Question

the chemical formula for an emerald is $\ce{be_{3}al_{2}(sio_{3})_{6}}$. an emerald can be described as
an impure
substance
because it is a
mixture.

a pure substance
because it is a
compound.

an impure
substance
because it is
chemically
combined.

a pure substance
because it is an
element.

Explanation:

1. Analyze each option:

• Option 1: Emerald with formula $\ce{Be_{3}Al_{2}(SiO_{3})_{6}}$ is not a mixture. A mixture has variable composition, but emerald has a fixed chemical formula, so this is incorrect.
• Option 2: A compound is a pure substance made of two or more elements chemically combined. Emerald has beryllium, aluminum, silicon, and oxygen in a fixed ratio (from its formula), so it is a compound and thus a pure substance. This is correct.
• Option 3: Saying it's an impure substance because it's chemically combined is wrong. Chemical combination leads to pure substances (compounds), not impure ones.
• Option 4: Emerald has multiple elements (Be, Al, Si, O), so it's not an element. An element has only one type of atom, so this is incorrect.

Answer:

B. a pure substance because it is a compound.