1. Brief Review of Elements & Compounds Are there elements?  Yes, our class knows of the existence of some elements such as hydrogen, oxygen, chorine, helium, mercury, carbon, etc. Are there compounds?  Yes, our class knows of the existence of some compounds such as water (H 2 O), carbon dioxide (CO 2 ), hydrogen chloride (HCl), etc. Can we cite evidence that supports the belief that some pure substances are “compounded” of simpler particles in a definite ratio?

2. Decomposition of water via electrolysis When water is broken down into its components, we clearly see the 2:1 volume ratio of hydrogen to oxygen Suggests that water is made of 2 atoms of hydrogen and 1 atom of oxygen

3. Formation of water experiment Video we watched showed the scientists trying to combine 2 volumes of hydrogen gas with 2 volumes of oxygen gas (2:2 volume ratio) Notice the “extra” oxygen that was unused Despite repeated trials, 2 volumes of hydrogen will combine with only 1 volume of oxygen Avogadro was one of many to repeat this experiment, which provided the basis for Avogadro’s hypothesis + +

4. So what does this experimental evidence suggest? Supports our definition that elements are made of only one kind of atom Some pure substances (compounds) can be broken down into simpler substances Those that cannot be broken down are said to be elemental So, not all the particles that form pure substances are simple; some contain smaller particles that combine (are compounded) in fixed proportions and are not merely simple mixtures of these small particles