What is an acid and also what is a base​

a chemical substance that neutralizes alkalis, dissolves some metals, and turns litmus red; typically, a corrosive or sour-tasting liquid of this kind.

the lowest part or edge of something, especially the part on which it rests or is supported.

Explanation:

An acid is a substance that donates protons (in the Brønsted-Lowry definition) or accepts a pair of valence electrons to form a bond (in the Lewis definition). A base is a substance that can accept protons or donate a pair of valence electrons to form a bond.

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most sediment washes or falls into the river as a result of mass movement and runoff. other sediment eroded from the bottom or sides of the river.