If we add HCL to a river that is alkalic (due to a NaOH spill for example) to neutralize it, would it kill the fish ? Or would it save them from the NaOH?

fresh water fish can tolerate a small amount of salt, so it really all depends on the quantity and fish breed.

in neutralization reaction: Acid+Base=>salt+water

so the answer is NO, it wouldnt kill the fish, had you mentioned the PH of water, the answer would have been different

That makes no sense whatsoever.

The death of the fish cannot possibly depend on whether or not someone mentions pH any more than it matters if they mention the dead parrot sketch.

What he means is that a pH swing is far more dangerous than a slight increase in salt content. When I collect fish i often add salt to the collection buckets to protect teh fish from injury even though the fish are coming from soft acidic water. If I added an alkaline substance to the water it would be more likely to kill the fish. At the very least ammonia is far more poisonous at high pH than it is at low pH. pH is more important to fish than dissolved solid content.

Salt matters to fish. Sea fish cannot survive in sweet water, and sweet water fish cannot survive in the sea.

However, I think that the NaOH is much more dangerous to fish... so if the NaOH spill is so small that the fish survived so far, then the little salt will do no harm I think.

Salt matters to fish. Sea fish cannot survive in sweet water, and sweet water fish cannot survive in the sea.

 

What do you mean by this? No one was suggesting dedicated marine fish can live in freshwater or that dedicated freshwater fish can live in the ocean. All surface water on the earth contains some salts, some more than others and fish can and do live in waters of varying salinity. Freshwater fish are probably more tolerant than marine fish to variations in salt content.

 

However, I think that the NaOH is much more dangerous to fish... so if the NaOH spill is so small that the fish survived so far, then the little salt will do no harm I think.

 

Sodium hydroxide is a powerful base, high pH is usually not well tolerated by freshwater fish, fish exposed to a high pH would be vulnerable to a rise in ammonia at the very least. The key here is to lower the pH slowly, most fish cannot tolerate a rapid change in pH. Salt is not the issue, pH is the issue. Combining HCl with NaOH will not result in a large amount of salt, if that much of either chemical was present the fish would be long dead.

What I meant to say is that if the salt concentration gets as high as in the ocean, then your sweet water fish will eventually die from the salt water, even in the totally unlikely case that they survive a short exposure to the NaOH. I agree that I could have expressed myself a bit better there