Which of the following best describes a chemical reaction, and which is an example of one?

• A. A process in which substances change their physical state but retain their chemical identity; for example, melting ice to form liquid water.
• B. A process in which energy is absorbed or released without any change in the composition of the substances; for example, heating a metal rod.
• C. A process in which a solid dissolves in a liquid to form a solution without any change in chemical composition; for example, dissolving salt in water.
• D. A process in which substances are transformed into new substances with different chemical properties; for example, the reaction of hydrogen and oxygen to form water: 2H_2 + O_2 → 2H_2O.

Answer: (D)

Verified step by step guidance

1. Understand that a chemical reaction involves the transformation of substances into new substances with different chemical properties and compositions.
2. Recognize that physical changes, such as melting ice or dissolving salt in water, do not alter the chemical identity of the substances involved.
3. Identify that processes involving only energy changes without changing the chemical composition (like heating a metal rod) are not chemical reactions.
4. Focus on the example where hydrogen gas reacts with oxygen gas to form water, represented by the balanced chemical equation: \$2H_2 + O_2 \rightarrow 2H_2O$.
5. Conclude that the best description of a chemical reaction is a process where substances are transformed into new substances with different chemical properties, as illustrated by the formation of water from hydrogen and oxygen.