Which one of the following reaction is an example of combination reaction?

Options

A. `2Mg+O+2rarr2MgO`
B. `CaO+H_2OrarrCa(OH)_2+heat`
C. `H_2+Cl_2rarr2HCl`
D. all of these

Answer

To determine which reaction is an example of a combination reaction, we first need to understand what a combination reaction is.

### Step-by-Step Solution:

1. **Define Combination Reaction**:
A combination reaction is a type of chemical reaction where two or more reactants combine to form a single product. This can be represented as:
\[ A + B \rightarrow AB \]
where \( A \) and \( B \) are the reactants and \( AB \) is the product.

2. **Identify the Options**:
Typically, a question like this would provide several reaction options. For example, you might see reactions like:
- \( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \)
- \( \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} \)
- \( \text{Na} + \text{Cl}_2 \rightarrow \text{NaCl} \)
- \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)

3. **Analyze Each Reaction**:
- For \( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \): Two reactants (C and O2) combine to form one product (CO2).
- For \( \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} \): Two reactants (H2 and O2) combine to form one product (H2O).
- For \( \text{Na} + \text{Cl}_2 \rightarrow \text{NaCl} \): Two reactants (Na and Cl2) combine to form one product (NaCl).
- For \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \): This is a decomposition reaction where one reactant breaks down into two products.

4. **Select the Combination Reaction**:
From the analysis, the first three reactions are examples of combination reactions because they involve two reactants combining to form a single product. The fourth reaction is not a combination reaction.

5. **Conclusion**:
Therefore, any of the first three reactions (C + O2, H2 + O2, or Na + Cl2) can be considered as examples of combination reactions.