Water Vapour Condensation
Experiment number : 1768
Goal of experiment
Our goal is to visualize a temperature increase caused by condensation of water vapour.
The described experiment can to be considered complementary to the experiment Odpařování vody a lihu (s termovizní kamerou) (only in Czech so far). Explanation of this experiment is that the evaporating liquid takes out latent heat of vaporization Lv from its surroundings, and thus the vapour has higher energy than the liquid of the same temperature. Logical reasoning leads us to the conclusion that the condensation (liquefaction) of gas gives off “excessive” energy as heat to become liquid. This heat is called the latent heat of condensation Lc of the gas and its size equals to the latent heat of vaporization Lv of liquid of the same temperature.
In our experiment, we show a local temperature increase above the water level in a cup; the water vapour condensates on a paper covering the cup.
Thermal imaging camera, cup with water at a temperature slightly lower than the ambient temperature (e.g. 2 °C), sheet of paper.
Fill a cup with water and cover the cup with a sheet of paper. Observe the temperature change of the paper by the thermal imaging camera.
The experiment is illustrated by the video below. At the place where the paper covers the water surface, we can see a temperature increase of about 1 °C. This increase is temporary, after a while the sheet of paper comes to thermal equilibrium with the surroundings.
In this experiment, the thermal imaging camera FLIR i7 was used. The temperature range of colour scheme was chosen in the interval from 19 °C to 25 °C, the emissivity was ε = 0.95.
It is important to set the range of the thermal imaging camera so that the temperature difference between maximum and minimum tepmerature is the smallest possible – we need to be able to detect really small changes in temperature in order of about 1 °C.
It is ideal to perform this experiment with water of temperature a few degrees lower than the temperature of the surroundings. For this purpose it is useful to use cold tap water; its typical temperature is 20 °C (of course it is recommended to try it before conducting the experiment). With cold water the experiment is quite demonstrative – although the water itself is colder than the surroundings, the paper is heated to a temperature greater that the temperature of the surroundings thanks to the latent heat of condensation.
If you choose to experiment with water of higher temperature (same as the temperature of the surroundings or higher), students can explain the temperature increase of the paper by heat exchange between the water and the paper. On the other hand, if you choose water that is too cold (five or more degrees lower that the temperature of the surroundings), the effect can be supressed and the experiment is unconvincing.
This experiment was inspired by a similar experiment on web page Infrared Tube that is dedicated to experiments with thermal imaging camera.