Physical Change is a change where there is no change in the composition of the substance and only the physical state of the substance takes place. Examples include the melting of a solid, The freezing of a liquid, sublimation of a solid to a gas, formation of a solution, evaporation, boiling process, crystallization, etc. In a physical change the molecular particles remain unchanged and no fragmentation of the molecules take place. Physical changes may be accompanied with energy being absorbed or given off during the physical change.
Chemical change involves a change in the composition as well as the physical state (perhaps). In a chemical change molecules fragment and recombine to form new molecules with a different composition. Evidence of Chemical Change include formation of evolution of gas at room temperature, the formation of a precipitate at the mixing of two solutions. Examples of Chemical Change include the souring of milk, fermentation, combustion (burning), cooking of foods, metal corrosion, electroplating of objects, etc.
One view in which we can perceive such a change is the "sub-microscopic" view. This view is what I call the "molecular" view. It is a view that we can only hope to envision with our mind's eye. We can't actually view the particles (molecules or ions), but by envisioning molecules and atoms dynamically where the particles are in constant movement we can better understand what is actually happening.
The relationship between the two viewpoints is important. Often we can't really understand what we are viewing macroscopically until we have developed a sub-microscopic or molecular view of that change.
There are a number of clues that will point to a Chemical change taking place. A chemical change will take place when the molecules or ions of matter undergo rearrangement or fragmentation where in many cases the fragments recombine to form new molecules or ions. The bottom line from a molecular view is that molecules undergo change in structure or shape. However from a "macro-scopic" view, the view that we can see in the laboratory what are some laboratory observations?
- A rapid evolution of bubbles at room temperature.
- A solid forms upon the mixture of two chemical solutions.
This is called "precipitation". Do not confuse this with another process that takes place which would be described as a physical change, solidification or crystallization. When a pure liquid becomes a solid, a solid does appear (macroscopically), but this is the physical change of liquid molecules into more rigidly fixed solid molecules. Since the molecules are the same (whether in the liquid or solid state) no chemical change is taking place.
A precipitation also results in a solid appearing (macroscopically). However the solid is caused as a result of two solutions (containing ion particles) being brought together. Two of the ions of opposite charge get together and a new substance is formed which is insoluble in the solvent present (usually water). This results in the new substance appearing as a solid. Since the new substance was not present before the change took place this would be described as a chemical change. The precipitate upon further examination of its properties would have a different property profile.
- Energy is being exchanged (absorbed or liberated). However, we have to be very careful with this observation since physical changes can also involve energy exchanges. The important thing is that if we reverse the energy exchange that the pure substances before and after the energy exchange are the same. How can we know that the pure substances are the "same"? That brings us to our fourth observation.
- The measurable properties of a pure substance before the change will be altered to a new value after the change if the change is truly a chemical change.
Every pure substance (elements and compounds) have a set of measurable properties such as melting point, boiling point, density, etc. I call this its' "property profile" of the pure substance. If the property profile is altered after the change has taken place then we can suspect that a new pure substance is present and a chemical change has apparently taken place. For example, if we have a pure substance "A" and a change takes place where the boiling point of a pure substance "B" is measured and is shown to be different than the boiling point of substance "A", then we have to conclude that a new substance "B" has formed that was not there initially.