Question

Give reason why copper is used to make hot water tanks and not steel(an alloy of iron).

Answer 1

it's just because Cu does not react with H2O (let it be cold,hot or steam) whereas Fe  does. We know that steel is an alloy of iron so,if water tanks are made of steel,iron would react vigorously with steam formed from hot water to form ferric/ferrous oxide +water 

Answer 2

copper is low in reactivity series and it will not react with H2O(steam,cold water)
but iron reacts with steam and hot water.the following reaction is given below:
3Fe(s) + 4H2O(g) ⇒ Fe3O4(s) + 4H2(g)

HOPE IT HELPS.....