A compound is formed when a _____ occurs between two or more elements.

• A. chemical bond
• B. physical mixture
• C. nuclear reaction
• D. phase change

Answer: (A)

Verified step by step guidance

1. Understand the key terms in the question: a 'compound' is a substance formed when elements combine in fixed proportions.
2. Recall that a 'chemical bond' is the force that holds atoms together in a compound, involving the sharing or transfer of electrons.
3. Recognize that a 'physical mixture' involves no chemical bonding; substances are simply mixed without forming new compounds.
4. Note that a 'nuclear reaction' involves changes in the nucleus of atoms, not the formation of chemical compounds.
5. Understand that a 'phase change' refers to changes in the state of matter (solid, liquid, gas) without forming new compounds.