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

• A. Melting of ice into liquid water
• B. Formation of a gas or bubbles
• C. Breaking a glass into smaller pieces
• D. Change in the shape of a substance without altering its composition

Answer: (B)

Verified step by step guidance

1. Understand the difference between physical and chemical changes: A physical change affects the form of a substance but not its chemical composition, while a chemical change results in the formation of one or more new substances with different properties.
2. Analyze each option to determine if it represents a physical or chemical change:
3. - Melting of ice into liquid water is a physical change because the chemical composition (H2O) remains the same.
4. - Formation of a gas or bubbles typically indicates a chemical reaction, as new substances (gases) are produced.
5. - Breaking a glass into smaller pieces and changing the shape of a substance without altering its composition are physical changes, as no new substances are formed.