Physics 22 Introduction to Heat & Temperature (6 of 6) Change of Phase & Latent Heat

Main ideas / lessons

• Phase changes require energy even when temperature stays constant. During melting or boiling (at fixed phase-change temperatures), the added/removed heat does not increase temperature; it is used to change the molecular state.

• Two key phase-change processes for water:

  • Melting (solid → liquid) at 0°C: requires the latent heat of fusion.
  • Evaporation/boiling (liquid → gas) at 100°C: requires the latent heat of vaporization.

• Large energy is needed compared to warming the same substance by 1°C:

  • Heating water by 1°C takes 1 calorie per gram (for 1 g, +1°C corresponds to ~1 cal).
  • Completely melting 1 g of ice at 0°C takes about 80 calories.
  • Completely evaporating 1 g of boiling water at 100°C takes about 540 calories.

Methodology / instructions (formulas and how to apply them)

• General principle for phase change at constant temperature:

[ Q = M \cdot L ]

where:

- **Q** = heat required (or removed) to complete the phase change  
- **M** = mass of the substance undergoing the phase change  
- **L** = latent heat (a material-specific constant)

Melting (latent heat of fusion)

• For water at 0°C:

[ Q = (1\,\text{g}) \cdot (80\,\text{cal/g}) = 80\,\text{cal} ]

• Meaning: add enough heat to convert all ice to liquid; the resulting liquid remains at 0°C because temperature does not change during the phase transition.
• Note: More precisely, this is cited as ~79.7 cal, but it is typically rounded to 80 cal.

Freezing (reverse process of melting)

• The energy requirement is symmetric: To freeze 1 g of water at 0°C into ice, you must remove 80 calories.

• This is also described as latent heat of fusion (just in reverse).

Evaporation / boiling (latent heat of vaporization)

• For water at 100°C:

[ Q = (1\,\text{g}) \cdot (540\,\text{cal/g}) = 540\,\text{cal} ]

• Meaning: add enough heat to convert all boiling water to vapor; the vaporization occurs while temperature stays at 100°C.

Condensation (reverse process of vaporization)

• The energy is symmetric in reverse: To condense 1 g of steam at 100°C into liquid water, you must remove 540 calories.

• This is described as latent heat of vaporization.

Conceptual explanation (what the energy is doing)

• Solid → liquid (melting):

  • Initially, atoms/molecules are locked in position due to bonding.
  • With added heat, molecules vibrate more, eventually breaking loose from fixed positions, allowing the solid to become a liquid.

• Liquid → gas (vaporization):

  • Continued energy increases vibrations until molecules can escape/jump free, producing gas/vapor.

• Why “no temperature change” happens during the phase change:

  • The heat goes into changing state (molecular arrangement) rather than increasing average kinetic energy (temperature).

Speakers / sources

• No specific speaker name is provided in the subtitles (only an implied instructor voice / narration).