Jan 2024

Thermos Mysteries: Why It’s Better at Maintaining Coldness

January 22, 2024
Thermos Mysteries: Why It’s Better at Maintaining Coldness

When it comes to maintaining the temperature of your favourite beverages, be it a hearty soup or an ice-cold cola, there’s no better tool for the job than a good old thermos. Also known as a vacuum flask, this simple container’s temperature-preserving properties undoubtedly seemed like magic when we first heard about it. However, after using it for a while, one cannot help but notice that it seems better at retaining the temperature of cold drinks than hot ones. If you are also perplexed about why this is the case, read on to discover the science behind this everyday mystery.

The Heat Transfer Process in a Vacuum Flask

As a refresher, a thermos is an insulating storage vessel consisting of two flasks placed within each other and joined at the neck of the container. This double-walled design is an invention stumbled upon by Sir James Dewar back in the 19th century during his studies on cryogenics. The trapped air between the containers can then be purged to create a vacuum within the empty space and prevent any heat transfer from taking place. Hence, any liquids stored in the inner flask are kept isolated from exterior conditions, enabling it to maintain its temperature for extended periods.

With that out of the way, let us focus on how heat transfer is affected in a thermos. As a refresher, energy is produced by the vibrational energy or movement of molecules and atoms. This generated energy, better known as heat energy, can then be transferred, such as when there is a temperature difference, and it generally flows from a hot object to a cool object.

As you might remember from what your physics tutor in Singapore taught you, heat transfer is an integral part of thermodynamics and thermal engineering. Not only that, but it also takes place all across the known universe, such as how the sun heats the earth or when roasting marshmallows over the campfire.

In the vacuum space of a thermos, there is a significant delay in the heat transfer between its contents and the exterior environment. For instance, when you store hot chicken soup for later, the heat energy from the inner container will gradually heat the cooler gas molecules in the vacuum layer, which could then slowly pass on to the outer wall and the temperature of the exterior environment. But, since there are no gas particles in the vacuum, the heat transfer does not readily occur. The same thing happens when you put in an ice-cold liquid, only this time things are reversed: the warmth of the exterior environment cannot easily penetrate the inner flask, keeping its contents cold for longer.

Why Thermoses Are Better at Maintaining Coldness

When it comes to heat transfer, the higher the temperature difference between two objects, the quicker the energy transfer (from hot to cold) between them occurs. Moreover, our perception of these temperature extremes is also a key consideration in this question. As a refresher, room temperature is around 20-22℃ while water boils at 100℃ and freezes at 0℃.

The difference between a thermos filled with an ice-cold drink is far smaller than that of a thermos full of hot coffee. Also, the thermos is not a perfect design, so it still loses heat, albeit at a snail’s pace. For example, whenever you open it to take a sip, a great deal of heat transfer occurs (i.e., cold water gets warmer or coffee cools down).

But between these two beverages, more energy will be transferred between the coffee and the colder outside air due to their sizable temperature difference. In contrast, only a small amount of energy transfer transpires between the warm, room-temperature air and the cold water due to the minimal temperature difference.

This is where our perception comes into play, as we tend to be more perceptive of the temperature changes in hot beverages compared to colder ones. Hence, many believe that vacuum flasks are more efficient at preserving cold temperatures. Still, when you get into the nitty-gritty and look at the percentage of heat energy transferred, the numbers will say that their efficiency is equivalent at both ends of the temperature spectrum.


In short, a thermos seems more effective at keeping things chilled due to how we perceive the temperature change. Since hot liquids are generally prepared much hotter and thus farther than room temperature compared to cold drinks, we are simply more aware of the difference in heat energy as the former cools down. As such, to avoid disappointment when taking your coffee to go, it is best to drink it all immediately upon opening your thermos for the first time before heat energy can spoil its warm goodness!

If you want to learn more about the physics behind everyday mysteries like thermos efficiency or the deep-seated questions about the known universe, sign up for our classes at Tuition Physics today! Our O-level and IP physics tuition in Singapore not only helps you ace your exams in physics but also equips you with the foundational knowledge to pursue the answers behind how everything works in our reality.

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