Step 1: The first pair tells us the relationship: hydrogen is a substance whose state at room temperature is gas. Step 2: Apply the same relationship to mercury: what is the state of mercury at room temperature? Step 3: Mercury is the only metal that is liquid at room temperature (about 25 °C); it is used in thermometers and barometers. Step 4: Bromine is the only non-metal that is liquid at room temperature. Answer: Liquid.