Chemical Weathering

 

Chemical weathering - mainly occurs in warm and wet climates.

 

Water, oxygen and acid can all weather rocks away.  They breakdown rocks by dissolving or reacting with some of the minerals in the rock.

 

Water attack

 

Water itself can cause chemical reactions with some rocks.  Water attacks one of the minerals in granite, for example.  The granite break down into tiny particles of clay.  These then get washed away.

 

Oxygen attack

 

Oxygen gas in the air can attack some rocks, particularly those containing iron.  The rusty brown streaks in some rocks are due to iron reacting with oxygen.  This leads to the formation of iron compounds which causes weakness in the rocks, hence the rocks wear away.

 

Chemical Weatherimg: Acid Rain

 

Chemical weathering is not just due to "acid rain" caused by pollution, it is also instigated by ordinary rain.  You probably think that rain-water is pure.  Actually it is slightly acidic.  This is because carbon dioxide gas dissolves in rain as it falls.  The resulting weak acid (carbonic acid) attacks rocks, mainly those containing calcium carbonate.  

 

Calcium carbonate, in limestone, reacts with carbonic acid:

 

Calcium carbonate  +  carbonic acid    calcium hydrogencarbonate

CaCo₃(s)            +            H₂CO₃(aq)            Ca(HCO₃)₂(aq)

Soluble calcium hydrogencarbonate is formed.  So the limestone is slowly worn away.

 

 

🧪 Knowledge Check: Chemical Weathering

Test your understanding of how acid rain reacts with rocks and the chemical processes involved.

1. Which gas from the air dissolves in rain water to make it slightly acidic?

• Oxygen
• Nitrogen
• Carbon dioxide

2. What is the name of the weak acid formed when carbon dioxide reacts with rain water?

• Carbonic acid
• Sulphuric acid
• Hydrochloric acid

3. Limestone and chalk are easily weathered because they both contain which chemical?

• Sodium chloride
• Calcium carbonate
• Magnesium sulphate

4. What happens to the rock after the acid rain reacts with it?

• It turns into a harder type of rock.
• It forms a soluble substance that is washed away.
• It changes colour but stays the same shape.

5. Which statement about chemical weathering is correct?

• It is a very slow process that takes many years.
• It only happens to volcanic rocks.
• It is caused by the wind hitting the rock.

Click to Reveal Answers

1. Carbon dioxide (Dissolving in water makes it acidic).
2. Carbonic acid (The specific weak acid mentioned on the page).
3. Calcium carbonate (The mineral found in both limestone and chalk).
4. It forms a soluble substance that is washed away (This is how the rock is slowly removed).
5. It is a very slow process that takes many years (As stated in the final paragraph of the guide).

 

Tags: Erosion, Weathering, Biological weathering, Acid attack, Water attack, Oxygen attack, How do weathering and erosion work together, what does weathering mean, chemical weathering acid rain