Gas Released When Alum is Added to Water

When alum, which is a compound known as aluminum potassium sulfate (KAl(SO4)2), is added to water, it undergoes a chemical reaction that produces carbon dioxide gas (CO2) as one of the products. This gas is released as bubbles and can be observed as effervescence.

Explanation:

1. The Reaction:
- When alum is added to water, it dissociates into its constituent ions: aluminum ions (Al3+), potassium ions (K+), and sulfate ions (SO42-).
- The reaction can be represented as follows: KAl(SO4)2 + H2O → Al3+ + K+ + 2SO42- + H2O

2. The Formation of Carbon Dioxide:
- The carbon dioxide gas is formed due to the presence of bicarbonate ions (HCO3-) in water. These bicarbonate ions react with the aluminum ions released from the alum.
- The reaction between the aluminum ions and bicarbonate ions can be represented as follows: Al3+ + 3HCO3- → Al(OH)3 + 3CO2

3. Release of Carbon Dioxide:
- The carbon dioxide gas is released as bubbles during the reaction, resulting in effervescence.
- This release of gas can be observed as the alum dissolves in water.

4. Other Possible Reactions:
- The other options mentioned in the question, such as Al(OH)3, CaSO4, and Ca(OH)3, are not the correct answers because they do not represent the gas released when alum is added to water.
- Al(OH)3 represents aluminum hydroxide, which is a white precipitate formed when aluminum ions react with hydroxide ions.
- CaSO4 represents calcium sulfate, which is also a white precipitate formed when calcium ions react with sulfate ions.
- Ca(OH)3 represents calcium hydroxide, which is a white precipitate formed when calcium ions react with hydroxide ions.

Conclusion:

When alum is added to water, it undergoes a chemical reaction that releases carbon dioxide gas. This gas is formed through the reaction between aluminum ions and bicarbonate ions present in water. The release of carbon dioxide can be observed as effervescence.