The Royal Society of Chemistry is offering £1,000,000 to anyone who can show them a 100% chemical-free substance, see here for details.
Challenge your class to find one, it should be easy as there are hundreds of products advertised as being “chemical free”. There’s a great blog here which names and shames marketers who use the phrase. (WARNING! The “F No” pun in the blog title makes this unsuitable for a younger audience but you can use the blog contents as inspiration.)
Use Pinterest or Diigo to collect examples of bad advertising.