Coal seams are far deep underground to be considered for opencast minings. These coal seams require underground mining. Since it is too deep underground, it can be used for mapping and correlation of sedimentary rocks or as a key bed.
A scientific theory changes when there is more information available about what the theory covers.
Answer:
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A substance that releases hydroxyl ions in water is a base. Acids on the other hand release hydrogen ions when dissolved in water, while salts release sodium. To define acids and bases the Arrhenius definition is used by scientists---this describes what substances are produce by each type of solutions. Under the Arrhenius definition salt and water can be used to predict the result of the neutralization of an acid with a base.
In Chemistry, Bronsted-Lowry definition has superseded the Arrehenius definition.
