Which of the following statements about chemical reactions is true?

• A. Chemical reactions do not involve changes in energy.
• B. Chemical reactions always result in the formation of new substances.
• C. Chemical reactions can be reversed without any change in the substances involved.
• D. Chemical reactions only occur between gases.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the nature of chemical reactions. Chemical reactions involve the rearrangement of atoms to form new substances, which means the original substances change into different substances with different properties.
2. Step 2: Consider the role of energy in chemical reactions. Chemical reactions often involve changes in energy, either releasing energy (exothermic) or absorbing energy (endothermic), so the statement that reactions do not involve energy changes is false.
3. Step 3: Evaluate the reversibility of chemical reactions. While some reactions are reversible, many are not easily reversed without changing the substances involved, so the statement that reactions can be reversed without any change is generally false.
4. Step 4: Assess the states of matter involved in chemical reactions. Chemical reactions can occur in all states of matter—solids, liquids, and gases—not only gases, so the statement that reactions only occur between gases is false.
5. Step 5: Conclude that the true statement is that chemical reactions always result in the formation of new substances, reflecting the fundamental definition of a chemical reaction.