Which equation best illustrates a synthesis reaction?

• A. 2Na + Cl2 -> 2NaCl
• B. NaCl -> Na + Cl2
• C. Na + Cl2 -> NaCl
• D. 2NaCl -> 2Na + Cl2

Answer: (A)

Synthesis reactions involve two or more substances combining to form a single new compound. The clearest and properly balanced example shows exactly how many atoms come together to make that compound.

The equation with 2 sodium atoms reacting with chlorine gas to form 2 units of sodium chloride demonstrates this: it shows two reactant species joining to make one product, and it’s balanced (2 Na on the left and 2 Na in NaCl on the right; 2 Cl atoms from Cl2 on the left match the 2 Cl atoms in NaCl on the right). This balance reflects the conservation of mass and makes the reaction a textbook synthesis.

The other options depict different ideas or are not properly balanced. A representation of a compound breaking down into simpler substances is a decomposition, not synthesis. An unbalanced equation like Na + Cl2 -> NaCl would not conserve atoms as written, unless it’s balanced as 2Na + Cl2 -> 2NaCl or Na + 1/2 Cl2 -> NaCl. So the balanced form shown here best illustrates a synthesis reaction.