Environmental Chemistry looks light but I am dropping marks in the details. Three things I'd like to settle properly. 1. NCERT says rain water is normally pH 5.6 due to dissolved CO2, and acid rain has pH below 5.6. If p

Why is normal rain slightly acidic (pH 5.6)? And classical smog vs photochemical smog, I keep swapping the details

Environmental Chemistry looks light but I am dropping marks in the details. Three things I'd like to settle properly.

NCERT says rain water is normally pH 5.6 due to dissolved CO2, and acid rain has pH below 5.6. If pure water is pH 7 why isn't "acid rain" pH below 7? I understand the CO2 part ( $CO_2 + H_2O \rightleftharpoons H_2CO_3$ ) but I don't see why 5.6 is the cut-off and not something else.
Smog. I know there are two types and I keep mixing the properties: one is called classical (London type) and one is photochemical (Los Angeles type). I have a vague feeling about temperature, humidity, and which one is "reducing" vs "oxidising" but I can't hold the table in my head.
Why is carbon monoxide so dangerous? I wrote "it is a poisonous gas" and my teacher wrote "how?" in red.

Also for photochemical smog, I know there are NO2, ozone, PAN, formaldehyde. But the sequence of reactions of NO2 with sunlight? If someone can write it compactly I will be very grateful.

6 answers & replies

[+]Vivek Mishra12thSample· 9d ago

Acid rain is any rain with pH less than 7, no? Because acid means below 7. I think 5.6 is just an average number.

[+][–]Lakshmi TripathiDropperSample· 9d ago
Vivek, not exactly. Clean rain isn't pH 7, because atmospheric CO2 dissolves in it and forms carbonic acid, which lowers the pH to about 5.6. So pH 5.6 is "normal" for unpolluted rain. Acid rain means something beyond CO2: extra acids from the oxides of sulphur and nitrogen, $SO_2 \rightarrow SO_3 \rightarrow H_2SO_4$ and $NO_2 \rightarrow HNO_3$ . That's why the definition is "below 5.6".
- [+][–]Meera Patel11thSample· 9d agoAuthor
  Thank you lakshmi, that's the answer I wanted: "normal rain is 5.6 because of CO2, acid rain is more acid than that". Smog table copied onto an index card.

[+]Riya Singh12thSample· 9d ago

Acid rain eats marble too, the Taj Mahal is the usual NCERT example: $CaCO_3 + H_2SO_4 \rightarrow CaSO_4 + H_2O + CO_2$ . Calcium sulphate is more soluble than the carbonate, so the surface slowly dissolves and the white marble gets damaged. I find the image sad, a poem in itself.

[+]Aman YadavDropperSample· 9d ago

Smog table, learn this and you're done:

	Classical (London)	Photochemical (Los Angeles)
Weather	cool, humid	warm, dry, sunny
Made from	smoke + fog + $SO_2$	NOx + hydrocarbons + sunlight
Nature	reducing smog	oxidising smog
Occurs	winter, early morning	summer, midday
Key products	sulphur compounds, smoke	$O_3$ , PAN, HCHO, acrolein

Reactions for the photochemical one: $NO_2 \xrightarrow{h\nu} NO + O$ $O + O_2 \rightarrow O_3$ $NO + O_3 \rightarrow NO_2 + O_2$

Then hydrocarbons react with ozone and NO to give formaldehyde, acrolein and PAN (peroxyacetyl nitrate).

[+]Farhan ShaikhDropperSample· 9d ago

CO: it binds with haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable than oxyhaemoglobin (I think that's the NCERT number). The blood then can't carry enough oxygen to tissues, headache, dizziness, and at high concentration, death. That's the "how" your teacher wanted.

Photochemical smog is also eye-irritating, causes breathing problems. Classical smog hurts the lungs more because of the sulphur dioxide.

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