Measurement of Temperature and Heat, Change of State and Latent Heat, Modes of Heat Transfer
Measurement of Temperature and Heat, Change of State and Latent Heat, Modes of Heat Transfer MCQ Questions
13.
Which of the following is true about the latent heat of vaporization?
A.
It is the heat required to change a liquid into a gas
B.
It is the heat required to change a solid into a liquid
C.
It is the heat released when a gas turns into a liquid
ANSWER :
D. Both a) and c)
Approach
The question asks about the definition of latent heat of vaporization. We need to understand the concept of latent heat and phase changes.
Step-by-step
- Understanding Latent Heat: Latent heat is the heat energy absorbed or released by a substance during a phase change (e.g., solid to liquid, liquid to gas) without a change in temperature.
- Latent Heat of Vaporization: Specifically, the latent heat of vaporization refers to the amount of heat energy required to change a unit mass of a substance from a liquid state to a gaseous state at a constant temperature (its boiling point). This process is called vaporization or boiling.
- Latent Heat of Condensation: Conversely, when a gas changes back into a liquid (condensation), an equal amount of heat energy is released. This is also referred to as latent heat of vaporization, but in the context of release rather than absorption. The magnitude is the same, but the direction of energy transfer is opposite.
- Analyzing the Options:
- Option A states it is the heat required to change a liquid into a gas. This aligns with the definition of latent heat absorbed during vaporization.
- Option C states it is the heat released when a gas turns into a liquid. This aligns with the definition of latent heat released during condensation, which is the reverse process of vaporization and involves the same amount of latent heat.
Correct Option: D) Both a) and c)
The latent heat of vaporization is the energy absorbed when a liquid turns into a gas, and it is also the energy released when a gas turns into a liquid. The magnitude of this energy is the same for both processes.
Incorrect Options
- A) It is the heat required to change a liquid into a gas. — This statement is correct but incomplete as it only covers one aspect (absorption) of the latent heat of vaporization.
- B) It is the heat required to change a solid into a liquid. — This describes the latent heat of fusion, not vaporization.
- C) It is the heat released when a gas turns into a liquid. — This statement is correct but incomplete as it only covers one aspect (release) of the latent heat of vaporization.
14.
What is the latent heat of sublimation?
A.
The heat required to change a solid into a gas
B.
The heat required to change a gas into a solid
D.
The heat required to change a solid into a liquid
ANSWER :
C. Both a) and b)
Approach
The question asks for the definition of the latent heat of sublimation. To answer this, we need to understand the process of sublimation and the concept of latent heat.
Step-by-step
- Sublimation: Sublimation is a phase transition where a substance changes directly from a solid to a gas without passing through the liquid phase. An example is dry ice (solid carbon dioxide) turning directly into carbon dioxide gas.
- Deposition (or Desublimation): Deposition is the reverse process of sublimation, where a substance changes directly from a gas to a solid without passing through the liquid phase. An example is the formation of frost.
- Latent Heat: Latent heat is the energy absorbed or released by a substance during a phase change at a constant temperature. This energy is used to break or form intermolecular bonds, not to increase the kinetic energy of the molecules (which would raise the temperature).
- Latent Heat of Sublimation: Therefore, the latent heat of sublimation refers to the amount of heat energy required to change a unit mass of a solid directly into a gas at a constant temperature and pressure. Conversely, it is also the amount of heat energy released when a unit mass of a gas directly changes into a solid at a constant temperature and pressure. This means it encompasses both the forward (solid to gas) and reverse (gas to solid) processes.
Correct Option:
C) Both a) and b)
- Option A describes the process of a solid changing into a gas, which is sublimation. The heat required for this is the latent heat of sublimation.
- Option B describes the process of a gas changing into a solid, which is deposition (or desublimation). The heat released during this process is also the latent heat of sublimation, just in reverse.
- Since latent heat is associated with the phase change itself, it applies to both the absorption of heat during sublimation and the release of heat during deposition. Therefore, both statements A and B correctly describe aspects related to the latent heat of sublimation.
Incorrect Options
- A) The heat required to change a solid into a gas — This is partially correct, as it describes the energy absorbed during sublimation. However, it doesn't cover the reverse process (deposition), which also involves the same latent heat.
- B) The heat required to change a gas into a solid — This is partially correct, as it describes the energy released during deposition. However, it doesn't cover the forward process (sublimation), which also involves the same latent heat.
- D) The heat required to change a solid into a liquid — This describes the latent heat of fusion, not sublimation. Fusion is the process of melting, where a solid changes into a liquid.
15.
Which of the following statements about latent heat is true?
A.
It is the heat required to change the state of a substance without changing its temperature
B.
It is the heat required to raise the temperature of a substance
C.
It is the heat released during a phase change
D.
It is the heat absorbed during a phase change
ANSWER :
A. It is the heat required to change the state of a substance without changing its temperature
Approach
The question asks for the correct definition of latent heat. We need to understand the concept of latent heat and how it differs from sensible heat, particularly in the context of phase changes.
Step-by-step
- Define Latent Heat: Latent heat is the energy absorbed or released by a substance during a phase transition (e.g., melting, freezing, boiling, condensation) without a change in its temperature. This energy is used to break or form intermolecular bonds, rather than increasing the kinetic energy of the molecules.
- Consider Phase Change: When a substance undergoes a phase change (e.g., ice melting into water at \(0^\circ \text{C}\), or water boiling into steam at \(100^\circ \text{C}\)), heat is either absorbed (endothermic process like melting or boiling) or released (exothermic process like freezing or condensation). During this process, the temperature of the substance remains constant until the entire substance has changed its state.
- Distinguish from Sensible Heat: Sensible heat, on the other hand, is the heat absorbed or released by a substance that results in a change in its temperature without a change in its state.
- Evaluate the Options:
- Option A states that latent heat is the heat required to change the state of a substance without changing its temperature. This aligns perfectly with the definition of latent heat.
- Option B describes sensible heat, not latent heat.
- Option C and D describe aspects of latent heat (released/absorbed during phase change) but miss the crucial detail that the temperature remains constant during this process. Latent heat can be absorbed or released, but the defining characteristic is the constant temperature during the state change. Option A is the most complete and accurate definition.
Correct Option:
A) It is the heat required to change the state of a substance without changing its temperature. This statement accurately defines latent heat. It emphasizes that this heat energy is used for a phase transition (changing state) and that, crucially, the temperature of the substance remains constant during this process.
Incorrect Options
- B) It is the heat required to raise the temperature of a substance — This describes sensible heat, not latent heat. Sensible heat causes a change in temperature, whereas latent heat causes a change in state at a constant temperature.
- C) It is the heat released during a phase change — While latent heat can be released during a phase change (e.g., during condensation or freezing), this statement is incomplete. Latent heat can also be absorbed (e.g., during melting or boiling). More importantly, it omits the critical aspect that the temperature remains constant during this process.
- D) It is the heat absorbed during a phase change — Similar to option C, this statement is incomplete. Latent heat can be absorbed (e.g., during melting or boiling), but it can also be released. It also omits the crucial detail of constant temperature during the phase change.
16.
What is the term used to describe the change of a gas into a solid without passing through the liquid phase?
Approach
The question asks for the specific term describing the phase transition where a gas directly changes into a solid without becoming a liquid first. This is a direct phase change, bypassing the intermediate liquid state.
Step-by-step
- Understand Phase Changes: Matter exists in different phases (solid, liquid, gas, plasma). Phase changes occur due to changes in temperature and pressure.
- Analyze the given condition: The condition specifies a change from gas to solid, explicitly stating that it bypasses the liquid phase.
- Recall relevant terms:
- Sublimation: Solid directly to gas.
- Deposition: Gas directly to solid.
- Melting/Fusion: Solid to liquid.
- Freezing/Solidification: Liquid to solid.
- Vaporization/Boiling/Evaporation: Liquid to gas.
- Condensation: Gas to liquid.
- Match the condition to the term: The process of a gas changing directly into a solid is known as deposition. A common example is the formation of frost, where water vapor (gas) in the air directly turns into ice (solid) on cold surfaces.
Correct Option: A) Deposition is the correct term for the phase transition from gas directly to solid, bypassing the liquid phase.
Incorrect Options
- B) Solidification: This term refers to the process where a liquid changes into a solid. For example, water freezing into ice. It involves the liquid phase, which is explicitly excluded in the question.
- C) Condensation: This term describes the process where a gas changes into a liquid. For example, water vapor turning into liquid water droplets (dew, clouds). This does not involve forming a solid directly from a gas.
- D) Sublimation: This term refers to the process where a solid changes directly into a gas without passing through the liquid phase. A common example is dry ice (solid carbon dioxide) turning directly into gaseous carbon dioxide. This is the reverse of the process described in the question.
17.
What happens to the density of water as it is heated from 0°C to 4°C?
Approach
This question tests knowledge about the anomalous expansion of water. Unlike most substances that expand when heated, water exhibits peculiar behavior between 0°C and 4°C. Understanding this anomaly is key to determining how its density changes in this temperature range.
Step-by-step
- General Behavior of Substances: Most substances expand when heated and contract when cooled. This is because heating increases the kinetic energy of molecules, causing them to move further apart, thus increasing volume and decreasing density. Conversely, cooling decreases kinetic energy, leading to contraction and increased density.
- Anomalous Expansion of Water: Water is an exception to this general rule in the temperature range of 0°C to 4°C. When water is heated from 0°C to 4°C, instead of expanding, it contracts. This means its volume decreases during this heating process.
- Density Formula: Density is defined as mass per unit volume:\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
- Applying to Water (0°C to 4°C): As water is heated from 0°C to 4°C, its mass remains constant, but its volume decreases. According to the density formula, if the volume decreases while mass is constant, the density must increase.
- Maximum Density at 4°C: Water reaches its maximum density at 4°C. If heated further beyond 4°C, it then behaves like most other substances, expanding and decreasing in density. If cooled below 4°C, it also expands and decreases in density until it freezes at 0°C.
Correct Option: C) It increases — As water is heated from 0°C to 4°C, it undergoes anomalous expansion, meaning its volume decreases. Since density is inversely proportional to volume (for a constant mass), a decrease in volume leads to an increase in density.
Incorrect Options
- A) It decreases — This would be true if water expanded upon heating from 0°C to 4°C, which is contrary to its anomalous behavior.
- B) It remains constant — Water's volume and thus its density do change significantly in this temperature range.
- D) It fluctuates — While the density changes, it does so in a consistent, predictable manner (increasing) within this specific temperature range, rather than fluctuating randomly.
18.
What is the process by which a solid changes directly into a gas without becoming a liquid?
Approach
The question asks to identify the process where a solid directly converts into a gas without passing through the liquid state. This is a fundamental concept in the states of matter and phase transitions.
Step-by-step
- Recall the definitions of various phase transitions.
- Sublimation is the process where a solid changes directly into a gas without first becoming a liquid. Examples include dry ice (solid carbon dioxide) turning into gaseous carbon dioxide, and iodine crystals turning into iodine vapor.
- Solidification (or freezing) is the process where a liquid changes into a solid.
- Condensation is the process where a gas changes into a liquid.
- Melting is the process where a solid changes into a liquid.
- Based on these definitions, the process described in the question perfectly matches the definition of sublimation.
Correct Option: A) Sublimation is the process by which a solid changes directly into a gas without becoming a liquid. This direct phase transition is observed in substances like dry ice (solid \(\text{CO}_2\)) and naphthalene balls.
Incorrect Options
- B) Solidification is the process where a liquid changes into a solid (e.g., water freezing into ice). This is the opposite of what the question describes.
- C) Condensation is the process where a gas changes into a liquid (e.g., water vapor forming liquid water droplets). This involves a gas-to-liquid transition, not solid-to-gas.
- D) Melting is the process where a solid changes into a liquid (e.g., ice turning into water). This involves an intermediate liquid state, which the question explicitly excludes.
