CO2 Makes Acid
Dissolve carbon dioxide in water to form carbonic acid
What this lesson covers
Why it matters
Clean rain is not neutral; it carries a faint sting of acidity from the air itself. Carbon dioxide dissolves in water droplets to form a weak acid.
The idea in plain words
Balance check: confirm the combination of one CO₂ and one H₂O into one H₂CO₃.
Predict first
Why does pure water become slightly acidic when exposed to air?
Carbon dioxide combines with water molecules to create carbonic acid (H₂CO₃), a weak acid responsible for the natural acidity of rain.
- CO₂ dissolves to form carbonic acid — correct
- Pure water slowly turns acidic by itself, even with no gas dissolved in it
- CO₂ reacts to form hydrochloric acid
- CO₂ simply lowers the pH without reacting
What you do
Bubble CO₂ into the water. Observe the formation of carbonic acid and note the slight acidity.
Check yourself
What is the chemical formula of the acid formed?
The reaction CO₂ + H₂O → H₂CO₃ produces carbonic acid.
Why is clean rain slightly acidic (pH ~5.6)?
Natural rain absorbs atmospheric CO₂, forming H₂CO₃ which lowers pH to ~5.6.
What type of reaction is CO₂ + H₂O → H₂CO₃?
Two reactants combine to form a single product, defining a combination reaction.
- H₂CO₃ — correct
- H₂SO₄
- HCl
- HNO₃
- Dissolved CO₂ forms weak carbonic acid — correct
- Rainwater contains dissolved dust
- Oxygen in the air makes it acidic
- It is a measurement error
- Combination — correct
- Decomposition
- Displacement
- Neutralization