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A thermal cycler is a chemistry lab device that increases or decreases the temperature of the material inside it. The lid of the device is heated to prevent condensation and evaporation of the mixture inside.

I understand the condensation bit, as a hot lid would keep the gaseous particles gaseous. But I don't understand how a hot lid can prevent evaporation of the mix. Is there a physics law that states liquids cannot evaporate if the temperature of the gas above them is higher than the liquid? Or is it about pressure? Or both?

Edit: Adding some more info since this was migrated to physics. This site provides a decent overview on thermal cyclers. The following paragraph that explains the basics is from there:

The basic idea of a thermal cycler is that it provides a thermally controlled environment for PCR samples. A thermal cycler usually contains a heating block with holes or depressions in it that receive sample tubes (though other types of sample vessels are now possible also; see below). For the PCR reactions to work properly, the block must change temperature at specific times, and spend specific durations of time at specific temperatures. The researcher programs the temperature cycling information into the thermal cycler either by computer or via a console on the instrument, or uses a preprogrammed routine built into the machine.

The main idea is that the machine cycles the temperatures of its contents, the temperatures usually range from around 50 to around 95 C.

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  • $\begingroup$ Does the lid just prevent the contents of the device from evaporating away into the room? Like the difference between water stored in a sealed bottle versus water heated in an open dish? $\endgroup$
    – rob
    Commented Jun 28, 2019 at 11:10
  • $\begingroup$ @rob No, the tubes inside the machine have lids themselves. The heated lid is a larger, thicker cover that is placed on top of all the tubes. $\endgroup$
    – Esoppant
    Commented Jun 28, 2019 at 12:00
  • $\begingroup$ are you aware of anyone measuring a loss of mass due to evaporation of water out of the sealed tubes when the lid is not heated? $\endgroup$
    – lamplamp
    Commented Nov 30, 2020 at 5:18

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The heated lid minimizes net evaporation by preventing condensation.

The liquid water in the sealed sample tube is in a dynamic equilibrium with the water vapour. Liquid water is continuously evaporating and water vapour is continuously condensing. At a constant temperature and pressure the partial pressure of the water vapour is constant.

If water vapour is allowed to condense on the lid then more water in the sample evaporates and we're effectively distilling water out of the sample, raising its concentration, which is undesirable.

By preventing condensation, the sample only loses the amount of water necessary to saturate the air in the tube.

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    $\begingroup$ A crucial point here that might need a bit more emphasis: At any point, the amount of water that is dissolved in air is negligible in amount compared to a single condensed droplet. $\endgroup$
    – Wrzlprmft
    Commented Mar 2, 2023 at 21:16
  • $\begingroup$ @Wrzlprmft Are you sure? According to the OP the temperature is 50°-90° C, so the vapour pressure of water is significant. At the low end of that range, I estimate that the air contains ~0.08 g/L of water. $\endgroup$
    – PM 2Ring
    Commented Mar 2, 2023 at 21:46
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    $\begingroup$ Taking values for PCR tubes (which are used in a thermal cycler) since I have them at hand: We have an air volume of about 50 μℓ, this makes for 4 μg of water dissolved in air, whereas the total amount of liquid used is about 20 μℓ, i.e., 20 mg. A droplet is on the order of magnitude of 1 μℓ, so 1 mg. That’s around a hundred times as much as what is dissolved in the air. $\endgroup$
    – Wrzlprmft
    Commented Mar 2, 2023 at 22:02
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We were discussing about this with a colleague, and we couldn't agree. I propose that it's due to the vapor going near the lid being heated more, therefore developing relatively more pressure than the column immediately under it, and therefore migrating to the lower pressure zone. This brings more vapor/air in its place that gets heated and returns down to the liquid surface. This functionally should stop more vapor from forming, creating an equilibrium that will be reached fairly quickly given the small tubes. Also, the higher vapor pressure due to the heat should reduce the vaporization of the water? I don't know though, i couldn't find a definitive explanation. What do you think?

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You were wise to be suspicious, because the answer is that thermal cyclers do not reduce evaporation relative to a similarly sealed chamber.

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