Changes of state Knowledge-Based Practice

Questions

1. 1.

   Which of the following changes increases the rate of evaporation of a liquid from an open vessel?

   • A) Decreasing the temperature of the liquid
   • B) Increasing the humidity of the surrounding air
   • C) Decreasing the surface area of the exposed liquid
   • D) Increasing the speed of airflow across the liquid surface
2. 2.

   What term is given to the amount of thermal energy required to change one kilogram of a solid into a liquid without any change in temperature?

   • A) Specific heat capacity
   • B) Specific latent heat of vaporization
   • C) Thermal conductivity
   • D) Specific latent heat of fusion
3. 3.

   Unlike most substances, what happens to the volume and density of pure water as it changes state from a liquid to a solid at its freezing point?

   • A) Its volume decreases and its density increases.
   • B) Its volume increases and its density decreases.
   • C) Its volume decreases and its density decreases.
   • D) Its volume increases and its density increases.
4. 4.

   How does the melting behavior of an impure solid differ from that of a pure crystalline substance?

   • A) It melts at a higher, equally sharp temperature without broadening.
   • B) It melts over a range of temperatures rather than at a single, sharp point.
   • C) It transitions directly into a gas without passing through the liquid state.
   • D) It maintains a completely fixed temperature until the entire sample has melted.
5. 5.

   Why is a change of state, such as melting or boiling, classified as a physical change rather than a chemical change?

   • A) It occurs without any transfer of thermal energy between the substance and its surroundings.
   • B) It alters the intramolecular chemical bonds to produce new substances with distinct formulas.
   • C) The chemical identity and composition of the substance's molecules remain unchanged.
   • D) It can only occur when matter is actively exchanged with the surrounding environment.
6. 6.

   Under what condition does a liquid actively boil throughout its bulk rather than merely evaporating at its surface?

   • A) When the average kinetic energy of the molecules exceeds the latent heat of vaporization
   • B) When the saturated vapour pressure of the liquid equals the external atmospheric pressure
   • C) When the rate of condensation at the surface exceeds the rate of evaporation
   • D) When all intermolecular attractive forces within the liquid are permanently eliminated
7. 7.

   When a pure substance undergoes a change of state in a sealed system, what happens to its total mass?

   • A) It increases during heating because the individual particles expand in size.
   • B) It remains constant because the total number and identity of the particles are conserved.
   • C) It decreases during melting because overcoming intermolecular forces destroys matter.
   • D) It changes in direct proportion to the change in the volume occupied by the substance.
8. 8.

   Why does exposure to steam at 100 °C typically cause a significantly more severe burn than an equal mass of liquid water at 100 °C?

   • A) Steam has a much higher specific heat capacity than liquid water.
   • B) Steam releases a large quantity of latent heat as it condenses into liquid on the skin.
   • C) Steam has a significantly higher thermal conductivity than liquid water.
   • D) Steam is at a higher temperature than the boiling liquid water at atmospheric pressure.
9. 9.

   During which change of state do the particles of a substance break free from fixed positions in a rigid lattice and gain the ability to slide past one another while remaining in close contact?

   • A) Vaporization
   • B) Condensation
   • C) Freezing
   • D) Melting
10. 10.

    On a temperature–time graph recorded while heating a pure substance at a constant rate, what does a horizontal plateau represent?

    • A) A phase transition is occurring at constant temperature as added energy changes intermolecular bonding.
    • B) The substance has reached thermal equilibrium with the heater and stops absorbing energy.
    • C) The average kinetic energy of the particles is increasing without any change in physical state.
    • D) The rate of heat supply is insufficient to cause any structural change among the particles.
11. 11.

    Why does the temperature of a liquid decrease as it undergoes spontaneous evaporation from an open container?

    • A) The molecules with the highest kinetic energy escape the liquid, lowering the average kinetic energy of the remaining molecules.
    • B) Thermal energy is absorbed from the liquid to break the strong intramolecular covalent bonds of the escaping molecules.
    • C) The remaining liquid molecules expand into the empty space left behind, converting their kinetic energy into mass.
    • D) Heat naturally flows from the cooler liquid to the warmer surrounding air during any phase transition.
12. 12.

    When pure water boils vigorously in an open container, what primarily fills the bubbles that form within the liquid and rise to the surface?

    • A) Dissolved atmospheric air that expands and escapes from the liquid
    • B) Water vapor produced by the liquid transitioning into the gas phase
    • C) Separate hydrogen and oxygen gases produced by chemical decomposition
    • D) Pockets of empty space containing a near-complete vacuum
13. 13.

    A pure liquid is freezing at its normal freezing point. How do the average kinetic energy and the microscopic potential energy of its particles change during this process?

    • A) Both the average kinetic energy and the microscopic potential energy decrease.
    • B) The average kinetic energy decreases, while the microscopic potential energy remains constant.
    • C) The average kinetic energy increases, while the microscopic potential energy decreases.
    • D) The average kinetic energy remains constant, while the microscopic potential energy decreases.
14. 14.

    Which statement correctly explains why the specific latent heat of vaporization of a substance is typically much greater than its specific latent heat of fusion?

    • A) Boiling requires increasing the average kinetic energy of the particles, whereas melting only affects their potential energy.
    • B) Particles in the gaseous state experience stronger intermolecular attractive forces than particles in the solid state.
    • C) Vaporization requires overcoming intermolecular attractions almost completely, whereas melting only partially disrupts the rigid structure.
    • D) A substance always experiences a temperature increase during vaporization, while its temperature remains constant during melting.
15. 15.

    How does a decrease in external atmospheric pressure affect the boiling point of a liquid?

    • A) The boiling point decreases because the vapor pressure can match the external pressure at a lower temperature.
    • B) The boiling point increases because molecules require additional thermal energy to overcome the lower pressure.
    • C) The boiling point remains unchanged because phase change temperatures are determined solely by chemical composition.
    • D) The boiling point decreases because the weaker external pressure directly weakens the liquid's intermolecular forces.
16. 16.

    When a pure liquid undergoes vaporization at its normal boiling point, which statement correctly describes what happens at the molecular level?

    • A) The average kinetic energy of the molecules increases as thermal energy is supplied.
    • B) Work is done to overcome intermolecular forces, increasing molecular potential energy while average kinetic energy remains constant.
    • C) The intramolecular covalent bonds within the molecules break, separating them into individual atoms.
    • D) The individual molecules expand significantly in size due to the absorption of latent heat.
17. 17.

    Which statement correctly compares evaporation and boiling in a liquid?

    • A) Boiling occurs only at the surface of the liquid, whereas evaporation takes place throughout the entire volume.
    • B) Evaporation requires the liquid to reach a fixed temperature, whereas boiling happens spontaneously at any temperature.
    • C) Boiling increases the potential energy of molecules, whereas evaporation permanently alters their chemical bonds.
    • D) Evaporation occurs only at the surface across a range of temperatures, whereas boiling occurs throughout the liquid at a specific boiling point.
18. 18.

    Which of the following changes of state releases thermal energy to the surroundings?

    • A) Sublimation
    • B) Vaporization
    • C) Melting
    • D) Condensation
19. 19.

    Which phase transition describes the direct conversion of a substance from a gas into a solid without passing through the liquid state?

    • A) Sublimation
    • B) Condensation
    • C) Vaporization
    • D) Deposition
20. 20.

    What happens to the temperature of a pure crystalline substance while it is actively melting at its normal melting point?

    • A) It steadily increases as thermal energy is continuously absorbed.
    • B) It fluctuates periodically while the crystal lattice breaks apart.
    • C) It remains constant until the entire solid has transitioned into liquid.
    • D) It decreases slightly because latent heat is consumed by the system.

Answers and step-by-step solutions appear after you finish the test.