Change of State | Jamb(UTME)

Latent Heat

1. Latent heat is the heat energy required to change the state of a substance without changing its temperature.
2. It is used to overcome intermolecular forces rather than raising the substance's temperature.
3. The two main types of latent heat are latent heat of fusion and latent heat of vaporization.
4. Latent heat is measured in joules (J).
5. During melting, latent heat breaks bonds between particles, allowing the solid to become a liquid.
6. During boiling, latent heat provides energy for particles to escape the liquid and become gas.
7. The formula to calculate latent heat is:
   
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   $Q = mL$
   where:
   • $Q$ = Heat energy,
   • $m$ = Mass of the substance,
   • $L$ = Specific latent heat.
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Specific Latent Heat of Fusion

8. Specific latent heat of fusion is the heat energy required to convert 1 kg of a solid into a liquid at its melting point without a temperature change.
9. It is measured in joules per kilogram $(J/kg )$.
10. For ice, the specific latent heat of fusion is approximately $334,000J/kg$.
11. Latent heat of fusion explains why ice melts without a temperature rise until all the ice turns to water.
12. The specific latent heat of fusion varies for different substances.
13. Materials with high latent heat of fusion, like water, absorb a lot of heat during melting.
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Specific Latent Heat of Vaporization

14. Specific latent heat of vaporization is the heat energy required to convert 1 kg of a liquid into gas at its boiling point without a temperature change.
15. It is also measured in joules per kilogram $(J/kg)$.
16. For water, the specific latent heat of vaporization is approximately $2,260,000J/kg$.
17. The large latent heat of vaporization of water makes it effective for cooling in processes like sweating.
18. This energy allows particles to overcome atmospheric pressure and intermolecular attractions to become gas.
19. Latent heat of vaporization explains why boiling takes time and energy.
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Melting

20. Melting is the process of a solid turning into a liquid when heated.
21. The temperature at which melting occurs is the melting point.
22. During melting, heat energy is absorbed to break the bonds holding the solid's structure.
23. The melting point is a characteristic property of a substance, such as $0^\circC$ for ice.
24. Melting is an example of a process requiring latent heat of fusion.
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Evaporation

25. Evaporation is the process where liquid particles escape into the gas phase at the surface, even below the boiling point.
26. It occurs faster at higher temperatures because particles have more kinetic energy.
27. Evaporation also depends on surface area, wind speed, and humidity.
28. It is a cooling process because faster particles leave the liquid, lowering its average energy.
29. Examples include the drying of clothes and cooling of water in ponds.
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Boiling

30. Boiling is the rapid conversion of a liquid to gas throughout the liquid at its boiling point.
31. The temperature at which boiling occurs is the boiling point.
32. Boiling requires latent heat of vaporization.
33. Unlike evaporation, boiling occurs uniformly throughout the liquid, not just at the surface.
34. Boiling happens when the vapor pressure of the liquid equals the atmospheric pressure.
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Influence of Pressure on Boiling and Melting Points

35. Increasing pressure raises the boiling point because particles need more energy to escape the liquid.
36. At high altitudes, where atmospheric pressure is lower, water boils at a lower temperature.
37. Pressure cookers work by increasing pressure, raising the boiling point of water, and cooking food faster.
38. Melting points of most substances increase slightly with pressure.
39. For substances like ice, increasing pressure lowers the melting point because the liquid phase is denser.
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Influence of Dissolved Substances on Boiling and Melting Points

40. Dissolved substances, like salt, increase the boiling point of a liquid—this is called boiling point elevation.
41. Adding salt to water raises its boiling point, making it hotter for cooking.
42. Dissolved substances lower the melting point of a liquid—this is called freezing point depression.
43. Salt is spread on icy roads to lower the melting point of ice, making it melt at lower temperatures.
44. The more solute added, the greater the effect on boiling or melting points.
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Real-Life Applications of Latent Heat

45. Ice melting absorbs latent heat, keeping drinks cold without warming quickly.
46. Sweating cools the body as latent heat is absorbed when sweat evaporates.
47. Steam engines use the latent heat of vaporization to convert water into steam for mechanical work.
48. Refrigerators use latent heat of vaporization to absorb heat from inside and release it outside.
49. Freezing foods preserves them by removing heat during the phase change from liquid to solid.
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Practical Observations of Phase Changes

50. A boiling kettle emits steam because water particles escape as vapor.
51. Ice remains at $0^\circC$ while melting because the absorbed heat is used for the phase change.
52. The temperature of boiling water remains constant until all the liquid turns to steam.
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Interesting Facts About Phase Changes

53. Metals like iron have very high latent heats, requiring significant energy to melt.
54. Freezing water releases latent heat, which helps stabilize the temperature of the surrounding environment.
55. Geysers erupt because latent heat builds up pressure in underground water.
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Thermodynamic Considerations

56. Latent heat is crucial in climate systems, transferring heat during evaporation and condensation.
57. Hurricanes derive energy from the latent heat of vaporization of ocean water.
58. Snow formation in clouds releases latent heat, affecting weather patterns.
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Key Differences Between Melting, Evaporation, and Boiling

59. Melting occurs at a fixed temperature, evaporation happens at any temperature, and boiling occurs at a specific boiling point.
60. All three processes involve latent heat but differ in energy requirements and mechanisms.
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