Question

water being separated with an electric current into hydrogen and oxygen gas tells you that
a. water is a mixture.

b. water is a compound.

c. water is an element.

d. water is a conductor

e. water is an isotope.

Explanation:

• A compound is a substance made of two or more different elements chemically bonded. Water ($H_2O$) can be decomposed into hydrogen and oxygen (different elements) via electrolysis, showing it is a compound.
• A: A mixture is physically combined substances, but water is chemically bonded, so A is wrong.
• C: An element can't be broken into simpler substances, but water breaks into H and O, so C is wrong.
• D: Pure water is a poor conductor; the question is about its composition, not conductivity, so D is irrelevant.
• E: Isotopes are variants of an element, not related to water's decomposition, so E is wrong.

Answer:

B. water is a compound.