Explain why distilled water is preferred to dilute sulphuric acid when changing an accumulator cell. 

During recharging, there can be some production of hydrogen and oxygen caused by splitting of the water part of the acid. This needs to be replaced with distilled water. The sulphuric acid level remains the same, but since water is being lost, the concentration goes up, so adding water restores it to how it was when the battery was new.