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Acidity Change of Dry Ice in Water

Procedure

1. Add a few mLs of universal indicator to 900mL of water in a 1 liter graduated cylinder. Color is red from dissolved CO₂ in the water.

2. Drop in a chunk of dry ice.

3. Add 5mLs of 3M NaOH. Color changes from purple to yellow as the pH drops. pH never gets low enough again to turn indicator red. Repeat cycle.

4. Reaction slows down with successive aliquots of NaOH showing the buffering effects of the salts formed.

Reactions

CO₂(s) --> CO₂(g)

CO₂(g) + H₂O --> H₂CO₃ <--> H⁺ + HCO₃^-(aq)

2NaOH (aq) + CO₂(g) --> Na₂CO₃(aq) + H₂O