Which of the following is a sign that a chemical reaction has occurred?

• A. Change in the physical state without new substances forming
• B. Formation of a gas or precipitate
• C. Melting of ice into liquid water
• D. Breaking a glass into smaller pieces

Answer: (B)

Verified step by step guidance

1. Understand that a chemical reaction involves the formation of new substances with different chemical properties, not just physical changes.
2. Recognize that physical changes, such as melting ice or breaking glass, do not produce new substances; they only change the form or state of the material.
3. Identify that the formation of a gas or a precipitate (a solid that forms from a solution) indicates a chemical change because new substances are created during the reaction.
4. Recall that a change in physical state alone (like ice melting) is a physical change, not a chemical reaction, since the molecular structure remains the same.
5. Conclude that the sign of a chemical reaction is the formation of new substances, such as gas bubbles or a precipitate, rather than just changes in physical state or size.