drawn across the membrane toward the hypertonic side, in an attempt to promote isotonicity. For example, if red blood cells are placed in a hypotonic environment, water will flow into the cells and cause them to expand.

Question 48

What equation can be used to calculate osmotic pressure?

Answer 48

π = nRTi
V

Where:

Π = osmotic pressure (atm)
n = number of moles of solute
R = ideal gas constant (L-atm/K*mol)
T = temperature (K)
i = the van’t Hoff factor of the solute
V = volume (L)

Question 49

At a constant temperature, how will the osmotic pressure of a 1 M NaCl solution compare to that of a 1 M CaCl₂ solution?

Answer 49

The osmotic pressure of the CaCl₂ solution will be higher.

CaCl₂ has a van’t Hoff factor of 3 (Ca⁺², 2 Cl^-) compared to 2 for NaCl (Na⁺, Cl^-). Since Π = iMRT, and M,R,T are all constant, the van’t Hoff factors determine the osmotic pressure difference.

Question 50

How will the osmotic pressure of a solution change when its temperature is increased?

Answer 50

The osmotic pressure will be greater at a higher temperature.

Since Π = iMRT, and i,M,R are all constant, the difference between the new and old temperatures determines the osmotic pressure difference. Osmotic pressure and temperature are directly proportional.

Question 51

Define:

colloid

Answer 51

A colloid is a system in which one substance is microscopically dispersed evenly throughout another substance.

Colloidal particles will not settle to the bottom due to gravity and time. Colloids can occur in any of the three phases of matter.

For example, air is a gaseous colloid composed of many gases, milk is a colloid of liquid butter fat in aqueous solution, and colored glass is a solid colloid of metal oxides in a silica matrix.

Question 52

What common characteristics distinguish colloids?

Answer 52

• Colloids have a continuous phase, or dispersion medium, and an internal phase, or dispersed medium.
• The internal phase particles are too small to easily extract physically, though it is possible to extract them chemically.
• Liquid and solid colloids scatter light; in other words, they are opaque or translucent. Only gas colloids can be colorless.

Question 53

What values define Standard Temperature and Pressure (STP)?

Answer 53

STP conditions require a temperature of 0ºC (273 K) and a pressure of 1 atm.

The MCAT occasionally refers to standard state conditions, which (among other stipulations) require a temperature of 25ºC and a pressure of 1 atm.

Question 54

List the assumptions of the kinetic molecular theory of gases.

Answer 54

The kinetic molecular theory assumes that gases act in an ideal fashion. Assumptions of this theory include:

1. A gas molecule has no volume of its own.
2. The collisions between gas molecules are completely elastic, so molecules do not lose energy due to dissipative forces.
3. The average kinetic energy of gas molecules is directly proportional to the temperature of the gas.

Question 55

Define:

absolute temperature

Answer 55

The absolute temperature is any temperature that is given in Kelvin (K).

Absolute zero, or 0 K, is the temperature at which all molecules cease movement completely. To convert Kelvin to Celsius:

T_K = T_C + 273.15

Question 56

Define:

Charles’ Law

Answer 56

Charles’ Law states that the volume of a gas is directly proportional to its temperature at constant pressure.

Shown above is the equation for Charles’ Law.

Question 57

If the pressure of an ideal gas system is held constant but the temperature is doubled, what does Charles’ Law predict will happen to the volume?

Answer 57

The system’s volume will also double.

Charles’ Law indicates that at a constant pressure, the temperature and volume of a gas are directly proportional. Remember that you must use temperature in Kelvin, not Celsius, for Charles’ Law calculations.

Question 58

Define:

Boyle’s Law

Answer 58

Boyle’s Law states that the volume of a gas is inversely proportional to its pressure at a constant temperature.

Shown above is the equation for Boyle’s Law.

Question 59

If the temperature of an ideal gas system is held constant but the pressure is reduced by 1/2, what does Boyle’s Law predict will happen to the volume?

Answer 59

The system’s volume will double.

Boyle’s Law indicates that at a constant temperature, the pressure and volume of a gas are inversely proportional. Remember that you must use temperature in Kelvin, not Celsius, for Boyle’s Law calculations.

Question 60

Define:

Avogadro’s Law

Answer 60

Avogadro’s Law states that the volume of a gas is directly proportional to its number of moles at a constant temperature and pressure.

V / n = constant

Where:

V = volume in L
n = number of moles

Question 61

If the pressure and temperature of an ideal gas system are held constant but the number of moles is decreased to 1/3 of the original value, what does Avogadro’s Law predict will happen to the system’s volume?

Answer 61

The system’s volume will also decrease to 1/3 of its original value.

Avogadro’s Law indicates that at a constant pressure and temperature, the number of moles and the volume of a gas are directly proportional.

Question 62

Define:

ideal gas law

Answer 62

The ideal gas law combines Charles’, Boyle’s, and Avogadro’s Laws into one:

PV = nRT

Where:

P = pressure (atm or Pa)
V = volume (L)
n = number of moles
R = the ideal gas constant: 0.082 L(atm)/mol(K) or 8.31 J/mol(K)
T = temperature (K)

Question 63

What is the volume of 1 mole of gas molecules at STP?

Answer 63

At STP, one mole of any gas has a volume of 22.4 L

This is called the standard molar volume, and is true for a mole of any ideal gas at STP. While this value can be calculated using the ideal gas law, it is worth memorizing.

Question 64

How is the partial pressure of a gas in a mixture calculated?

Answer 64

P_A = x_AP_total

Where:

P_A = pressure from gas A
x_A = mole fraction of A (a ratio of the moles of A to the total moles of gas)
P_total = total pressure of the sytem due to all gases present

Question 65

Define:

Dalton’s Law

Answer 65

P_total = P_A + P_B + P_C + …

Dalton’s Law states that the total pressure of a system of ideal gases is equal to the sum of the partial pressures of all gases in the mixture.

Question 66

How is the average kinetic energy of an ideal gas calculated?

Answer 66

KE_avg = (3/2)nRT

Where:

KE_avg = average kinetic energy (J)
n = number of moles of gas
R = ideal gas constant: 0.082 L(atm)/mol(K) or 8.31 J/mol(K)
T = temperature (K)

Question 67

When do gases deviate most from ideal behavior?

Answer 67

Gases deviate from ideal behavior at:

1. low temperatures
2. high pressures (low volume)

Question 68

Why do low temperatures and high pressures cause a gas to deviate from ideal behavior?

Answer 68

Gases deviate from ideal at low temperatures because the molecules have very low kinetic energy and their motion will be affected by intermolecular attractive forces.

Gases deviate from ideal at high pressures, or very low volumes, because the molecules get forced closer together, and will start to exhibit characteristics more like a liquid than a gas.

Question 69

What does the van der Waals equation of state measure?

Answer 69

The van der Waals equation can be used to calculate the degree of deviation from ideal gas behavior.

(P + a(n/V)²)(V - nb) = nRT

Where:

P = pressure (atm or Pa)
V = volume (L)
n = number of moles
R = ideal gas constant: 0.082 L(atm)/mol(K) or 8.31 J/mol(K)
T = temperature (K)
a = attraction between molecules due to intermolecular forces
b = actual volume taken up by gas molecules

Note that you do not need to have this equation memorized, but you should know what it measures.

Question 70

Which will deviate more from ideal behavior: a polar or a nonpolar gas?

Answer 70

Polar gases will deviate more from the ideal state.

The larger the attractive forces, the more a gas will demonstrate real, or nonideal, behavior. If attractive forces are large enough under certain conditions, the gas can condense into a liquid.

Question 71

Which will deviate more from ideal behavior: a real gas made up of small molecules, or a real gas made up of large molecules?

Answer 71

Gases made up of larger molecules will deviate more from ideal behavior.

An ideal gas is assumed to have particles with no volume of their own. The larger the particles of a real gas, the less ideal its behavior can be.