4.4 Physical and Chemical Changes

3 min read•january 3, 2023

Hope Arnett

AP Chemistry 🧪

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Review of Chemical Equations

Chemical equations show the products that the combination of reactants yields, but the equation doesn’t show how the reactants form those products. What we don’t see are the underlying chemical and physical changes that allow molecules to rearrange to change properties or create new substances. All it does is represent a chemical reaction!

Chemical Changes

Generally, chemical changes involve intramolecular (literally meaning “inside molecule”) bonds. This includes breaking and/or forming ionic or covalent bonds between elements during a chemical reaction. Let’s look at a chemical reaction using Lewis Dot Diagrams to visualize it.

The synthesis reaction above shows an interaction between two magnesium atoms and an oxygen molecule. The bonds that are broken are red, and the bonds formed are green. Here, we can see that the oxygen molecule's covalent bonds had to break, and then new ionic bonds formed between the magnesium and oxygen to form two molecules of MgO.

Note that ionic bonds are not represented like covalent bonds with lines as shown in the above diagram. They are rather expressed with brackets and charges around the brackets to show the transfer of electrons.

Remember that chemical changes are usually accompanied by a chemical reaction to show this breaking and forming of bonds. Examples of chemical changes include:

Burning: The chemical reaction that occurs when a substance reacts with oxygen to produce heat and light. Think about combustion reactions!
Rusting: The chemical reaction that occurs when iron reacts with water and oxygen to form iron oxide (rust).
Digestion: The chemical reactions that occur in the body to break down food into nutrients that can be absorbed and used by cells.

Physical Changes

Physical changes are usually intermolecular changes (literally meaning “between molecules”), such as phase changes. Some examples are freezing water and cutting paper.

The ice’s molecules maintain the same atomic structure (H2O), but more hydrogen bonds between each water molecule are formed. Conversely, the paper’s molecules keep the same atomic structure, but the interaction between the paper molecules is altered, or broken.

Since physical changes are only a transformation that changes a substance's properties without undergoing a chemical change, they are reversible.

However, some reactions can go either way and sometimes physical processes involve the breaking of chemical bonds. For example, the reaction between salt and water, or the dissolution of salt in water, involves breaking the bonds between ions, but it also creates ion-dipole interactions between the water and ions.

The easiest way to distinguish between a chemical and physical bond is to think about what forces/bonds are breaking or forming. If intramolecular bonds, such as covalent and ionic bonds, are being broken and formed, there must be a change at a molecular level. However, if intermolecular forces are involved and changing, it is simply a physical change.