So the answer is: unreasonable, because both copper and cuprous oxide can be dissolved in dilute nitric acid to form a blue solution; Reasonable, because cuprous oxide and dilute sulfuric acid produce Cu and Cu2+, resulting in blue solution;
(2) According to the conservation law of copper, 2Cu~Cu2O
128 144
b-a？ c-a
128 144=b？ ac？ A, the solution is c=9b? A8, so if the red powder contains Cu2O, a, b and c, the mathematical relationship is C < 9b? a8，
Burning at high temperature in air until the quality is constant requires at least two weighing, plus the previous two weighing, so the experimental scheme needs at least four weighing.
So the answer is: c < 9b? a8，4；
Three. (3) Method for testing the air tightness of the device: close the piston and inject a certain amount of water into the test tube from the long-necked funnel to make the liquid level in the funnel higher than that in the test tube. If the liquid level in the funnel does not drop after standing, it means that the air tightness of the device is good; When the liquid level drops, the device will leak air.
So the answer is: close the piston and inject a certain amount of water into the test tube from the long-necked funnel to make the liquid level in the funnel higher than that in the test tube. If the liquid level in the funnel does not drop after standing, it means that the air tightness of the device is good; When the liquid level drops, the device leaks;
(4) When judging whether cuprous oxide is contained according to whether copper sulfate turns blue, attention should be paid to the interference factors of water, so the prepared hydrogen should be dried. A drying device can be added between the hydrogen generator and the hard glass tube to eliminate the interference of moisture in the air. A drying tube filled with alkali lime can be connected behind the drying tube filled with copper sulfate, and the air in the device should be exhausted before the experiment to prevent interference.
So the answer is: ① ② ③.