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Why does a lid change the amount of condensation on a glass?

Does putting a lid on a glass that is filled with ice change the amount of condensation on the outside of the glass and why? We did this at my infants school but nobody knew the answer. Is it the same for my Dad’s bottles of beer?
Joseph Matthew Knight from Hertfordshire (Age 5-14)

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2 Responses

  1. Hi Young Scientist :).
    When a ‘change in state’ occurs energy has done the work.
    In this case, the work is the condensation which has formed upon the outer surface of the glass, and has been created by the deposit of liquid water (obtained from the ‘invisible’ water vapour floating in the air around the container).
    A few different activities combine their actions in order to produce this outcome, all of which play essential roles in the process and below is a list of the major contributors:-
    Most importantly a reduction in heat energy (the temperature) at the outer surface of the glass container has cooled the air layer with which it is in contact – (heat naturally passes into cooler zones). The lowered temperature in this gas reduces the potential vapour density that can be supported by it. The surplus water vapour is then resolved in the less energetic state of. liquid water.
    Now consider how the rate of cooling of the glass may be influenced by the use of a lid… I think the two important factors that are involved are the rate at which evaporation occurs from the surface and the rate at which gas escapes from the liquid. Both of these processes require energy to drive them and, by exchange, the final outcome would be observed as a reduction in the temperature of the liquid. By a combination of convection and conduction the container would be cooled and thereby the outer surface would become an area for condensation to form. The presence of a lid would reduce the rate at which these two effects would occur by limiting air currents, by sealing and by insulation. Thus I think that the increase in condensation rate on a glass container which contains a ‘gaseous’ liquid and, to a lesser extent if a still, iced liquid is involved, when it is removed from a fridge, may be caused by the absence of such a lid
    I hope that this analysis may provide the answer to your question.
    Good Luck,
    Rockno3.

  2. HI Again Young Scientist :). Further to my answer above, may I attach a little more detail of the progression of consideration which led to my conclusions and I will include some of .the ‘facts and figures’ too.

    Firstly, all increases in temperature and all consequent changes of state e.g. from a solid to a liquid or the‘vaporisation’ of a liquid to a gas require an input of energy. The energy (heat) that is required is taken from the nearest available warmer body of material. The common unit for the measurement of heat energy is the ‘calorie’, where one calorie is the amount of heat that is required to raise the temperature of one gram (normally one cubic centimetre) of water by one degree in Celsius (or Centigrade). The terms are abbreviated as, cal for calories and grams weight as g. The temperature is quoted in 0C.

    The measurement of how much heat is required to change the temperature of a unit mass of a substance by one degree is characteristic and is called its Specific Heat.
    For instance, it would take twice as many calories to heat some liquid water by one degree than it would to heat the same mass of ice by one degree.

    When any material is heated to the temperature where it changes state, the actual level of the temperature in degrees remains steady, whilst the energy is taken up in the conversion process. Thus water normally boils at 100 0C but there is a time delay (which increases with the quantity involved) between the moment when that temperature is obtained and the moment when the liquid actually begins to boil.
    It follows that ice water will remain at 0 0C until all the ice is melted but heat energy will continue to be taken from adjacent warmer materials throughout the ‘change of state’, (melting/freezing of water requires 80 Cal /g). In a similar sequence, degassing and vaporisation also involve energy exchanges which are intrinsically related to ‘vapour pressure / temperature’ etc. in the ‘air’ above the liquid. Boiling/condensation of water will require 540 Cal /g in latent heat.
    Bearing these factors in mind, I concluded that the presence of a lid on a container of ice or a container of a pressurised beer would influence the rate at which condensation would form after their removal from a fridge by limiting such factors as described above.

    I hope this detail has helped to justify my opinion.
    Good Luck,
    Rockno3.

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