The best explanation as to why onion cells do not burst, but red blood cells often do when placed in distilled water.

the red blood cells have only a cell membrane, which does not protect them from bursting . the onion cells do not have a cell wall that could protect them from bursting . the onion cells have a cell membrane, which can protect them from bursting 

when a reversible reaction is occurring in both directions at the same rate; the products and reactants do not change in concentration. a chemical reaction where the compound is broken into its constituent parts.

sulfur dioxide

explanation: acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. these substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.