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.