In a reaction, a change is termed as physical change if no new species are formed and the change can be observed (or felt). So any phase change (changing from solid to liquid or gas, etc.) is a physical change. A chemical change takes place when a new chemical is formed by reactions. Physical changes can be more easily reversed as compared to a chemical change.
In the given cases here, only the burning of plastic is a chemical change, while melting of aluminum or silver is a physical change. Plastic is a combination of a number of substances and when heated, chemical changes takes place and new chemicals are formed (an example is the foul gas that usually accompanies burning of plastic). We can also see that all the properties of plastic- color and flexibility, changed after heating. In comparison, aluminum and silver are pure substances and even upon heating (and melting in case of aluminum), the substances stay the same (at the atomic level) and retain some of their properties (such as shiny color). Hence these two substances underwent a physical change.
Hope this helps.