Even though H2O and H2S have same molecular structure, H2S smells bad, but H2O not. Why ???

Question

H2S has more electrons than H2O and the angle is little bit different.

Answer 1

Actually, H2O and H2S are quite different. H2O is a clear colorless liquid in room temperature, but H2S is a gas. H2O has hydrogen bonding network. Hydrogen sulfide, similar to carbon monoxide, is a chemical asphyxiant that interferes with oxygen transport and cellular respiration and thereby cause tissue hypoxia.

Also, we can consider pseudohalides and halides. Chemically, pseudohalides behave similar to halides while most pseudohalides, like the most famous poison cyanide, is highly toxic.

Humans are very sensitive to sulfides [the distinctive smell]; that's why even if H2S is more toxic than HCN chemically, HCN is actually more lethal than H2S because we can detect the smell of H2S and leave.

Answer 2

Biologically speaking, your lungs cannot tell the difference. That is extremely bad. There is recorded evidence of entire dinosaur herds dying due to the inability to distinguish between H2O and H2S. Therefore these species die off in these conditions. However, if a species can distinguish this noxious gas, they live to breed on.

Now, poisonous gas is not the only reason this adaptation was made. The smell of rotting eggs is H2S. This gas signifies a rotting egg and thereby an egg that is unfit for consumption.

Humans developed a sense that H2S is poisonous. The human brain is not very complex. Don't listen to the people who say it is. The brain finds the easiest way to get you to do what it wants. A foul odor is a great way to get you to leave something alone. Therefore, olfactory senses adapted to recognize H2S and the human brain has registered the smell of H2S as a foul odor to prevent you from getting injured.

Now, away from the biology and into the chemistry. Oxygen has only one common oxidation state, -2. Sulfur on the other hand has many. Therefore, sulfur reacts much more. When H2S hits the liquid of your nasal passages, it is converted to Sulfurous Acid in your nose. This is the burning sensation you feel. This acid will quickly wear away at your linings until you cannot smell it anymore. This is how you "get used to the smell". This is a bad sign that alludes to the damage being done to your nose and lungs.

In short, H2O in your body stays H2O. H2S in your body QUICKLY converts to H2SO3.

Answer 3

A minor correction to the poster who says that sulfur compounds are what are responsible for the smell of rotting meat and poop.

That is not correct.

Rotting flesh/meat smell is due to two chemicals. One is called cadaverine and the other is called putracine. Both are diamines and contain no sulfur. Their names say it all.

The smell to poop or feces is due to two chemicals: indole and skatole. These are chemically similar to triptophane, one of the essential amino acids for your body. The name "skatole" is the Greek word for feces, by the way, so it is an appropriate name. Neither indole nor skatole contain any sulfur. Ironically indole and skatole are also responsible for the smell of orange blossoms, and are commonly used in perfumes. The difference is their concentrations, in dilute amounts they have a very pleasant flowery smell, but in concentrations they are the unpleasant smell of feces.

Chemically, the reason why H2S behaves so differently from H2O, water, is that it doesn't form hydrogen bonds. I don't know the psysiology of the nose to know whether that would be a factor in why H2S smells so bad to us, but I suspect that it must have something to do with it as it is the main chemical difference. Both sulfur and oxygen have two lone pairs. The angle of the hydrogen atoms is about the same, both molecules are "bent". The mass of the sulfur is more than the oxygen but that should make its boiling point higher. But because it doesn't form hydrogen bonds, it has a much lower boiling point and exists as a gas at room temperature. H2S is the smell of rotten eggs.

Answer 4

First of all, H2S is much more volitile, so there is more of it to smell. Mostly though, our bodies olfactory system has, for what ever reason, decided to make us percieve it as a "bad smell". Most sulfur compounds, espicially those with an SH bond really stink. They are called mercaptins or thiols. This is what skunks spray and what makes poop and rotting meat stink. Beyond this, you may want to ask a MD.

Answer 5

The receptors in your nose read H2S as a bad smell.

Answer 6

Most volatile sulphur compounds have a bad smell. This is because poo poos, fart gas and rotten food contain these compounds. It's nature's way of telling us not to eat or sniff them.

Answer 7

you are smelling the sulfur in the dihydrogen sulfide while oxygen is a pure colorless odorless gas in teh dihydrogen oxide.

Answer 8

Sulphur is the smelly substance.

Answer 9

You are smelling the Sulfur. Sulfur is what is in flatulence.