Which chemical can show that carbon dioxide is being produced?​

Carbon dioxide is produced whenever an acid reacts with a carbonate. This makes carbon dioxide easy to make in the laboratory. Calcium carbonate and hydrochloric acid are usually used because they are cheap and easy to obtain. Carbon dioxide can be collected over water, as shown in the diagram.

I believe that the correct answer would be:

“observation of the elements”

 

Moseley's changes about the periodic table were actually based mainly on observation of the elements. He worked on X-rays which involves the elements. He had arranged the periodic table according to the element's protons without regard to other properties.