Identify the substance given to the technician.

The substance would be sodium nitrate, to figure this out all you need to do is read each sentence carefully and see if whatever the lab technician notices happening to the substance does it match with any of the substances listed in the table, if it does not match, any one description we can eliminate that compound as a choice. Carry this method of elimination basically until you come to the last sentence. Since the substance melts at temperature above the melting point, it can't be potassium chromate as it would change state at a temperature of 974 degrees Celsius, not high of temperature for the change, as the oven is at 450 degrees Celsius only.