Which of the following represents a precipitation reaction?

Options

A. `CuCO_3 to CuO +CO_2`
B. `FeCl_3 + 3NH_4OH to Fe(OH)_3 + 3NH_4Cl`
C. `CaO + H_2O to Ca(OH)_2` + Heat
D. `HNO_3 + KOH to KNO_3 + H_2O`

Explanation

To determine which of the given reactions represents a precipitation reaction, we need to identify if any of the reactions lead to the formation of an insoluble solid (precipitate) from soluble reactants.

### Step-by-Step Solution:

1. **Understand Precipitation Reactions**: A precipitation reaction occurs when two soluble salts react in solution to form an insoluble salt, which is the precipitate.

2. **Analyze the Given Reactions**:
- **Reaction 1**: FeCl3 + NH4OH → Fe(OH)3 (precipitate) + NH4Cl
- In this reaction, iron(III) hydroxide (Fe(OH)3) is formed, which is insoluble in water and thus precipitates out of the solution. This indicates a precipitation reaction.

- **Reaction 2**: CO2 + H2O → H2CO3 (soluble)
- This reaction forms carbonic acid, which is soluble in water. No precipitate is formed here.

- **Reaction 3**: HNO3 + KOH → KNO3 + H2O (soluble)
- This reaction forms potassium nitrate, which is also soluble. No precipitate is formed here.

3. **Identify the Precipitate**: From the analysis, only the first reaction produces a precipitate (Fe(OH)3), which is white in color.

4. **Conclusion**: The reaction that represents a precipitation reaction is the one involving FeCl3 and NH4OH.

### Final Answer:
The reaction FeCl3 + NH4OH represents a precipitation reaction as it forms Fe(OH)3, an insoluble precipitate.