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Why is it considered Acid Rain with pH <5.6

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Why is it considered Acid Rain with pH

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2

1

$begingroup$

I recently read in a book that rain is considered Acid Rain if the pH falls below 5.6. However a substance is acidic when the pH is below 7; so why is the boundary for Acid Rain 5.6?

I was thinking pH between 5.6-7 would be too dilute to have an effect on limestone and other materials that's why the pH has to be under 5.6. But I wasn't sure if this is true so I wanted to find out.

inorganic-chemistry acid-base everyday-chemistry

share|improve this question

asked 8 hours ago

muhammad haidermuhammad haider

8155 bronze badges

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Because "normal rain" still is a bit acidic.
  $endgroup$
  – Mithoron
  4 hours ago
  

  
  
  
  

add a comment | 

2

1

$begingroup$

I recently read in a book that rain is considered Acid Rain if the pH falls below 5.6. However a substance is acidic when the pH is below 7; so why is the boundary for Acid Rain 5.6?

I was thinking pH between 5.6-7 would be too dilute to have an effect on limestone and other materials that's why the pH has to be under 5.6. But I wasn't sure if this is true so I wanted to find out.

inorganic-chemistry acid-base everyday-chemistry

share|improve this question

asked 8 hours ago

muhammad haidermuhammad haider

8155 bronze badges

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Because "normal rain" still is a bit acidic.
  $endgroup$
  – Mithoron
  4 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

I recently read in a book that rain is considered Acid Rain if the pH falls below 5.6. However a substance is acidic when the pH is below 7; so why is the boundary for Acid Rain 5.6?

I was thinking pH between 5.6-7 would be too dilute to have an effect on limestone and other materials that's why the pH has to be under 5.6. But I wasn't sure if this is true so I wanted to find out.

inorganic-chemistry acid-base everyday-chemistry

share|improve this question

asked 8 hours ago

muhammad haidermuhammad haider

8155 bronze badges

$endgroup$

share|improve this question

asked 8 hours ago

muhammad haidermuhammad haider

8155 bronze badges

share|improve this question

asked 8 hours ago

muhammad haidermuhammad haider

8155 bronze badges

asked 8 hours ago

muhammad haidermuhammad haider

8155 bronze badges

asked 8 hours ago

muhammad haidermuhammad haider

8155 bronze badges

3 Answers
3

active

oldest

votes

6

$begingroup$

The $mathrmpH$ of pure water (rain as well as distilled water) in equilibrium with the atmosphere ($p_ceCO2= pu10^-3.5 atm$) can be calculated as follows.

$$[ceH2CO3^*]=K_mathrm Hcdot p_ceCO2$$

where $[ceH2CO3^*]$ is the total analytical concentration of dissolved $ceCO2$, i.e. $[ceH2CO3^*]=[ceCO2(aq)]+[ceH2CO3]$, and
$K_mathrm H= pu3.39times10^-2 mol: L^-1: atm^-1$ is Henry's law constant for $ceCO2$.

$$beginalign
log[ceH2CO3^*]&=log K_mathrm H+log p_ceCO2\
&=-1.5-3.5\
&=-5.0
endalign$$

The commonly used first acid dissociation constant of carbonic acid $mathrm pK_mathrm a1=6.3$ (at $25 mathrm^circ C$) actually is a composite constant that includes both the hydration reaction
$$ceH2O + CO2(aq) <=> H2CO3$$
and the protolysis of true $ceH2CO3$
$$ceH2CO3 <=> H+ + HCO3-$$
For a weak acid
$$beginalign
log[ceH+]&approxfrac12left(log K_mathrm a+log[ceH2CO3^*]right)\
&=frac12left(-6.3-5.0right)\
&=-5.65\
mathrmpH&=5.65
endalign$$

Thus, pure rain in equilibrium with the atmosphere has about $mathrmpH=5.65$. Any acid rain with lower $mathrmpH$ would be caused by additional acids.

share|improve this answer

edited 16 mins ago

Mathew Mahindaratne

10.6k11 gold badge1212 silver badges3838 bronze badges

answered 5 hours ago

Loong♦Loong

36.3k88 gold badges9191 silver badges193193 bronze badges

$endgroup$

add a comment | 

1

$begingroup$

You are forgetting an important component of the air: carbon dioxide. When it dissolves in pure water (=rain water), it makes it acidic. It is not considered that harmful.

Acid rain has a negative connotation; it is mainly caused by anthropogenic activities. The low pH of acid rain is due to sulfur oxides and nitrogen oxides and it is indeed below 5.7.

share|improve this answer

answered 8 hours ago

M. FarooqM. Farooq

5,00666 silver badges1919 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  but why is it considered acid rain only below pH 5.7? Why not below 7?
  $endgroup$
  – muhammad haider
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  $begingroup$
  Because natural rain, which is free from all types of anthropogenic pollution has a pH < 7. You need to "distinguish" natural acidity vs. excessive acidity due man's activities. Otherwise what is the point of calling each an every rain as acid rain. Also remember there is little bit of nitric acid formed from lightning which is the plant food. That will not be termed as acid rain either. As I said, acid rain has a negative meaning.
  $endgroup$
  – M. Farooq
  5 hours ago
  

  
  
  
  

add a comment | 

0

$begingroup$

Under atmospheric pressure, dissolved carbon dioxide can reach an equilibrium state in water that yields a pH of as low as 5.7

share|improve this answer

answered 6 hours ago

Joseph HirschJoseph Hirsch

64944 silver badges1515 bronze badges

$endgroup$

add a comment | 

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6

$begingroup$

The $mathrmpH$ of pure water (rain as well as distilled water) in equilibrium with the atmosphere ($p_ceCO2= pu10^-3.5 atm$) can be calculated as follows.

$$[ceH2CO3^*]=K_mathrm Hcdot p_ceCO2$$

where $[ceH2CO3^*]$ is the total analytical concentration of dissolved $ceCO2$, i.e. $[ceH2CO3^*]=[ceCO2(aq)]+[ceH2CO3]$, and
$K_mathrm H= pu3.39times10^-2 mol: L^-1: atm^-1$ is Henry's law constant for $ceCO2$.

$$beginalign
log[ceH2CO3^*]&=log K_mathrm H+log p_ceCO2\
&=-1.5-3.5\
&=-5.0
endalign$$

The commonly used first acid dissociation constant of carbonic acid $mathrm pK_mathrm a1=6.3$ (at $25 mathrm^circ C$) actually is a composite constant that includes both the hydration reaction
$$ceH2O + CO2(aq) <=> H2CO3$$
and the protolysis of true $ceH2CO3$
$$ceH2CO3 <=> H+ + HCO3-$$
For a weak acid
$$beginalign
log[ceH+]&approxfrac12left(log K_mathrm a+log[ceH2CO3^*]right)\
&=frac12left(-6.3-5.0right)\
&=-5.65\
mathrmpH&=5.65
endalign$$

Thus, pure rain in equilibrium with the atmosphere has about $mathrmpH=5.65$. Any acid rain with lower $mathrmpH$ would be caused by additional acids.

share|improve this answer

edited 16 mins ago

Mathew Mahindaratne

10.6k11 gold badge1212 silver badges3838 bronze badges

answered 5 hours ago

Loong♦Loong

36.3k88 gold badges9191 silver badges193193 bronze badges

$endgroup$

add a comment | 

6

$begingroup$

The $mathrmpH$ of pure water (rain as well as distilled water) in equilibrium with the atmosphere ($p_ceCO2= pu10^-3.5 atm$) can be calculated as follows.

$$[ceH2CO3^*]=K_mathrm Hcdot p_ceCO2$$

where $[ceH2CO3^*]$ is the total analytical concentration of dissolved $ceCO2$, i.e. $[ceH2CO3^*]=[ceCO2(aq)]+[ceH2CO3]$, and
$K_mathrm H= pu3.39times10^-2 mol: L^-1: atm^-1$ is Henry's law constant for $ceCO2$.

$$beginalign
log[ceH2CO3^*]&=log K_mathrm H+log p_ceCO2\
&=-1.5-3.5\
&=-5.0
endalign$$

The commonly used first acid dissociation constant of carbonic acid $mathrm pK_mathrm a1=6.3$ (at $25 mathrm^circ C$) actually is a composite constant that includes both the hydration reaction
$$ceH2O + CO2(aq) <=> H2CO3$$
and the protolysis of true $ceH2CO3$
$$ceH2CO3 <=> H+ + HCO3-$$
For a weak acid
$$beginalign
log[ceH+]&approxfrac12left(log K_mathrm a+log[ceH2CO3^*]right)\
&=frac12left(-6.3-5.0right)\
&=-5.65\
mathrmpH&=5.65
endalign$$

Thus, pure rain in equilibrium with the atmosphere has about $mathrmpH=5.65$. Any acid rain with lower $mathrmpH$ would be caused by additional acids.

share|improve this answer

edited 16 mins ago

Mathew Mahindaratne

10.6k11 gold badge1212 silver badges3838 bronze badges

answered 5 hours ago

Loong♦Loong

36.3k88 gold badges9191 silver badges193193 bronze badges

$endgroup$

add a comment | 

$begingroup$

The $mathrmpH$ of pure water (rain as well as distilled water) in equilibrium with the atmosphere ($p_ceCO2= pu10^-3.5 atm$) can be calculated as follows.

$$[ceH2CO3^*]=K_mathrm Hcdot p_ceCO2$$

where $[ceH2CO3^*]$ is the total analytical concentration of dissolved $ceCO2$, i.e. $[ceH2CO3^*]=[ceCO2(aq)]+[ceH2CO3]$, and
$K_mathrm H= pu3.39times10^-2 mol: L^-1: atm^-1$ is Henry's law constant for $ceCO2$.

$$beginalign
log[ceH2CO3^*]&=log K_mathrm H+log p_ceCO2\
&=-1.5-3.5\
&=-5.0
endalign$$

The commonly used first acid dissociation constant of carbonic acid $mathrm pK_mathrm a1=6.3$ (at $25 mathrm^circ C$) actually is a composite constant that includes both the hydration reaction
$$ceH2O + CO2(aq) <=> H2CO3$$
and the protolysis of true $ceH2CO3$
$$ceH2CO3 <=> H+ + HCO3-$$
For a weak acid
$$beginalign
log[ceH+]&approxfrac12left(log K_mathrm a+log[ceH2CO3^*]right)\
&=frac12left(-6.3-5.0right)\
&=-5.65\
mathrmpH&=5.65
endalign$$

Thus, pure rain in equilibrium with the atmosphere has about $mathrmpH=5.65$. Any acid rain with lower $mathrmpH$ would be caused by additional acids.

share|improve this answer

edited 16 mins ago

Mathew Mahindaratne

10.6k11 gold badge1212 silver badges3838 bronze badges

answered 5 hours ago

Loong♦Loong

36.3k88 gold badges9191 silver badges193193 bronze badges

$endgroup$

share|improve this answer

edited 16 mins ago

Mathew Mahindaratne

10.6k11 gold badge1212 silver badges3838 bronze badges

answered 5 hours ago

Loong♦Loong

36.3k88 gold badges9191 silver badges193193 bronze badges

edited 16 mins ago

Mathew Mahindaratne

10.6k11 gold badge1212 silver badges3838 bronze badges

edited 16 mins ago

Mathew Mahindaratne

10.6k11 gold badge1212 silver badges3838 bronze badges

answered 5 hours ago

Loong♦Loong

36.3k88 gold badges9191 silver badges193193 bronze badges

answered 5 hours ago

Loong♦Loong

36.3k88 gold badges9191 silver badges193193 bronze badges

1

$begingroup$

You are forgetting an important component of the air: carbon dioxide. When it dissolves in pure water (=rain water), it makes it acidic. It is not considered that harmful.

Acid rain has a negative connotation; it is mainly caused by anthropogenic activities. The low pH of acid rain is due to sulfur oxides and nitrogen oxides and it is indeed below 5.7.

share|improve this answer

answered 8 hours ago

M. FarooqM. Farooq

5,00666 silver badges1919 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  but why is it considered acid rain only below pH 5.7? Why not below 7?
  $endgroup$
  – muhammad haider
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  $begingroup$
  Because natural rain, which is free from all types of anthropogenic pollution has a pH < 7. You need to "distinguish" natural acidity vs. excessive acidity due man's activities. Otherwise what is the point of calling each an every rain as acid rain. Also remember there is little bit of nitric acid formed from lightning which is the plant food. That will not be termed as acid rain either. As I said, acid rain has a negative meaning.
  $endgroup$
  – M. Farooq
  5 hours ago
  

  
  
  
  

add a comment | 

1

$begingroup$

You are forgetting an important component of the air: carbon dioxide. When it dissolves in pure water (=rain water), it makes it acidic. It is not considered that harmful.

Acid rain has a negative connotation; it is mainly caused by anthropogenic activities. The low pH of acid rain is due to sulfur oxides and nitrogen oxides and it is indeed below 5.7.

share|improve this answer

answered 8 hours ago

M. FarooqM. Farooq

5,00666 silver badges1919 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  but why is it considered acid rain only below pH 5.7? Why not below 7?
  $endgroup$
  – muhammad haider
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  $begingroup$
  Because natural rain, which is free from all types of anthropogenic pollution has a pH < 7. You need to "distinguish" natural acidity vs. excessive acidity due man's activities. Otherwise what is the point of calling each an every rain as acid rain. Also remember there is little bit of nitric acid formed from lightning which is the plant food. That will not be termed as acid rain either. As I said, acid rain has a negative meaning.
  $endgroup$
  – M. Farooq
  5 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

You are forgetting an important component of the air: carbon dioxide. When it dissolves in pure water (=rain water), it makes it acidic. It is not considered that harmful.

Acid rain has a negative connotation; it is mainly caused by anthropogenic activities. The low pH of acid rain is due to sulfur oxides and nitrogen oxides and it is indeed below 5.7.

share|improve this answer

answered 8 hours ago

M. FarooqM. Farooq

5,00666 silver badges1919 bronze badges

$endgroup$

share|improve this answer

answered 8 hours ago

M. FarooqM. Farooq

5,00666 silver badges1919 bronze badges

answered 8 hours ago

M. FarooqM. Farooq

5,00666 silver badges1919 bronze badges

answered 8 hours ago

M. FarooqM. Farooq

5,00666 silver badges1919 bronze badges

0

$begingroup$

Under atmospheric pressure, dissolved carbon dioxide can reach an equilibrium state in water that yields a pH of as low as 5.7

share|improve this answer

answered 6 hours ago

Joseph HirschJoseph Hirsch

64944 silver badges1515 bronze badges

$endgroup$

add a comment | 

0

$begingroup$

Under atmospheric pressure, dissolved carbon dioxide can reach an equilibrium state in water that yields a pH of as low as 5.7

share|improve this answer

answered 6 hours ago

Joseph HirschJoseph Hirsch

64944 silver badges1515 bronze badges

$endgroup$

add a comment | 

$begingroup$

Under atmospheric pressure, dissolved carbon dioxide can reach an equilibrium state in water that yields a pH of as low as 5.7

share|improve this answer

answered 6 hours ago

Joseph HirschJoseph Hirsch

64944 silver badges1515 bronze badges

$endgroup$

share|improve this answer

answered 6 hours ago

Joseph HirschJoseph Hirsch

64944 silver badges1515 bronze badges

answered 6 hours ago

Joseph HirschJoseph Hirsch

64944 silver badges1515 bronze badges

answered 6 hours ago

Joseph HirschJoseph Hirsch

64944 silver badges1515 bronze badges