There are many cases where the distinctions between physical changes and chemical changes are unclear. For example:
The dissolution of salt in water: This seems like a physical change because we know we can recover the salt from the water. However, if we look at the microscopic level, we see that the two types of atoms in salt, sodium and chlorine, separate from one another. In this example, we don’t have a new substance, therefore this salt in solution doesn’t fit the microscopic definition of a chemical change; but we also don’t have the substance in its original form — a stack of alternating sodium and chlorine atoms. Does this mean the change is half chemical and half physical? Though it has aspects of a chemical change, scientists would still classify the dissolution of salt as a physical change.
The creation of a metal alloy: If we melt two types of metal together, we create an alloy metal that has different properties than either of its components (e.g., heat conductivity, electrical conductivity, density, etc.). This might lead us to think that we’ve witnessed a chemical change. In fact, a new particle is not created by melting two metals together. This indicates they did not undergo a chemical reaction. Brass, for example, is about 60% copper and 40% zinc, and is composed of individual copper and zinc atoms (i.e., there is no “smallest unit” that is still brass). There is no such thing as a brass molecule.
The heating and cooling of certain rubbers and plastics: One might think that exposing certain rubbers and plastics to heat or cold would cause a chemical reaction because the properties change (e.g., the materials become more rigid and brittle). While chemical reactions do take place, they simply bind together different parts of the large molecules that compose rubber and plastic. These new bonds add to the rigidity of the material, but the particles of the substances remain the same.
So, to find which change is a physical change and which change is a chemical change, we have to make sure that we put the factors of the changes into account to make sure that they are not mixed-up.