a. What does `[H_(9)O_(4)]^(o+)` stand for ? Draw its structures.
b. Can sodium bicarbonate make water hard?
c. Hard water is softened before using in boilers. Why?
d. What is sequestration? How is hard water made soft by sequestration?

Explanation

(a) `H_(9)O_(4)^(+)` stands for hydrated proton . In `[H_(9)O_(4)]^(+) `, `H^(+)` is tetrahedrally surrounded by four water molecules as shown.
(b) `NaHCO_(3)` cannot make water hard because soaps themselves are sodium salts of fatty acids which are soluble in water.
(c) Hard water on boiling gives precipitates of `MgCO_(3), CaCO_(3) and CaSO_(4)` which form scales in boilers . The formation of these boiler scales causes rapid deterioration of the boiler due to over heating . Further , these boiler scales are non-conducting . Therefore, more fuel is consumed . Therefore, in order to avoid formation of these scales, hard water is softened before using in boilers.
(d) Sequestration means to render ineffective . The hardness of water is due to the presence of `Ca^(2+)` and `Mg^(2+)` ions. These ions are rendered ineffective by treating them with sodium hexametaphosphate , commercially called calgon. When calgo is added to hard water, the `Ca^(2+) and Mg^(2+)` ions present in it combine with sodium hexametaphosphate to form soluble complexes of calcium and magnesium salts . These complex calcium and magnesium ions do not form any precipitate with soap and hence readily produce lather with soap solution. In this way, hard water is softened by sequestration.