Consider the following reactants. Which produces a gas?

1. NH4Cl(aq) + AgNO3(aq) →
2. Na2CO3(aq) + BaCl2(aq) →
3. Na2S(aq) + HCl(aq) →
4. All of these would produce a gas.

All three of these reactions are double replacement. Therefore, all you need to do is to write the reaction and look up each compound to see what state it is. For example,

NH4Cl(aq) + AgNO3(aq) → NH4NO3(aq) + AgCl(s) etc.

To determine which of the given reactants produces a gas, we need to examine the products of each reaction.

1. NH4Cl(aq) + AgNO3(aq) → No gas is produced. The products are NH4NO3(aq) and AgCl(s).

2. Na2CO3(aq) + BaCl2(aq) → No gas is produced. The products are NaCl(aq) and BaCO3(s).

3. Na2S(aq) + HCl(aq) → Gas is produced. The products are NaCl(aq) and H2S(g).

4. All of these would produce a gas. This statement is incorrect as only the reaction between Na2S(aq) and HCl(aq) produces a gas.

Therefore, the correct answer is option 3: Na2S(aq) + HCl(aq) → NaCl(aq) + H2S(g).

To determine which reaction produces a gas, we need to consider the possible products of each reaction and identify if any of them are gases.

1. NH4Cl(aq) + AgNO3(aq) → The possible products of this reaction are NH4NO3(aq) and AgCl(s). Neither of these products is a gas.

2. Na2CO3(aq) + BaCl2(aq) → The possible products of this reaction are NaCl(aq) and BaCO3(s). Neither of these products is a gas.

3. Na2S(aq) + HCl(aq) → The possible products of this reaction are NaCl(aq) and H2S(g). H2S is a gas, so this reaction produces a gas.

4. All of these would produce a gas. This option is incorrect as not all of the reactions listed would produce a gas. Only reaction 3 produces a gas.

Therefore, the correct choice is:
3. Na2S(aq) + HCl(aq) →