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

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In this case, we have to consider whether copper and iron are resistant to hot water. In general, metals combine with steam generating metal hydroxides and hydrogen gas.

Complete answer:
Copper and iron both are metals. As iron is more reactive than copper, it readily reacts with the steam of hot water giving corresponding oxides. Copper is rather less reactive and does not combine with hot water steam.
The reactions of copper and iron with hot water steam are written as:
\[Cu(s) + {H_2}O\xrightarrow{{}}noproduct\]
\[3Fe(s) + 4{H_2}O\xrightarrow{{}}F{e_3}{O_4}(s) + 4{H_2}(g)\]
In the first reaction, the metallic copper does not undergo any reaction with water, and no product is formed. But in the second reaction, the metallic iron reacts with water giving precipitate of iron oxide known as a ferrosoferric oxide or iron (II, III) oxide.
If hot water tanks are made of copper the tanks will remain as such after use. But if steel (an alloy of iron) is used in place of copper then the iron will react with steam which will result in making the surface or body of the container become weak after use.
As a result, the tanks made of steel go deformed and will start leakage of water after regular usage. But the tanks made of copper will not have any deformation after the use of longer periods.
So copper is used instead of steel to make hot water tanks.

Note: Please consider the tendency of metals to produce metal oxide in cases where metals and hot water are kept together. Generally aluminum, iron, and zinc react with hot water steam but copper, silver and gold metals are resistant to hot water steam.