Draw the point
★ Metals cannot emit odor molecules through vapor, so we cannot smell metal.
★ The smell produced after touching the metal by hand is the odor of aldehydes or ketones formed after the body's secretions come into contact with the metal.
★ The taste of table salt comes from the sodium and potassium ions in it.
Figureworm creative
In ancient Times, China had an industry prejudice against merchants, calling it "full of "copper smell (xiù) smell". The Tang Dynasty poet Pi Rixiu also used copper odor to represent the merchant in the article "Wuzhong Bitter Rain Because of a Hundred Rhymes to Send Lu Wang": "Wuzhong copper stinks households, 70,000 boiling like a gourd." It has been explained that this statement is because the merchant has been exposed to copper money for many years and has been contaminated with that unique taste. In fact, if we think about it carefully, we will have an impression, and we will encounter similar phenomena in our lives, such as: iron products smell a unique "rusty smell". So does metal really have a taste? Today we will talk to you about this topic from the principle of taste generation and feeling.
01
The nose smells
Two conditions need to be met
I believe that everyone has such an experience: when cooking, pour some white wine into the pot, you will smell a unique fragrance, in fact, the aroma captured by our noses is derived from the ester compounds formed by alcohol and oil under high temperature conditions. Ester compounds are organic compounds with a low boiling point, and when stir-frying, the high temperature makes them become gas molecules and diffuse into the air. We can smell the fragrance because the ester compound molecules in the air bind to the receptor sensor in the nose and receive the smell.
It can be seen that smelling a substance requires two conditions: the substance can volatilize or evaporate to form gas molecules or vapors, and the gas or vapor can bind to the receptor sensor in the nose.
Metal atoms are connected by metal bonds and have a strong binding force, so unlike organic matter, most metals have a high melting point and boiling point, are solid at room temperature and are extremely difficult to volatilize.
However, as the saying goes, there are no absolutes in the world! There is a metal with a very low melting point, which is mercury, also known as mercury. Mercury has a melting point of -39°C and remains liquid at zero. Although the boiling point exceeds 350 ° C, it can partially become mercury vapor at room temperature, mainly because the electron sublayers of mercury atoms are in a full state, reducing the force between mercury atoms. But even then, mercury vapor can't smell. Moreover, mercury is a highly toxic substance that can also cause cancer. The flowing liquid inside the mercury thermometer commonly used in the home is mercury, if the thermometer is broken, do not smell the taste, you should immediately pour milk on the breaking area, and wear gloves to wipe it with a paper towel in time.
02
Where does the smell of metal come from?
Then again, since metals can't emit odor molecules through vapor, where do we smell metallic odors come from? After touching the coin or iron frame with your hand, smell the tip of your finger with your nose, it really smells!
Scientists have found that the "metallic taste" produced after touching the metal by hand is actually an odorous aldehyde or ketone substance formed by the catalytic oxidation reaction of the human secretion after contact with the metal.
Among them, after the iron product comes into contact with the skin, the sweat secreted by the human body will promote the oxidation of trace amounts of iron in the iron product into ferrous ions, and further catalyze the decomposition of peroxide ester on the surface of the skin into 1-octen-3-one (1-octen-3-one)[1]. The so-called rust smell is actually the odor produced by the 1-octen-3-one molecule formed by the contact of iron products with the skin.
An interesting digression: Rust smells a lot like blood, but the odor molecules of the two are different. The researchers found that trans-4,5-epoxy-2-decenal (trans-4,5-epoxy-(E)-2-decenal) emits a special odor that is easily associated with blood, the so-called bloody smell [2].
To sum up: the so-called metallic taste is not produced by the metal itself volatilization, but is achieved by chemical action to form odorous organic molecules. Except for a small number of metals that can catalyze the decomposition of organic matter, other metals cannot produce odors. In other words, the metallic smell you smell is actually your own "body odor."
03
Metal can't smell
Can you taste it?
Oyster lovers should know that there are some varieties of oysters that taste a unique "metallic taste", not only smell like, but also taste more like! Does this mean that some metals can sense taste with their tongues?
Indeed, there are some metals that can be tasted, and sodium and potassium are common in life. Condiment table salt is known to be salty, its main component is sodium chloride, may also contain potassium chloride. But in fact, strictly speaking, the taste of our tongue is not metallic sodium and potassium, but sodium and potassium ions. Sodium and potassium ions can interact with the corresponding ion channel receptors on the surface of taste bud cells, form a weak nerve current through osmotic pressure difference, and stimulate the brain to produce a salty feeling.
Ordinary metals, because their atoms cannot detach from the metal surface, cannot stimulate the taste even if they are contained in the mouth. However, I believe that everyone may have such an experience: taking a metal spoon in your mouth, always feels a hint of sweetness, which seems to contradict the previous theory?
In fact, this so-called sweetness is not really a metallic taste. Since most metal products cannot achieve 100% purity, the metal will be doped with trace amounts of impurity metal or carbon. In the electrolyte environment of the oral cavity, different active metals will form a system similar to the primary battery after contact, and cause a certain weak current stimulation to the lips or tongue, so that the brain thinks that it is sweet. Therefore, it is recommended that you do not always take the metal spoon in your mouth in the future, because the sweetness you think is not sweet, but electricity.
summary:
The metallic smell taste tasted by the tongue is not necessarily real, and the metallic smell smelled by the nose must not be real.
Resources:
1. Dietmar Glindemann, Andrea Dietrich, Hans-Joachim Staerk, Peter Kuschk. The Two Odors of Iron when Touched or Pickled: (Skin) Carbonyl Compounds and Organophosphines[J]. Angewandte Chemie International Edition, 2006, 45(42), 7009-7009.
2. Sara Nilsson, Johanna Sjoberg, Mats Amundin, Constanze Hartmann, Andrea Buettner, Matthias Laska. Behavioral Responses to Mammalian Blood Odor and a Blood Odor Component in Four Species of Large Carnivores[J]. PLoS ONE, 2014, 9(11), e112694.
Author | Zhiqin Yuan is an associate professor at Beijing University of Chemical Technology
Audit | Bin Xiong is an associate professor at the School of Chemistry and Chemical Engineering, Hunan University
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Source: Scientific Debunking
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