A synthesis reaction, also known as a combination reaction, occurs when two or more reactants combine to form a single product.
Let's analyze the options given:
A. \( \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{H}_2\text{CO}_3 \) - This is a synthesis reaction because two reactants combine to form one product (carbonic acid).
B. \( \text{HCl} + \text{LiOH} \rightarrow \text{LiCl} + \text{H}_2\text{O} \) - This is a neutralization reaction, not a synthesis reaction.
C. \( 2\text{Na} + \text{MgCl}_2 \rightarrow 2\text{NaCl} + \text{Mg} \) - This is a single replacement reaction, not a synthesis reaction.
D. \( \text{CO}_2 \rightarrow \text{C} + \text{O}_2 \) - This is a decomposition reaction, not a synthesis reaction.
The only equation that represents a synthesis reaction is:
A. \( \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{H}_2\text{CO}_3 \).