If you were asked to convert 25 mg to the unit hg, which of the following would be the first fraction used in the conversion? ten to the negative third power hg over one g ten to the negative third power mg over one hg ten to the negative third power mg over one g ten to the negative third power g over one mg

The conversion factor is added to the original given unit so that you can end up with the final unit. Basically, the conversion unit does not change the value because the factor is just equal to 1. You just manipulate the units by cancelling them.

Among the given choices, the fraction which is equal to 1 is 10⁻³ g/ 1 mg, because there are 1,000 mg per 1 g. In reverse, that would be 10⁻³ of a gram.

Sorry i dont understand the question can you rephase it?

Not a  mixture of two common elements that can be separated by physical means.