What is the evolution of gas?

Evolution of gas occurs during a double displacement reaction where one of the products is a gas. 

For example, carbonic acid breaks down to water and carbon dioxide

H₂CO₃  ---> H₂O + CO₂

sulfurous acid breaks down to water and sulfur dioxide

H₂SO₃ --> H₂O + SO₂

ammonium hydroxide breaks down to water and ammonia gas.

NH₄OH --> NH₃+ H₂O