Earth Science - Other

Why does your Tongue Stick to Metal

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Your tongue is kept moist by the water on it, water which freezes at 32°F or 0°C. When you stick your tongue out to touch cold metal of some sort, and your tongue sticks to the metal, this is because the water on your tongue, between the skin and the metal, freezes as a result of the temperature of the metal being at 32°F or 0°C or below that temperature. The lower the temperature of the metal is, the faster your tongue will stick to the metal. So, why is it, you may ask, if you stick your tongue to a very cold plastic object, does it not stick, if the sticking of your tongue is just based on the water on your tongue freezing? The answer to this question depends on another concept in chemistry known as thermal conductivity.

Thermal conductivity can be simply defined as the ability of a certain material to conduct heat. Plastic or rubber objects, even at very low temperatures of -300°F (-184.44°C), will not stick to your moist tongue as a result of the thermal conductivity of these materials. Materials that have a high thermal conductivity, such as many metals, are able to transfer a large amount of heat from a material of higher temperature to a material of lower temperature. This implies that the cold metal that your tongue sticks is able to extract heat faster that this heat is able to be supplied by your blood. The heat that is able to be conducted away from the object of higher temperature to the one of lower temperature can be scientifically described as the product of the temperature difference between the two materials involved – in this case your tongue and the metal – and the thermal conductivity of the metal. This equation highlights that it is not just the temperature difference between your tongue and the metal that causes it to stick to the metal, but also the thermal conductivity, as previously mentioned.

Plastics and rubber have a thermal conductivity about a hundred times lower than that of metal, and so cannot extract heat from your tongue as efficiently as metal does. This inability to remove heat from your tongue prevents the water on it from freezing, and hence is unable to make your tongue stick to the plastic or rubber. Sticking your tongue to metal can be painful as a result of the high thermal conductivity making it difficult to “unstick”, so if you are going to try to stick your tongue to something, it better be something like plastic or rubber so that you do not hurt yourself.

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