Question 1

Which of the following best describes a pure substance as defined in thermodynamics?

Accepted Answer

A substance with a fixed chemical composition throughout, which can exist in more than one phase as long as each phase is homogeneous.

Question 2

At 1 atm pressure, what is the temperature at which water starts to boil, and what is this temperature called?

Accepted Answer

$$100^\circ C$$, called the saturation temperature

Question 3

Which of the following statements about the phase change of water at constant pressure is correct?

Accepted Answer

The temperature remains constant during the phase change from liquid to vapor, while the specific volume increases.

Question 4

What is the term for the state in which a substance exists as a mixture of saturated liquid and saturated vapor?

Accepted Answer

Saturated liquid-vapor mixture

Question 5

On a T-v diagram, what does the critical point represent?

Accepted Answer

The intersection of the saturated liquid line and the saturated vapor line, where the properties of saturated liquid and vapor become identical.

Question 6

Which of the following best describes the triple point of water?

Accepted Answer

The unique combination of temperature and pressure where solid, liquid, and vapor phases coexist in equilibrium.

Question 7

Given a mixture of saturated liquid and saturated vapor, the quality $$x is $$defined as:

Accepted Answer

$$x = \frac{m_{vapor}}{m_{total}}$$

Question 8

If a piston-cylinder device contains water at $$20^\circ C$$ and 1 atm, and heat is added at constant pressure until the temperature reaches $$100^\circ C$$, what is the state of the water just before it starts to vaporize?

Accepted Answer

Saturated liquid

Question 9

Which of the following equations correctly expresses the enthalpy per unit mass in terms of internal energy, pressure, and specific volume?

Accepted Answer

$$h = u + Pv$$

Question 10

In the context of phase diagrams, what does the P-v diagram represent?

Accepted Answer

The relationship between pressure and specific volume, showing regions of different phases and phase transitions.

Question 11

Which of the following best describes the process of vaporization at the molecular level?

Accepted Answer

Molecules in the liquid phase gain enough energy to overcome intermolecular forces and enter the vapor phase.

Question 12

What is the significance of the enthalpy of vaporization $$h_{fg} in $$thermodynamics?

Accepted Answer

It is the amount of energy needed to vaporize a unit mass of saturated liquid at a given temperature or pressure.

Question 13

A sample of water is at its critical point. Which of the following statements is true?

Accepted Answer

The properties of saturated liquid and saturated vapor are identical.

Question 14

Which of the following is NOT a correct phase change process as shown on a P-T diagram?

Accepted Answer

Vapor $$\rightarrow$$ solid: melting

Question 15

If the quality $$x of a $$saturated liquid-vapor mixture is 0.25, what does this mean?

Accepted Answer

25% of the mixture is saturated vapor and 75% is saturated liquid.

Question 16

Which of the following best explains why the temperature remains constant during the boiling of water at 1 atm?

Accepted Answer

The heat added is used to overcome intermolecular forces and convert liquid to vapor, not to increase temperature.

Question 17

Which property is NOT identical for saturated liquid and saturated vapor at the critical point?

Accepted Answer

Question 18

A P-v diagram for water shows a dome-shaped region. What does the region under the dome represent?

Accepted Answer

Mixture of saturated liquid and saturated vapor

Question 19

Which of the following is true about the specific volume of a substance during the phase change from liquid to vapor at constant pressure?

Accepted Answer

It increases.

Question 20

In a real-world scenario, why does water boil at a lower temperature on top of Mount Everest compared to sea level?

Accepted Answer

The atmospheric pressure is lower, so the saturation temperature is lower.

Phase Changes and Properties of Pure Substances in General Chemistry

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