The process by which all matter change is either by a change in phase as discussed in the previous lecture. Examples would be the melting of ice or the solidification of metals. In these situations there is no change in composition.
Chemical reactions are where the composition changes.
You start out with what are called reactants and produce what are called products. Demo of silver nitrate solution and lead iodide which forms lead nitrate precipitate. See Trefil book for picture of the demo and other demo. Use the board to explain the reactions.