(1) describes the harm of acid rain to crops, buildings and human body, and makes exploratory experiments with soybean seedlings to simulate acid rain, and obviously explores the influence of acid rain on plant growth. Therefore, if these seeds do not germinate (regardless of their own conditions, dormancy, etc. ), it can be explained that they are affected by simulated acid rain.

(2) The experiments we do are generally control experiments, so it is required that ① variables should be unique. In the designed experiment, we obviously found the lack of control experiment design. As a control experiment, we should ensure the uniqueness of variables, because the experiment explores the influence of acid rain on plant growth. Therefore, the only variable should be acid rain, and another pot of soybean seedlings with the same conditions should be planted and sprayed with clear water every day.

So the answer is: (1) simulated acid rain.

(2)②; Clear water; Simulated acid rain