Introduction

In topic 4.1, we discussed the two different types of reactions: physical and chemical reactions. In this topic, we will look deeper into the difference between the two, namely, showing the difference between a macroscopic change (what you see with your eyes) versus a microscopic change (what happens at a particle level).

Review of Physical vs Chemical Changes

First, let’s revisit the difference between physical and chemical reactions. 

Physical Changes

A physical change occurs when a substance changes its form/shape but retains its chemical identity. For example, if a substance undergoes a phase change (e.g. melting, evaporating, etc.), that is considered a physical change since it is the same substance but in a different phase of matter. Moreover, a substance can undergo a physical change by changing the physical appearance of the substance (e.g. cutting, folding, turning it into a powder by grinding, etc.). Mixing two substances that do not react with each other is also considered a physical change because the chemical bonds are not altered. The main takeaway with physical changes is that no new substances are formed, and existing chemical bonds are maintained.

Chemical Changes

A chemical change occurs when one or more substances in a reaction form a new chemically distinct substance. There are numerous indicators that strongly suggest (but do not prove) that a chemical reaction has occurred. They are:

The main takeaway with chemical changes is that old chemical bonds are broken and new chemical bonds are formed in a reaction.

Ex: Iron rusting ($4\text{Fe}(s)+3{\text{O}}_2(g)\to 2{\text{Fe}}_2{\text{O}}_3(s)$)

Ex: Combustion of methane (${\text{CH}}_4(g)+2{\text{O}}_2(g)\to {\text{CO}}_2(g)+2{\text{H}}_2\text{O}(g)$) 

Dissolution

Although there are many different types of physical and chemical changes, the process of dissolution may be difficult to categorize. If we consider the example of table salt dissolving in water, would we categorize that as a physical or chemical change? To answer this, we need to first look at the change that occurs as the ionic solid becomes dissolved in water.

Although the ionic attractions holding the crystal lattice together are disrupted during dissolution, these are intermolecular (noncovalent) electrostatic interactions rather than covalent bonds within a molecule. The separated ions are then stabilized by ion–dipole interactions with water molecules. Since the chemical identities of $\mathrm{Na}{}^{+}$ and $\mathrm{Cl}{}^{-}$ do not change and no new substances are formed, the process is classified as a physical change. 

This is a summary: In AP Chemistry, the process of dissolution is treated as a physical change as the chemical identity of the substance does not change.

Practice Problems