Explanation:
When a metal reacts with hydrochloric acid (HCl), it produces hydrogen gas and a salt of the metal. However, not all metals react with HCl. In this case, the metal that does not react with HCl is copper (Cu) and mercury (Hg).

Reactivity Series:
The reactivity of metals can be determined by their position in the reactivity series. The reactivity series is a list of metals arranged in order of their reactivity, with the most reactive metal at the top and the least reactive metal at the bottom.

Copper and Mercury:
Copper and mercury are both less reactive metals and are located towards the bottom of the reactivity series. They do not react with HCl because they are less reactive than hydrogen. When HCl is added to copper or mercury, no reaction occurs, and no hydrogen gas is produced.

Magnesium:
Magnesium (Mg) is a highly reactive metal and is located towards the top of the reactivity series. When magnesium reacts with HCl, it produces hydrogen gas and magnesium chloride. The reaction can be represented by the following equation:
Mg + 2HCl → MgCl2 + H2

Zinc:
Zinc (Zn) is also a reactive metal, but it is less reactive than magnesium. When zinc reacts with HCl, it produces hydrogen gas and zinc chloride. The reaction can be represented by the following equation:
Zn + 2HCl → ZnCl2 + H2

Iron:
Iron (Fe) is a moderately reactive metal and is located in the middle of the reactivity series. When iron reacts with HCl, it produces hydrogen gas and iron chloride. The reaction can be represented by the following equation:
Fe + 2HCl → FeCl2 + H2

Conclusion:
In summary, copper and mercury do not react with HCl because they are less reactive than hydrogen. Magnesium, zinc, and iron, on the other hand, do react with HCl to produce hydrogen gas and the respective metal chlorides.