Grade 6 → Physical and chemical changes ↓
Difference Between Physical and Chemical Changes
In the wonderful world of chemistry, everything around us is made up of tiny building blocks called atoms and molecules. These can undergo changes that are divided into two main categories: physical changes and chemical changes. Understanding the difference between these two types of changes is essential to knowing how matter interacts in our universe.
What is a physical change?
A physical change affects the appearance of a chemical substance, but not its chemical composition. This means that a substance that undergoes a physical change remains the same substance. These changes are usually reversible and involve changes in physical properties such as shape, size, phase (e.g. solid, liquid, gas) and density.
Examples of physical change
- Melting ice to make water
- Boiling water into steam
- Breaking the glass
- Masturbation
- Crushing the can
Visual example: melting snow
What is a chemical change?
On the other hand, a chemical change occurs when a substance combines with another substance to form a new substance. This process often involves making or breaking chemical bonds and results in substances with new chemical properties. Chemical changes are usually not easily reversible.
Examples of chemical change
- Burning wood produces ash and gases
- Rusting of iron
- Baking the cake
- Souring of milk
- Making Carbon Dioxide Gas by Mixing Baking Soda and Vinegar
Visual example: vinegar and baking soda reaction
Indicators of Chemical Change
Identifying a chemical change can sometimes be challenging, but there are several signs we can look for. Here are some signs that a chemical change has occurred:
- Discoloration (e.g., rusting of iron)
- Formation of precipitate (solid substance formed from solution)
- Gas production (bubbles or smoke)
- Change in temperature (the reaction may release or absorb heat)
- Changes in light or sound (e.g., explosions or light emissions)
Lesson example: baking a cake
Baking a cake is a delicious example of a chemical change. When you combine ingredients such as flour, eggs, sugar and baking powder, the heat from the oven causes the baking powder to release carbon dioxide gas. This gas makes the cake rise and gives it a fluffy texture. Also, proteins and carbohydrates undergo reactions that give the cake its delightful taste and aroma.
Caking Baking Reaction: Baking soda (NaHCO3
) + acid (e.g., vinegar) → carbon dioxide (CO2
)
Detailed comparison
Reversibility
One of the primary differences between physical and chemical changes is reversibility. Physical changes, such as melting or freezing, can generally be reversed. For example, water can freeze into ice and then melt back into water.
Chemical changes usually cannot be reversed by simple physical means. For example, once wood is burned to ashes, it cannot be turned back into wood.
Energy transformation
Both physical and chemical changes can involve energy changes.
- The energy changes in physical changes are usually small, such as the energy needed to change the phase of water.
- Chemical changes often involve significant energy transformations, such as the release of heat during combustion.
Changes in structure
The composition of matter remains the same during physical changes. Chemically, ice and water are both H2O
In chemical changes the composition of matter changes. For example, burning methane in oxygen produces carbon dioxide and water.
Methane combustion reaction: CH4 + 2O2 → CO2 + 2H2O
Visual representation of the differences
Consider the following simplified visualization:
Conclusion
Understanding the difference between physical and chemical changes is a foundational concept in chemistry. While both types of changes can occur naturally, seeing them in action—such as water boiling, bread baking, or leaves changing color—helps us understand the processes at work. Identifying whether a change is physical or chemical gives us information about the nature of matter and the universe. It also allows chemists to purposefully manipulate substances, contributing to advances in technology, medicine, and countless other fields.