"which piece of evidence would suggest an unknown substance is an ionic compound?
"

it conducts electricity when dissolved in water.
it is a liquid at room temperature.
it is a gas at room temperature.
it has a low melting point.

The correct answer is It conducts electricity when dissolved in water.

When dissolved, they split into ions that are charged and have a positive or a negative charge. They can be either liquid or solid depending on the size of the molecules, while gasses don't occur. They have high melting and boiling points.