Which of the following is not a combination reaction?

Options

A. `Fe + S to FeS`
B. `CaO + CO2 to CaCO_3`
C. `NH_3 + HCl to NH_4Cl`
D. `AgNO_3 +NaCl to AgCl + NaNO_3`

Explanation

To determine which of the following reactions is not a combination reaction, we first need to understand what a combination reaction is. A combination reaction occurs when two or more reactants combine to form a single product.

Let's analyze the options provided:

1. **Fe + S → FeS**:
- Here, iron (Fe) and sulfur (S) combine to form iron sulfide (FeS). This is a classic example of a combination reaction.

2. **CaO + CO2 → CaCO3**:
- In this reaction, calcium oxide (CaO) and carbon dioxide (CO2) combine to form calcium carbonate (CaCO3). This is also a combination reaction.

3. **NH3 + HCl → NH4Cl**:
- Ammonia (NH3) and hydrochloric acid (HCl) combine to form ammonium chloride (NH4Cl). This is another example of a combination reaction.

4. **AgNO3 + NaCl → AgCl + NaNO3**:
- In this reaction, silver nitrate (AgNO3) and sodium chloride (NaCl) react to form silver chloride (AgCl) and sodium nitrate (NaNO3). This reaction involves the exchange of ions between the two reactants, which characterizes it as a double displacement reaction.

From the analysis above, we can conclude that the fourth option (AgNO3 + NaCl → AgCl + NaNO3) is not a combination reaction, but rather a double displacement reaction.

### Final Answer:
The reaction that is not a combination reaction is **AgNO3 + NaCl → AgCl + NaNO3**.