Specific latent heat – measuring fusion and vaporization of water
PhysicsThermal Physics & GasesAges 17–18Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Thermal Physics & Gases
An electric heater of power P heats m kg of ice from −10 °C until it has all melted, boiled and vaporized. A thermometer and a stopwatch give a T(t) graph with two flat sections at 0 °C and 100 °C. From the duration of these plateaus students calculate λ = P·Δt/m and L = P·Δt/m, compare them with 3.34·10⁵ J/kg and 2.26·10⁶ J/kg, and see the effect of heat losses.
Lesson: Specific latent heat of fusion and of vaporization
What it shows
During a change of state a substance absorbs energy without any change in temperature: the energy separates the particles rather than making them move faster. The energy needed per kilogram is the specific latent heat, so Q = m·λ for melting and Q = m·L for vaporization. Here ice starting at −10 °C is heated at a known power, giving a heating curve with flat sections at 0 °C and 100 °C. Timing each plateau gives λ and L. Because part of the heater's energy is lost to the surroundings, the raw results come out too large; multiplying by the heat transfer efficiency H corrects them.
How to use
Set Heater power, Mass of ice and Heat transfer efficiency H, then press Run. The run is sped up, but the clock shows the real experiment time. Watch the graph level off at 0 °C and 100 °C; the table records each plateau automatically. Students can also time a plateau themselves with the Start timer and Stop timer buttons. Compare the measured values with the accepted ones, first at H = 100% and then at a lower efficiency.
Parameters you can change
- Heater power 200–2000 W
- Mass of ice 0.05–0.5 kg
- Heat transfer efficiency H 50–100 %
- Time speed-up 10–200 ×
Questions to explore
- Why does the temperature stay at 0 °C while the ice melts, even though the heater keeps supplying energy?
- About how many times longer is the boiling plateau than the melting plateau, and why?
- Do heat losses make the measured λ larger or smaller than the true value, and why?
