The effect of dew on a cup upon the temperature increase of cold drink in the cup

When a cup is filled with cold drink, dew is produced on the cup surface. Because this is an exothermic process, the latent heat is released from the dew to the cup raising the temperature of the cup and cold drink. We studied this phenomenon by measuring the rate of temperature increase of the cold water in a cup as a function of the mass of water, initial temperature of water, relative humidity and temperature of the room. We coated the cup with hydrophilic or hydrophobic material to control the amount of condensation, and relate the rate of temperature increase to the amount of condensation. Our analysis of the data based on the equation ‘amount of heat = heat capacity × mass × temperature change’ showed that the rate of temperature increase was greater when the cup surface was hydrophilic. By comparing the rate of temperature increase against time with the Newton’s law of cooling, we found that the heat transfer toward inside of the cup was better when the cup surface was hydrophilic. We propose that the observed results are because on the hydrophilic surface the dew drops flatten and release the latent heat more easily and the contact area between the dew drops and the cup surface is larger. When the humidity of the room was low, the temperature increase was slowed because of the evaporation of dew which takes away the heat from the cup.