AAT Bioquest

How do I tell if it's a physical or chemical change?

Posted May 13, 2022


The basic difference between the two is that a physical change only affects the appearance or physical properties of a substance whereas a chemical change affects the chemical composition of the substance. In trying to tell whether it’s a physical or chemical change, these are some of the things to look for: 

Physical change 

  • The appearance of the substance has undergone a change but its chemical composition remains the same. 
  • The new substance may look different but it is fundamentally the same as the original substance. 
  • The change is reversible. For example, water can be frozen to form ice, which can be melted again to form water. 
  • Examples of physical change include but are not limited to shape, size, texture, color, temperature and change of state (gas, liquid, or solid). 

Chemical change 

  • The physical appearance of the substance has undergone a change and so has its chemical composition and properties resulting in an entirely different substance with a new chemical formula. 
  • The new substance is completely different from the original substance. This is because the process involves breaking or forming interatomic bonds of the different substances, which changes the chemical composition of all substances in the reaction.  
  • The change is irreversible. For example, wood gets converted to ash, carbon dioxide and water. These individual substances cannot be converted back to form wood. 
  • Examples of chemical change include but are not limited to formation of bubbles or precipitate, noticeable odor, or change in temperature or color. 

In most cases, it’s easy to tell whether the change is physical or chemical by simple observation. However, because there are some common features, some changes are a little more complex to determine. The best way to be absolutely certain whether a change is physical or chemical is by determining the composition of the original substance and the new substance before and after a reaction. This can be done by performing chemical analyses, such as mass spectroscopy, on the substance. 

Additional resources

Using UPLC-QTOF-MS to analyze the chemical changes between traditional and dispensing granule decoctions of San-Ao-Tang