Let's analyze the given chemical reactions to determine which one produces hydrogen gas. Hydrogen gas (\(\text{H}_2\)) is typically produced when a reactive metal reacts with an acid. However, the nature of the acid also plays a crucial role.

Understanding Hydrogen Gas Production

Hydrogen gas can be produced through various chemical reactions. A common method in chemistry involves the reaction of certain metals with acids. When a metal that is more reactive than hydrogen in the reactivity series reacts with a dilute non-oxidizing acid, it displaces hydrogen from the acid, forming a salt and releasing hydrogen gas.

Analyzing Each Reaction Option

Option 1: Magnesium and Hydrochloric Acid

Magnesium (\(\text{Mg}\)) is a reactive metal, positioned above hydrogen in the metal reactivity series. Hydrochloric acid (\(\text{HCl}\)) is a strong, non-oxidizing acid (in this context). When magnesium reacts with hydrochloric acid, a displacement reaction occurs.

The reaction is:

\(\text{Mg(s)} + \text{2HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}\)

In this reaction, magnesium displaces hydrogen from hydrochloric acid, forming magnesium chloride (a salt) and releasing hydrogen gas.

Option 2: Copper and Dilute Nitric Acid

Copper (\(\text{Cu}\)) is a less reactive metal, positioned below hydrogen in the metal reactivity series. While copper reacts with nitric acid (\(\text{HNO}_3\)), nitric acid is a strong oxidizing agent. Unlike non-oxidizing acids like HCl or \(\text{H}_2\text{SO}_4\) (dilute), nitric acid typically oxidizes the metal, and the nitrogen in the acid gets reduced. Hydrogen gas is usually *not* produced.

The reaction with dilute nitric acid typically produces nitric oxide (\(\text{NO}\)) gas or nitrogen dioxide (\(\text{NO}_2\)) gas, not hydrogen.

A possible reaction with dilute nitric acid is:

\(\text{3Cu(s)} + \text{8HNO}_3\text{(dilute)} \rightarrow \text{3Cu(NO}_3)_2\text{(aq)} + \text{2NO(g)} + \text{4H}_2\text{O(l)}\)

No hydrogen gas is produced in this reaction.

Option 3: Calcium Carbonate and Hydrochloric Acid

Calcium carbonate (\(\text{CaCO}_3\)) is a metal carbonate. The reaction between a metal carbonate and an acid produces a salt, water, and carbon dioxide gas (\(\text{CO}_2\)), not hydrogen gas.

The reaction is:

\(\text{CaCO}_3\text{(s)} + \text{2HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\)

Carbon dioxide gas is released, not hydrogen gas.

Option 4: Zinc and Nitric Acid

Zinc (\(\text{Zn}\)) is a reactive metal, above hydrogen in the reactivity series. It reacts with nitric acid (\(\text{HNO}_3\)). However, as mentioned before, nitric acid is a strong oxidizing agent. Although zinc is reactive, the strong oxidizing power of nitric acid causes the nitrogen in the acid to be reduced, leading to the production of various nitrogen oxides (\(\text{NO}_2\), \(\text{NO}\), \(\text{N}_2\text{O}\)) or even ammonium nitrate (\(\text{NH}_4\text{NO}_3\)) depending on the concentration and reaction conditions, instead of hydrogen gas.

A reaction with concentrated nitric acid might be:

\(\text{Zn(s)} + \text{4HNO}_3\text{(conc.)} \rightarrow \text{Zn(NO}_3)_2\text{(aq)} + \text{2NO}_2\text{(g)} + \text{2H}_2\text{O(l)}\)

A reaction with dilute nitric acid might produce \(\text{NO}\) or \(\text{N}_2\text{O}\). Very dilute nitric acid might produce hydrogen with very reactive metals, but it is not the typical product compared to other acids like HCl or \(\text{H}_2\text{SO}_4\). Given the options, the reaction with hydrochloric acid is the clear producer of hydrogen gas.

Conclusion

Based on the analysis of each reaction, the reaction between Magnesium and Hydrochloric acid is the one that will produce hydrogen gas. Reactive metals reacting with typical non-oxidizing acids displace hydrogen to form the gas.

Revision Table: Reactions and Products

Additional Information: Metal Reactivity and Acid Types

The ability of a metal to displace hydrogen from an acid depends on its position in the reactivity series. Metals placed above hydrogen in the series are generally reactive enough to displace hydrogen from dilute non-oxidizing acids like HCl and \(\text{H}_2\text{SO}_4\). Examples include Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead.

Metals placed below hydrogen, such as Copper, Silver, Gold, and Platinum, do not react with dilute non-oxidizing acids to produce hydrogen gas.

The type of acid is also important. While acids like HCl and dilute \(\text{H}_2\text{SO}_4\) behave as typical proton donors, nitric acid (\(\text{HNO}_3\)) is a strong oxidizing agent. Its oxidizing power usually leads to the reduction of the nitrate ion (\(\text{NO}_3^-\)) rather than the reduction of hydrogen ions (\(\text{H}^+\)) to form \(\text{H}_2\) gas, even with reactive metals (except for very specific conditions with extremely dilute nitric acid and very reactive metals like Mn or Mg). Other oxidizing acids, like hot concentrated sulfuric acid, also behave differently.

• Reactivity Series Example (simplified): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au
• Non-oxidizing Acids: HCl, dilute \(\text{H}_2\text{SO}_4\). React with active metals (> H) to give salt + \(\text{H}_2\).
• Oxidizing Acids: \(\text{HNO}_3\) (various concentrations), hot concentrated \(\text{H}_2\text{SO}_4\). React with metals, but typically produce reduced forms of the acid element (like N oxides or S oxides) and water, not \(\text{H}_2\) gas.