As many times as you possibly can. Only once, so that you have just one set of results. The theory is tested and upheld in several different experiments. All of these must happen together. The theory must fit in with other accepted theories.
Elements are the simplest type of a substance.
Living things carry out certain functions which are called life processes. A food chain is really just one of many paths through a food web. Food chains are used for land ecosystems and food webs for ocean ecosystems. Food chains show just consumers, but food webs also show producers. Food chains always end with decomposers, but food webs never do.