Heat
Short Questions
1. Why does the temperature remain constant during the melting of ice despite heat input?
Answer : Heat is absorbed as latent heat of fusion to break molecular bonds without increasing kinetic energy.
2. How does pressure affect the melting point of ice in regelation?
Answer : Increased pressure lowers the melting point, causing ice to melt at a temperature below 0°C.
3. What is the significance of water’s maximum density at 4°C?
Answer : It ensures denser water sinks, preserving aquatic life by maintaining liquid water under ice.
4. How does the latent heat of vaporization differ from sensible heat?
Answer : Latent heat changes phase at constant temperature, while sensible heat changes temperature without phase change.
5. Why does regelation allow ice to reform after a wire passes through it?
Answer : Water refreezes when pressure is removed, restoring the melting point to 0°C.
6. What happens to water’s density when cooled from 10°C to 0°C?
Answer : Density increases from 10°C to 4°C, then decreases as water expands below 4°C.
7. How does Hope’s apparatus confirm water’s anomalous behavior?
Answer : It shows water at 4°C sinks due to maximum density, with colder water rising above it.
8. Why does air at 20°C hold less water vapor than at 40°C before saturation?
Answer : Warmer air has a higher capacity to hold water vapor before reaching saturation.
9. What indicates that air has reached its dew point?
Answer : Condensation begins as the air becomes saturated with water vapor, forming droplets.
10. How is relative humidity related to absolute humidity?
Answer : Relative humidity is the ratio of actual vapor mass (absolute humidity) to the saturation vapor mass.
11. Why do rocks crack in cold regions due to water’s properties?
Answer : Water’s anomalous expansion below 4°C exerts pressure when freezing, cracking rocks.
12. What role does latent heat play in the formation of dew?
Answer : Latent heat is released when water vapor condenses into droplets at the dew point.
13. Why is the calorie defined for a specific temperature range (14.5°C to 15.5°C)?
Answer : Water’s specific heat varies slightly with temperature, requiring a standard range for consistency.
14. How does specific heat capacity affect thermal equilibrium in a calorimeter?
Answer : It determines the heat absorbed or released to reach a common temperature during exchange.
15. Why does iron penetrate deeper into wax than lead in the specific heat experiment?
Answer : Iron’s lower specific heat allows it to absorb more heat, melting more wax.
16. What is the effect of high relative humidity on cloud formation in airplane trails?
Answer : High relative humidity slows the dissipation of condensed vapor trails, forming persistent clouds.
17. How does the principle of heat exchange ensure no net energy loss?
Answer : Heat lost by a hot object equals heat gained by a cold object in an isolated system.
18. Why is the specific heat of water higher than that of metals like copper?
Answer : Water’s hydrogen bonds require more energy to increase molecular motion compared to metals.
19. What happens to latent heat during the condensation of steam?
Answer : Latent heat of vaporization is released as steam condenses into water at 100°C.
20. How does the anomalous behavior of water prevent pipes from bursting in some cases?
Answer : Water’s contraction from 0°C to 4°C reduces volume, delaying freezing and pressure buildup.
Long Questions
1. Explain the concept of latent heat during the transformation of ice to water.
Answer : Latent heat of fusion is absorbed by ice at 0°C to weaken molecular bonds, converting it to water without raising temperature. This heat, typically 80 cal/g for ice, is used to change the phase from solid to liquid. The temperature remains constant until all ice melts, as shown in the temperature vs. time graph.
2. How does regelation allow a wire to pass through an ice slab without breaking it?
Answer : Pressure from the wire lowers the melting point of ice, causing it to melt into water beneath the wire. Once the wire passes, the pressure is removed, and the water refreezes, reforming ice. This process, called regelation, keeps the ice slab intact as the wire moves through.
3. Why does water exhibit anomalous behavior between 0°C and 4°C, and what is its significance?
Answer : Unlike most liquids, water contracts when heated from 0°C to 4°C, reaching maximum density at 4°C. Beyond 4°C, it expands, reducing density, due to changes in hydrogen bonding. This behavior preserves aquatic life in cold climates by keeping denser 4°C water at the bottom of lakes.
4. How does Hope’s apparatus demonstrate the anomalous behavior of water?
Answer : Hope’s apparatus shows water at the bottom (T1) cools faster to 4°C, then remains stable as denser water sinks. The upper water (T2) cools to 0°C first due to less dense water rising below 4°C. The intersection of T1 and T2 curves at 4°C indicates maximum density.
5. Explain how relative humidity is determined and its relation to dew point.
Answer : Relative humidity is the ratio of actual water vapor mass to the mass needed for saturation at a given temperature, expressed as a percentage. At the dew point, air is saturated, and relative humidity reaches 100%. Higher relative humidity indicates air is closer to saturation, affecting weather perception.
6. Why do water droplets form on a cold bottle taken out of a refrigerator?
Answer : The cold bottle’s surface cools surrounding air to its dew point, causing water vapor to condense into droplets. This mirrors dew formation, where cool surfaces at night reach the dew point of atmospheric air. It demonstrates the presence of water vapor in the air.
7. How does the principle of heat exchange apply in a calorimeter to measure specific heat?
Answer : In a calorimeter, heat lost by a hot object equals heat gained by water and the calorimeter until thermal equilibrium is reached. Using Q = m × c × ΔT, the specific heat of the object is calculated from known masses and temperatures. This ensures no heat is lost to the environment in an isolated system.
8. Why does aquatic life survive in cold regions despite freezing temperatures?
Answer : Due to water’s anomalous behavior, 4°C water, being densest, sinks, forming a layer above which ice forms at 0°C. Ice, less dense, floats and insulates the water below, maintaining a stable 4°C environment. This allows aquatic organisms to survive in liquid water beneath the ice.
9. Calculate the heat required to convert 1 kg of ice at 0°C to water at 20°C, and explain the process.
Answer : Heat for melting ice: Q1 = 1 × 80 = 80 kcal; heat to raise water temperature: Q2 = 1 × 1 × 20 = 20 kcal; total Q = 100 kcal. Latent heat of fusion is absorbed at 0°C to change phase, then sensible heat increases temperature. This reflects the energy needed for phase and temperature changes.
10. How does specific heat capacity affect the heating of different substances, and why is it significant?
Answer : Specific heat capacity determines how much heat a substance absorbs to raise its temperature, varying across materials (e.g., iron vs. copper). Higher specific heat, like water’s 1 cal/g°C, means more heat is needed, affecting thermal regulation. This property is critical in applications like cooling systems and climate moderation.
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