Question

which property of argon makes it useful for potassium-argon dating? it decays into potassium, it is an unstable element, it remains trapped in rock, it is useful only for dating newer rock.

Explanation:

For potassium - argon dating, the key property of argon is that it is a gas that escapes from molten rock (magma or lava) and then, when the rock solidifies, any argon that accumulates in the rock can be measured. The process relies on the fact that potassium - 40 decays into argon - 40. When the rock is molten, argon (being a gas) escapes. Once the rock cools and solidifies, argon starts to accumulate from the decay of potassium - 40. So the property that makes argon useful for potassium - argon dating is that it escapes from molten rock.

Let's analyze the other options:

• "It is an unstable element": Argon is a stable element. Potassium - 40 is the unstable isotope that decays, not argon. So this is incorrect.
• "It remains trapped in rock": Argon is not trapped in molten rock; it escapes. It gets trapped only when the rock solidifies. So this is incorrect.
• "It is useful only for dating newer rock": Potassium - argon dating is used for dating old rocks (like volcanic rocks) that are millions of years old, not just newer rocks. So this is incorrect.

Answer:

The correct option is "It escapes from molten rock" (the first option among the given choices).