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Question: Why doesn't water smell?
It is true that most humans cannot smell pure water and we know better than drinking water that we can actually detect a smell! But it is also true that some animals can indeed smell water; they wouldn't survive without this aptitude. So, the question is: If the water that animals smell is the same water that we can't smell, does the water have a smell or not?
In order to talk about smell we have to first discuss olfaction. Olfaction is one of our five senses. It uses a large part of our brain. It not only enhances taste but also warns us of dangerous situations. Olfaction enables us to detect odors and odors can elicit emotions like fear, sadness and love. In humans, the olfaction region is a small area with less than 0.5 square inches in each of the two nasal passages. Olfaction allows animals with olfactory receptors to distinguish among an enormous number of chemical compounds, called odorants, at very low concentrations.
Odorants are volatile chemical compounds that are carried by inhaled air into the nose. In order to provide sensory properties this compound must be partially soluble in water and fat among other characteristics. Once inhaled the odorants have to be detected. There are several theories on olfaction. Most of them are based on the fact that a specific odor is due to the binding of the odorant molecule to the receptor site in the olfactory system. This binding is based on the molecule's shape and size. The nose then, sends a message to be decoded in the brain. Some odorants, like perfumes, triggers a big response and some don't. The mammal olfactory system uses a combination of receptors to create a specific smell response. This is how approximately one thousand receptors can describe thousands of different odors. The water, H2O, is a small molecule with an angular shape.
So, why does water not smell?
Basically, because it does not prompt a response from the human olfactory system, as it does in other vertebral animals and some organisms. Does it leave the humans in disadvantage in Nature? I don't think so; it did not prevent our ancestors to locate and use water. They combined all five senses and used a very important tool: Intelligence.
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