Lead-acid batteries use dilute H₂SO₄ as electrolyte. The electrodes are Pb and PbO₂. During discharge, both plates form PbSO₄ and the acid concentration decreases. Recharging reverses the reaction.
Explanation:
In K2Cr2O7, potassium has an oxidation state of +1 and oxygen is -2. Let the oxidation state of Cr be x. The sum of oxidation states in a neutral molecule is zero: 2(+1) + 2(x) + 7(-2) = 0. Solving this gives 2 + 2x - 14 = 0, so 2x = 12, and x = +6.
Explanation:
Gypsum (CaSO₄·2H₂O) is added to clinker to control the rapid reaction of tricalcium aluminate with water, which would otherwise cause flash (quick) setting. Gypsum slows down the hydration of C₃A, thus increasing the setting time and allowing workability. Without gypsum, cement would set almost immediately upon mixing with water, making placement impossible.
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