Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

Theme B — The Particulate Nature of Matter

B.1.9 Specific Latent Heat — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 11 of 53 1 quiz

📚 IB Physics HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read
Lesson summary

B.1.9 Specific Latent Heat — IB PHYSICS SL notes: Use Q = mL to calculate the energy for changes of state and describe how latent heats are measured.Specific latent heat L = the amount of energy required to change the state of 1 kg of a substance without changing its temperature.…

  • What the symbols mean
  • Specific latent heat of fusion Lf
  • Specific latent heat of vaporisation Lv
  • Using the right one
  • Steam scalds
  • Heating ice at −10 °C to steam at 100 °C

Part 1 of 7Definition

Specific latent heat L
Specific latent heat L the amount of energy required to change the state of 1 kg of a substance without changing its temperature.
  • The unit is J kg−1.
Equation
  • Q = mL, with Q the energy in J, m the mass that changes state in kg and L = Q/m.
    What the symbols mean: E or Q = energy to change state without a change of temperature (J) · m = mass (kg) · L = specific latent heat (J kg−1)
Size of L
  • Size of L the higher L is, the more energy is needed to change the state.
Temperature
  • Temperature it does not change during the phase change.

Part 2 of 7Fusion and vaporisation

1Specific latent heat of fusion Lf
  • Specific latent heat of fusion Lf the energy absorbed when 1 kg of solid melts, or released when 1 kg of liquid freezes, at constant temperature.
2Specific latent heat of vaporisation Lv
  • Specific latent heat of vaporisation Lv the energy absorbed when 1 kg of liquid evaporates,
    • or released when 1 kg of gas condenses, at constant temperature.
3Water
  • Lf = 3.34 × 105 J kg−1 and Lv = 2.26 × 106 J kg−1.
4Using the right one
  • Using the right one melting and freezing use Lf, and boiling and condensing use Lv.

Part 3 of 7Why L<sub>v</sub> is larger than L<sub>f</sub>

Fusion
  • Fusion the intermolecular forces only need to be partially overcome, and the particles stay in contact.
Vaporisation
  • Vaporisation the forces must be completely overcome as the particles separate.
  • Extra work is done pushing back the atmosphere.
Result
  • Lv > Lf for every substance, nearly seven times for water.
Steam scalds
  • Steam scalds condensation releases 2.26 MJ per kilogram on the skin before it cools.

Part 4 of 7Multi-stage problems

1Heating ice at −10 °C to steam at 100 °C
  • Heating ice at −10 °C to steam at 100 °C add each stage.
  • Stage 1: Warm the ice to 0 °C: mciceΔT.
  • Stage 2: Melt: mLf.
  • Stage 3: Warm the water to 100 °C: mcwaterΔT.
  • Stage 4: Boil: mLv.
2Mixing
  • Mixing energy lost by the hot body equals energy gained by the cold one, including latent terms.
3Ice added to warm water
  • Ice added to warm water find the energy the water releases in cooling to 0 °C and compare with the energy needed to melt all the ice.
  • If it releases more, all the ice melts and the final temperature is above 0 °C.
  • If not, some ice survives and the final temperature is 0 °C.
4Heater problems
  • Heater problems use P = E/t, so Pt = mL.
5Plan
  • Plan sketch temperature against energy, with slopes for mcΔT and plateaus for mL.

Part 5 of 7Measuring latent heat

1
For Lv, boil water with an electric heater of known power on a balance and record the mass lost in time t.
2
Apply Pt = mLv after steady boiling is reached.
3
Repeat at a different power and subtract to remove background losses.
4
For Lf, put a heater in a funnel of crushed ice and collect the meltwater for time t.
5
Compare with an identical setup without the heater to correct for melting from the room, then use VIt = mLf.
  • Errors
    • losses to the surroundings, readings not in a steady state and vapour carrying liquid droplets.

Part 6 of 7Practical importance

Practical importance
Ice as coolant
  • Ice as coolant each kilogram absorbs 334 kJ at 0 °C.
Steam
  • Steam it is used for heating and sterilising.
Refrigerator
  • Refrigerator evaporation of the refrigerant absorbs energy.
Sweating
  • Sweating evaporation cools the body.
Damp cloth and glass of ice water
  • Damp cloth and glass of ice water evaporation cools a surface and condensation releases energy and gathers droplets.

Part 7 of 7More from the specification

1
Phase change thermodynamics in solids
  • Phase change thermodynamics in solids latent heat of fusion breaks intermolecular bonds without changing temperature when melting crystalline solids on Earth.
2
Latent Heat and Constant Phase Temperature
  • When a substance undergoes a phase change at fixed melting or boiling points, added thermal energy breaks intermolecular bonds without altering the average particle kinetic energy or temperature.
3
Thermal Conduction and Convection
  • Conduction occurs primarily in solid conductors (such as copper) via lattice vibrations and free electrons,
  • whereas convection transfers heat through circulatory bulk density currents in fluids.
4
Stefan-Boltzmann Law and Stellar Luminosity
  • The total thermal power (luminosity) emitted by a black-body object of surface area A and absolute temperature T through electromagnetic radiation is given by L = sigma * A * T4.
5
Specific Heat Capacity vs Latent Heat
  • Specific heat capacity measures thermal energy needed to change temperature without a phase change,
    • (Q = m * c * Delta T)
  • whereas specific latent heat measures energy for phase transitions at fixed temperature (Q = m * L).
6
Intensity
  • Power per unit area perpendicular to the wave direction.
7
Kinetic energy
  • K = ½mv2, energy of motion.
8
Apparent brightness
  • The power received per unit area at the observer
9
Thermal equilibrium
  • Constant temperature because energy is absorbed and lost at equal rates
10
Thermal conductivity
  • Material constant relating the rate of conduction to the temperature gradient and area
11
Potential energy
  • Energy stored by the configuration of a system.
12
Thermal
  • A convection current in the air.
13
Radiation
  • Transfer by electromagnetic waves, requiring no medium
14
Copper
  • Metal widely used for electrical wires.
15
Convection
  • Transfer of thermal energy by the movement of a heated fluid
16
Conduction
  • Transfer of thermal energy by lattice vibrations and free electrons
17
Luminosity
  • The total power of radiation emitted by a star.
18
Capacity
  • Charge a cell can supply, in A h or C.
19
Wavelength
  • Distance between a point on one wave and the same point on the next wave

Key terms

TermMeaning
Specific latent heatEnergy per kilogram to change state at constant temperature
Latent heat of fusion (Lf)Energy per kilogram to melt a solid or freeze a liquid
Latent heat of vaporisation (Lv)Energy per kilogram to boil a liquid or condense a gas
Phase changeChange of matter from one state to another
PlateauFlat section of a heating curve at constant temperature
Intermolecular potential energyEnergy stored in the forces between particles

Avoid these mistakes

1
Using Lf for boiling — boiling uses Lv.
2
Saying the temperature rises while a substance changes state — it stays constant.
3
Assuming all the ice melts in a mixing problem — check the energy available first.
4
Using grams with J kg−1 — convert to kilograms.
5
Forgetting that condensation and freezing release the same energy — they give out mL.

Last updated:

\n