Bluetree Education Group

Why do cold drinks “sweat”?

In this article, I would be sharing about the thought process which leads us to the answer of why cold drinks “sweat”?

Recently, a P5 student asked our teacher as we discussed the topic on water and their changes of states.

This is the question:

Why does a glass of cold drink seem to “sweat”? Especially when the drink is colder, it “sweats” even more?

First, let’s understand what are the processes that different states of water can undergo.

First – Boiling/Evaporation and Melting

Boiling, evaporation and melting are processes which require heat gain, meaning that heat must be supplied to the state of water to allow these processes to occur.

Boiling and evaporation occur when water (liquid state) gains heat from the surroundings and changes to water vapour (gaseous state). Boiling happens only at a fixed temperature when the substance is pure while evaporation happens at any temperature if the liquid state of a substance is present. For pure water, it boils at 100 oC whereas water can evaporate between 0 oC and 100 oC. Melting occurs when ice (solid state) gains heat from the surroundings and changes to water (liquid state). Melting of pure ice takes place at 0 oC.

Second – Condensation and Freezing

Condensation and freezing are processes which require heat lost, meaning that heat must be removed from the state of water to allow these processes to occur.

Condensation occurs when water vapour (gaseous state) loses heat to cooler surroundings and changes to water (liquid state). Condensation happens only when there is a temperature difference between water vapour (of a higher temperature) and a surface (of a lower temperature). The greater the temperature difference, the greater the rate of condensation, the more water droplets form on the cooler surface. On the other hand, freezing occurs when water (liquid state) loses heat to the cooler surroundings and changes to ice (solid state).

 

Let’s go back to the questions:

a) Why does a glass of cold drink seem to “sweat”?

The phenomenon of “sweating” is in fact the condensation of the water vapour in the surrounding air on the outer surface of the glass as the water vapour loses heat to the cooler outer surface of it.

Students may find it hard to visualise the process of condensation as water vapour (a gas) in the atmosphere is invisible! What parents can do to show that the air around us has water vapour is to breathe out/blow out air onto a mirror (cooled mirror works better). Students can immediately see the effect of condensation through the appearance of tiny water droplets on the mirror. Otherwise, we can explain that Singapore is a country with a humid environment, in which humid means that the amount of water vapour in the air in Singapore is very high (60-90%).

When students can recognise where the water vapour is present, the next step would be to identify the cooler surface. The cooler surface is relative to the temperature of the water vapour present. Even if the water vapour is at room temperature (~25 oC), the glass with a cold drink (~10 oC) in it has a surface that is relatively cooler than the water vapour, hence a temperature difference is present for condensation to occur.

Similarly, if a glass is at room temperature (~25 oC) and it in contact with hot steam (~90 oC) from the boiling water that it contains, the glass is still cooler than the hot steam, hence the glass is identified as the cooler surface (though

it may not feel cold to us). Therefore, in this situation, a glass of hot water can have condensation too, but the water droplets would be found on the inner surface of the glass instead of the outer surface (see Figure 1).

As the water droplets are formed from the condensation of water vapour, the water droplets will be formed at the where the water vapour is, on the cooler surface (see Figure 1 for better illustration).

b) Why does a glass of colder “sweat” even more?

There is more “sweating” due to the greater rate of condensation of the water vapour in the surrounding air on the even cooler outer surface of the glass as the water vapour loses more heat to the cooler outer surface of it.

As we have mentioned above, a greater temperature difference between the water vapour and the cooler surface, the greater the rate of condensation, hence more water droplets will form on the even cooler surface.

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