This article explores the criteria for distinguishing between physical and chemical changes, and discusses the dissolution of carbon dioxide in water and the dissolution of salt. It analyzes these changes from a variety of perspectives, focusing on both phenomena as changes in the properties of matter and changes in molecular structure.
“Is the dissolution of carbon dioxide in water a physical change or a chemical change?” and ”Is the process of diluting concentrated sulfuric acid in water to make dilute sulfuric acid a physical change or a chemical change?”
When asked these questions, most people would answer that it is a physical change, but unfortunately, the correct answer is a chemical change. We usually know the definition of physical change as “a phenomenon in which the properties of a substance do not change, but only its state or appearance changes,” and chemical change as “a phenomenon in which a substance changes into a new substance that is completely different from its original properties.” However, we don’t know exactly what we mean by “properties of matter”.
The properties of a substance are the properties of the structure of the molecules or ions that make up the substance. Therefore, if the structure of the molecules or ions changes, the properties of the substance will change. For example, carbon dioxide (CO2) combines with water to form carbonic acid ions (HCO3-) to form carbonated water. This is a chemical change because carbon dioxide and water have completely different properties, so the dissolution of carbon dioxide in water is a chemical change. On the other hand, when sugar dissolves in water, it only changes the color and shape of the sugar, so this dissolution is a physical change.
So, how can we more clearly distinguish between physical and chemical changes? A physical change is usually a change in the state or appearance of a substance and does not involve a structural change at the atomic or molecular level. The properties of the substance are maintained, and the changed state can be easily reversed back to its original state. For example, the melting of ice into water is a physical change; the molecular structure of water does not change, only its state from solid to liquid.
Chemical changes, on the other hand, are when the bonds between the molecules or atoms of a substance are reorganized to form a completely new substance. In this case, the properties of the original substance disappear, and a substance with new properties appears. Chemical changes can often manifest as changes in energy, color, gas production, or precipitate formation, and these changes are usually difficult to reverse. For example, the oxidation of iron to rust is a chemical change in which the properties of iron are completely changed and a new compound is formed.
Still, you might be tempted to ask whether the change from concentrated sulfuric acid to dilute sulfuric acid is a physical change, since it’s just a change in the concentration of sulfuric acid. However, this is also not the case. Even without considering the molecular structure, concentrated sulfuric acid is dehydrating, whereas dilute sulfuric acid is not, and mixing concentrated sulfuric acid with water generates a lot of heat. We can also add the fact that water acts as a base in the reaction between concentrated sulfuric acid and water as another piece of evidence.
So, what kind of change is salt dissolving in water? Again, it’s easy to say that it’s a physical change, but unlike sugar dissolving in water, it’s not a physical change because the ionic bonds of salt (NaCl) are broken, separating it into sodium ions (Na+) and chloride ions (Cl-). Some may argue that the change of salt dissolving in water is a physical change because heating the salt water causes a reversible reaction that precipitates salt again, but reversible reactions are also common in chemical changes.
However, it is also difficult to categorize the dissolution of salt in water as a chemical change. Sure, when salt dissolves in water, the ionic bonds are broken, and the result is a change from a non-current-carrying conductor to a current-carrying electrolyte. However, new chemical bonds are not formed. Therefore, if we define chemical change as “the breaking of chemical bonds between the particles that make up a substance and the organization of the particles into new chemical bonds,” then the dissolution of salt in water can be considered a physical change. In short, the same phenomenon can be viewed as a physical change or a chemical change, depending on your frame of reference.
