What describes synthesis in chemical reactions?

• A. Two elements combine. A + B = AB
• B. A compound splits. AB = A + B
• C. Two elements exchange electrons to form ions.
• D. A substance changes phase without chemical change.

Answer: (A)

Synthesis is when two elements or simpler substances join to form one compound. That idea is captured by the statement that two elements combine to form a single product. A common example is sodium reacting with chlorine to form sodium chloride, illustrating two substances coming together to make one new substance.

This differs from decomposition, where a compound breaks apart into simpler substances. It also differs from reactions where elements exchange electrons to form ions (a redox or displacement type reaction), which involves electron transfer rather than simply building one product from two reactants. Finally, changing phase without chemical change is a physical change, not a chemical reaction, so it doesn’t describe synthesis.