Physical Reaction: Definition & Examples

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  • 0:00 Definition of Physical…
  • 2:32 Types of Physical Reactions
  • 4:18 Examples of Physical Reactions
  • 5:44 Lesson Summary
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Lesson Transcript
Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Did you know the cycle of freezing water to make ice and melting ice to produce water is considered a physical reaction? In this lesson, we'll learn more about physical reactions, define what a physical reaction is, and look at some examples.

Definition of Physical Reactions

I must admit, I'm a big fan of lemon meringue pie. Although it is a very well-known dessert, would you be shocked if I told you the process of making meringue involves a physical reaction? It sure does! But, before we talk about some examples, though, let's discuss the definition of a physical reaction.

A physical reaction occurs when molecules undergo a molecular rearrangement to produce a physical change. The molecules are not chemically altered. As a reminder, molecules are two or more atoms linked by chemical bonds. But wait, why is a chemical reaction different from a physical reaction if they are both reactions? Great question!

A chemical reaction occurs when molecules are chemically altered as a result of interacting with one another, such as their bonds breaking. So, the difference is that with a physical reaction, a chemical change in the molecule is not observed, but a physical change is observed. Speaking of the phrase 'physical change,' physical reactions are commonly referred to as physical changes.

The diagram here provides a great illustration of what a physical reaction is, and why it is different from a chemical reaction.


As you can see in both (A) and (B), several molecules decided to meet up and mingle with each other. These molecules interact with each other so well that something sparks from the 'conversation.' This conversation produces a physical change in A and a chemical change in B. Both sets of molecules met, mingled, talked, and sparked a change, but the takeaway point is that the changes were different.

The molecules in A lowered the temperature in the room and decided to get closer to one another, but they did not change chemically (i.e. formed bonds with other atoms, lost atoms, etc.). Now, this is an entirely different story for our molecule friends in B; they met, mingled, talked, and sparked a big change. They changed in such a way that new molecules were formed! This is because some type of alteration to the structure of the molecules occurred while they were mingling and talking (i.e. bonds were formed between atoms, new molecules were made, etc.). This is the main difference between chemical and physical reactions. Always remember that the original make up, or 'DNA' of a molecule, never changes in a physical reaction, but the arrangement of molecules can change.

Types of Physical Reactions

There are many, many types of physical reactions and many ways to describe the change occurring as a result of a physical reaction. However, I will only touch upon two of them here:

Boiling Point

When molecules interact with each other the physical change can cause an increase in temperature. The boiling point occurs when molecules heat up in such a way that the pressure surrounding them causes bubbles in the liquid to form. Think of taking water, cranking up the heat, and watching it bubble away at high temperatures. A physical reaction happened with those water molecules. The hydrogen and oxygen atoms in the water mingled. When heated, those water molecules did not change their chemical composition, but the rearrangement of their molecules did cause a physical change. In this case, it was the boiling water.

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