======Carbon Dioxide Equilibrium in Water======


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**Alternative titles:** Effect of Pressure on Carbon Dioxide Solubility
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====Summary====                   
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====Procedure====  
  -Draw carbonated water into a large syringe and add a few drops of bromcresol green indicator.  
  -Seal the syringe tightly with a cap to trap the liquid and gas.  
  -Observe the initial color of the solution; under pressure, CO2 remains dissolved and the solution is more acidic.  
  -Gently pull back on the plunger to reduce pressure inside the syringe.  
  -Watch for bubble formation as CO2 comes out of solution and note the color change of the indicator as the acidity decreases.  
  -Push the plunger back in to increase pressure again; observe that some CO2 redissolves, shifting the indicator color back toward acidic.




====Links==== 
Le Chatelier and Pressure Changes using carbonated water - Linda Hanson: 
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LeChatelier's Principle and the Solubility of Carbon Dioxide - FlinnScientific: 
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====Variations====
  *Repeat with different indicators (e.g., universal indicator or phenol red) to compare color ranges.  
  *Try using sparkling water at different temperatures; colder samples retain more CO2 and show slower color change.



====Safety Precautions====
  *Do not pull the plunger out completely; carbonated liquid may spray.  
  *Keep the syringe upright to minimize spillage during pressure changes.  
  *Wear safety glasses and protect work surfaces from staining by the indicator.




====Questions to Consider====
  *Why does the indicator show a color change when the syringe pressure is lowered? (Less CO2 dissolves, lowering H+ concentration and increasing pH.)  
  *How does Henry’s Law explain the behavior of CO2 in the syringe? (Gas solubility is proportional to its partial pressure above the liquid.)  
  *Why does the solution become acidic when CO2 is dissolved? (CO2 reacts with water to form carbonic acid, releasing H+ ions.)  
  *What differences would you expect if you repeated the demonstration with warm carbonated water? (Less CO2 would dissolve, so the solution would be less acidic and lose its fizz faster.)