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

• A. Formation of a gas or precipitate
• B. Melting of a solid
• C. Change in the physical state without new substances forming
• D. Dissolving sugar in water

Answer: (A)

Verified step by step guidance

1. Understand that a chemical reaction involves the transformation of substances into new substances with different properties.
2. Recognize that physical changes, such as melting or dissolving, do not produce new substances; they only change the state or appearance of the material.
3. Identify that the formation of a gas or a precipitate (a solid that forms from a solution) indicates new substances are being created, which is a hallmark of a chemical reaction.
4. Recall that melting of a solid and dissolving sugar in water are physical changes because the chemical identity of the substances remains the same.
5. Conclude that the formation of a gas or precipitate is a clear sign that a chemical reaction has taken place.