The enamel present on teeth becomes much harder due to the conversion of $[3 Ca_3(PO_4)_2, X]$ into $[3 Ca_3(PO_4)_2, Y]$. What are $X$ and $Y$?

$CF_xCly$ [where $x + y = 4$] compounds are not useful because .....

Given below are two statements.
Statement $I$: The non-biodegradable fly ash and slag from the steel industry can be used by the cement industry.
Statement $II$: The fuel obtained from plastic waste is lead-free.
In the light of the above statements,choose the most appropriate answer from the options given below:

$A$ water sample has $ppm$ level concentration of following anions:
$F^- = 10; \, SO_4^{2-} = 100; \, NO_3^- = 50$
The anion/anions that make/makes the water sample unsuitable for drinking is/are:

Sewage containing organic waste should not be disposed of in water bodies because it causes major water pollution. Fishes in such polluted water die because of:

Salts of chromium and arsenic are generated as waste from which industry?