Evaporation and boiling sim
Weigh a dish of water or ethanol on a balance as it evaporates, changing the temperature, the surface area and a desk fan's air speed; watch a wet-bulb thermometer cool below a dry one; then heat water on a hotplate until bubbles form all through it and the temperature stays at the boiling point.
Thermal properties of matter · covers the O-Level, IGCSE topic
What the sim shows
Model. Evaporation: rate = A × k(u) × φ × (ρ_sat(θ) − ρ_air), where A is the surface area, k(u) = 0.004 + 0.006 u m/s grows with the air speed u (chosen values), φ is 1 for water and 0.61 for ethanol (its vapour spreads more slowly), ρ_sat is the vapour's density just above the surface (from the saturated vapour pressure, by the Antoine equation) and ρ_air is the vapour the room air already holds (humidity × ρ_sat at 20 °C for water; none for ethanol). The lamp holds the dish's liquid at its setting; with the lamp off the liquid is taken to be at room temperature, 20 °C (a real dish runs a little colder). The balance reads to 0.01 g with seeded scatter of up to ±0.006 g. The wet bulb: C dθ/dt = (h + h_rad) A (20 − θ) − L φ k A (ρ_sat(θ) − ρ_air), with h_rad = 5.5 W/(m² K) for radiation from the room (chosen), C = 0.5 J/K, A = 3.0 cm², k = 0.006 + 0.012 u m/s and h = ρc Le^(2/3) k (chosen sizes; latent heats 2.45 MJ/kg for water and 0.92 MJ/kg for ethanol at about 20 °C); the wick keeps the muslin wet. Thermometers are read to 0.5 °C (seeded scatter up to ±0.15 °C). Boiling: 200 g of water, c = 4200 J/(kg °C), heat loss 1 W per °C above 20 °C; at the boiling point it stays at the boiling point and boils away at (P − loss) ÷ L, L = 2.26 MJ/kg; evaporation below the boiling point is left out of the energy sums. Boiling points from the same vapour-pressure equation: 100.0 °C at 101 kPa, 90.0 °C at 70 kPa, 72.1 °C at 34 kPa (beyond the syllabus). Latent heats and vapour-pressure constants from memory. Time moves only when you press a wait button.
- The part to watch: the dish of liquid, the wet bulb's muslin, or the thermometer in the boiling water
- Top-pan balance, heat lamp, desk fan, hygrometer, wet-and-dry thermometers, hotplate, retort stand, barometer, stopwatch
Use it in your lessons
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