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

Copper does not react with cold water, hot water, or steam. However, iron reacts with steam. If the hot water tanks are made of steel (an alloy of iron), then iron would react vigorously with the steam formed from hot water. $$ 3Fe + 4H_{2}O \rightarrow Fe_{3}O_{4}+H_{2}O $$
That is why copper is used to make hot water tanks, and not steel.