That is a law, specifically the law of conservation of mass. This states that matter can neither be created nor destroyed in a chemical change. This was proved in Lavosier's experiment where he determined that the total lass of the products formed by burning were equal to the mass of the matter burned and oxygen consumed.
Terrarium.
In a closed system, water can be boiled until it produces water vapor (steam). If the steam is then condensed through a coil of tubing and collected in another container, the same volume of water winds up in the second container that you started with originally. An action created and then reversed to it's original point. Note that this is in a closed system.
Water will evaporate from any source, especially when heat is present. If in a closed container it will just turn to condensation. But if in a lake or stream, It will be absorbed into the clouds.
It is extremely dangerous, it can cause rapid freezing if it comes into contact with living tissue (skin). Also, if you put it in a closed container (like a Thermos bottle with the top screwed on tightly) that isn't strong enough to handle the buildup of pressure, it can actually cause the container to explode.
The relative humidity in a closed vessel will not change due to pressure. Assuming water is the fluid, the relative humidity will stabilize to 100%.
observation
In a closed container the mass of reactants (wood) and products (carbon dioxide, water, ash) remain unchanged; this is the low of mass conservation.
Since you have specified a closed container, the amount of gas doesn't change, and the volume that it occupies doesn't change. The only thing that is likely to change is the temperature, and changes in temperature will cause corresponding changes in pressure. And if the pressure gets too high, your closed container is going to explode. So avoid overheating.
evaporate
Water can be evaporated from a sealed container.
small container
the particles of gas bouncing into the walls of the container.
They can't live in a closed container because there is no oxygen.
Lowering the temperature will cause a decrease in gas pressure in a closed container.
They move Freely in the closed container.
The particles will gain kinetic energy and will move faster in the closed container. This will increase the number of collisions they make with the walls of the container, thus increasing the pressure.
in a closed container and then refrigerate.