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_3 Cl`
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 understand what a precipitation reaction is. A precipitation reaction occurs when two solutions are mixed, and an insoluble solid (precipitate) forms as a result of the reaction.

### Step-by-Step Solution:

1. **Identify the Reactions**: Look at the provided reactions and identify the reactants involved in each one.

2. **Understand Precipitation**: Recall that a precipitation reaction produces a solid that separates from the solution. This solid is called a precipitate.

3. **Analyze Each Reaction**:
- **First Reaction**: Check if there is a formation of a solid. If CO2 gas is released, it indicates that no precipitate is formed.
- **Second Reaction**: Examine the reactants (FeCl3 and NH4OH). In this case, when these two react, they can form a precipitate (such as NH4Cl).
- **Third Reaction**: If the reaction involves heating and water, it is less likely to produce a precipitate.

4. **Conclusion**: Based on the analysis, the second reaction (FeCl3 + NH4OH) is the one that represents a precipitation reaction, as it leads to the formation of a solid precipitate.

### Final Answer:
The reaction that represents a precipitation reaction is the second option: **FeCl3 + NH4OH**.