(1) Sodium hydroxide has the property of absorbing carbon dioxide. In one device, sodium hydroxide can absorb carbon dioxide. Except for carbon dioxide, all other devices are the same, and the variable is carbon dioxide, so the second device is clean water.

(2) Plants can't carry out photosynthesis in the dark, but they can breathe, so put the experimental device in the dark for 2 ~ 3 days, and geranium can only breathe to consume starch in leaves;

(3) The difference between the first device and the second device is that there is clear water in the second device and sodium hydroxide solution in the first device. Because sodium hydroxide solution can absorb carbon dioxide in the air, but clean water does not have this function, so the second device has carbon dioxide, and the first device has no carbon dioxide. Therefore, the only variable of the blade environment in the first device is carbon dioxide. Because there is no carbon dioxide in the environment of the leaves in the first device, which is one of the raw materials for photosynthesis, the leaves in one device can't be photosynthetic normally and can't make organic starch due to the lack of raw materials, so they can't turn blue under the action of iodine, while the leaves in the second device turn blue under the action of iodine because there is carbon dioxide in the environment to make starch through photosynthesis.

(4) From the above experiments, we can know that one of the raw materials of photosynthesis is carbon dioxide, and one of the products is starch.

So the answer is:

(1) Clear water;

(2) breathing; Starch (or organic matter);

(3) invariable blue;

(4) carbon dioxide; Starch (no extra points for organic matter).