The reason why the liquid level rises is that the rust of nails consumes oxygen in the test tube, making the pressure in the test tube lower than atmospheric pressure, and the liquid level in the right conduit of the device rises under the action of external atmospheric pressure.
So the answer is: the rust of nails consumes oxygen in the test tube, making the pressure in the test tube lower than atmospheric pressure;
[Analysis and Verification]:
(1) Because ferric chloride is easily soluble in water, the black solid substance at the bottom of the test tube is not ferric chloride.
Therefore, the answer is: ferric chloride is soluble in water;
(2) Because the nail is ferroalloy and contains carbon, the black solid substance at the bottom of the test tube contains carbon.
Therefore, the answer is: nails are carbon-containing iron alloys;
(3) Because iron reacts with acid to generate gas, we can add dilute hydrochloric acid or dilute sulfuric acid. If there are bubbles, it means iron; if there are no bubbles, it means there is no iron. The experimental steps, phenomena and conclusions are shown in the table.
Experimental steps Experimental phenomena Experimental conclusion Dilute hydrochloric acid (or dilute sulfuric acid) has bubbles (no bubbles). Knowledge link: Salt solution can accelerate the rust rate of iron.
So the answer is: salt water.