In basic chemical reaction classification, which of the following balanced equations is an example of a synthesis (combination) reaction?

Introduction / Context:
Classifying chemical reactions into types such as synthesis, decomposition, single displacement, and double displacement is an important skill in introductory chemistry. A synthesis or combination reaction is one in which two or more simple substances combine to form a single more complex product. This question asks you to identify which given chemical equation best represents a synthesis reaction. Recognising examples helps learners connect abstract definitions to real chemical processes.

Given Data / Assumptions:
- Four balanced equations are provided as options. - We must select the equation that represents a synthesis or combination reaction. - We assume normal definitions used in school and college level chemistry.

Concept / Approach:
In a synthesis reaction, two or more reactants combine to form a single product. A classic example is the combination of elements hydrogen and oxygen to form water. In contrast, a decomposition reaction involves a single compound breaking into simpler substances. Neutralisation and double displacement reactions involve exchange of ions between compounds. The key pattern for synthesis is many reactants forming one product. By examining the number and type of reactants and products in each equation, we can identify which one matches the synthesis pattern.

Step-by-Step Solution:
Step 1: Look at Option A, CaCO3 -> CaO + CO2. One reactant produces two products, which matches the pattern of a decomposition reaction, not synthesis. Step 2: Look at Option B, 2H2 + O2 -> 2H2O. Two reactants, hydrogen and oxygen, combine to form a single product, water, which matches the definition of a synthesis reaction. Step 3: Look at Option C, HCl + NaOH -> NaCl + H2O. Here an acid and a base react to form salt and water, which is a neutralisation and double displacement reaction. Step 4: Look at Option D, AgNO3 + NaCl -> AgCl + NaNO3. Two compounds exchange ions to produce two new compounds, which is a double displacement reaction. Step 5: Conclude that Option B is the only equation that clearly shows two simple reactants combining into one compound, so it is the correct synthesis reaction.

Verification / Alternative check:
To verify, recall common textbook examples of synthesis reactions. The formation of water from hydrogen and oxygen, the formation of ammonia from nitrogen and hydrogen, and the formation of magnesium oxide from magnesium and oxygen are standard illustrations. These all show multiple reactants forming a single product. The equation in Option B fits this pattern exactly. The other equations either break a compound apart or exchange ions between compounds, confirming that they are not synthesis reactions.

Why Other Options Are Wrong:
Option A shows CaCO3 breaking down into CaO and CO2, which is a decomposition reaction, the opposite of synthesis. Option C represents an acid base neutralisation, where HCl reacts with NaOH to form NaCl and water. This is typically classified as a double displacement reaction, not a synthesis reaction. Option D involves AgNO3 and NaCl exchanging ions to form AgCl and NaNO3, another clear example of a double displacement reaction rather than a combination of reactants into a single product.

Common Pitfalls:
A common mistake is to focus only on the number of reactants, thinking that two substances reacting must mean synthesis. However, two reactants can also participate in double displacement or neutralisation reactions that produce more than one product. Another pitfall is not paying attention to the products side of the equation. Always remember that in a synthesis reaction, multiple reactants form exactly one product. Checking both sides of the equation carefully will help you avoid misclassification.

Final Answer:
The correct answer is: 2H2 + O2 -> 2H2O.