1A.7: Physical and Chemical Changes

Learning Objectives

• Differentiate between physical and chemical changes at the particulate level
• Classify changes as physical or chemical

Change is happening all around us all of the time. Just as chemists have classified elements and compounds, they have also classified types of changes. Changes are either classified as physical or chemical changes.

Physical & Chemical Change

Chemists learn a lot about the nature of matter by studying the changes that matter can undergo. Chemists make a distinction between two different types of changes that they study - physical changes and chemical changes.

Physical Change

Physical changes are changes in which no bonds are broken or formed. This means that the same types of compounds or elements that were there at the beginning of the change are there at the end of the change.Physical changes involve moving molecules around, but not changing them. Some types of physical changes include:

• Changes of state (changes from a solid to a liquid or a gas and vice versa)
• Separation of a mixture
• Physical deformation (cutting, denting, stretching)
• Making solutions (special kinds of mixtures)

Figure \(\PageIndex{1}\): Ice Melting is a physical change. When liquid water (\(\ce{H2O}\)) freezes into a solid state (ice), it appears changed; However, this change is only physical as the the composition of the constituent molecules is the same: 11.19% hydrogen and 88.81% oxygen by mass. Image used with permission (Public Domain; Moussa)

For the most part, physical changes tend to be reversible - in other words, they can occur in both directions. You can turn liquid water into solid water through cooling; you can also turn solid water into liquid water through heating. However, as we will later learn, some chemical changes can also be reversed.

Chemical Change

Chemical changes occur when bonds are broken and/or formed between molecules or atoms. This means that one substance with a certain set of properties (such as melting point, color, taste, etc) is turned into a different substance with different properties. Chemical changes are frequently harder to reverse than physical changes.

Figure \(\PageIndex{1}\): Burning of wax to generate water and carbon dioxide is a chemical reaction. Image used with permission (CC-SA-BY-3.0; Andrikkos)

One good example of a chemical change is burning paper. In contrast to the act of ripping paper (physcial change), the act of burning paper actually results in the formation of new chemicals (carbon dioxide and water, to be exact).

Evidence of Chemical Change

We can't actually see molecules breaking and forming bonds, although that's what defines chemical change. We have to make other observations to indicate that a chemical change has happened. What kind of macroscopic observations can indicate a chemical change?

Observations that help to indicate chemical change include:

• Temperature changes when no energy is added or removed (it can increase or decrease)
• Light is given off
• Unexpected color changes (a substance with a different color is made, rather than just mixing the original colors together)
• Bubbles are formed (but the substance is not boiling - you made a substance that is a gas at the temperature of the beginning materials, instead of a liquid)
• Different smell or taste (do not taste your chemistry experiments, though!)
• A solid forms if two clear liquids are mixed (look for floaties - technically called a precipitate)

Example 1.7.1

Label each of the following changes as a physical or chemical change. Give evidence to support your answer.

1. Boiling water
2. A nail rusting
3. A green solution and colorless solution are mixed. The resulting mixture is a solution with a pale green color.
4. Two colorless solutions are mixed. The resulting mixture has a yellow precipitate.

Solution

1. Physical: boiling and melting are physical changes. When water boils no bonds are broken or formed. The change could be written: \(\ce{H_2O} \left( l \right) \rightarrow \ce{H_2O} \left( g \right)\)
2. Chemical: The dark grey nail changes color to form an orange flaky substance (the rust); this must be a chemical change. Color changes indicate chemical change. The following reaction occurs: \(\ce{Fe} + \ce{O_2} \rightarrow \ce{Fe_2O_3}\)
3. Physical: because none of the properties changed, this is a physical change. The green mixture is still green and the colorless solution is still colorless. They have just been spread together. No color change occurred or other evidence of chemical change.
4. Chemical: the formation of a precipitate and the color change from colorless to yellow indicate a chemical change.

 

Test Yourself 

 

Query \(\PageIndex{1}\)

 

Contributors and Attributions

• Robert E. Belford (University of Arkansas Little Rock; Department of Chemistry). The breadth, depth and veracity of this work is the responsibility of Robert E. Belford, [e-mail]. You should contact him if you have any concerns. This material has both original contributions, and content built upon prior contributions of the LibreTexts Community and other resources, including but not limited to:

• Ronia Kattoum (Learning Objectives)
• Elena Lisitsyna (H5P interactive modules)