Question

In A skillet is lined with a thin layer of cooking oil followed by a layer of unpopped popcorn kernels. Upon heating, the kernels all pop thereby escaping the skillet. Identify any physical or chemical changes.

Short answer

Heating the oil is a physical change; popping the kernels involves both physical and chemical changes.

Step-by-step solution

Understand the Scenario

A physical change involves changes in physical properties without altering the chemical identity of the substance. A chemical change results in new substances being formed with different properties than the original.

Analyze the Heating of Oil

Heating the oil causes it to change state from a liquid to a gas, but it remains chemically the same substance. Therefore, this is a physical change.

Analyze the Popping of Popcorn

The kernels popping is due to the water inside them turning into steam and causing pressure, leading to the kernel exploding. The inner parts are physically and chemically altered, hence it is both a physical and chemical change.

Key concepts

Physical Change

Physical changes occur when the form of a substance is altered, but its chemical structure remains the same. These transformations can include changes in texture, appearance, or phase, such as melting or boiling. For example, when you heat the oil in a skillet, it transitions from a liquid to a gas. However, the chemical makeup of the oil does not change. It's still the same substance, just in a different state. This is a hallmark of a physical change since the identity of the substance remains untouched.
When discussing physical changes, it is important to note:

• No new substance is produced.
• The change can often be reversed, such as freezing or condensing.
• Physical properties like shape, size, and phase are altered.

Recognizing these characteristics can help distinguish between physical and chemical changes in various scenarios.

Chemical Change

A chemical change results in the formation of one or more new substances with different properties from the original. This change happens at the molecular level, involving the breaking and forming of chemical bonds. When popcorn kernels pop, there is both a physical and chemical change. While the steam builds pressure and eventually causes the outer shell to burst (a physical transition), the heat also induces a chemical change within the kernel. The inside becomes fluffy and changes in texture due to new substances forming through molecular interactions.
Some key indicators of a chemical change include:

• Formation of a gas or precipitate.
• Color change that is not due to mixing.
• Temperature change that is not caused by an external factor.

Both the formation of new substances and the creation of gas (steam) in the popcorn illustrate these principles.

Phase Transition

Phase transitions are specific examples of physical changes where a substance changes from one state of matter to another. Common transitions include melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), and condensation (gas to liquid). In our skillet scenario, heating the oil causes it to vaporize.
During a phase transition:

• The substance retains its chemical identity despite the change in physical form.
• Energy is absorbed or released, often in the form of heat.
• The transition can typically be reversed, such as steam condensing back to water.

Understanding phase transitions helps to identify situations where only physical changes are occurring, reinforcing the concept of unchanged chemical identity.

Popcorn Chemistry

Popcorn is a fascinating example of chemistry interacting with physics. At the core of this process is the kernel, which contains water and starch surrounded by a hard shell. When heated, the water inside turns into steam, creating pressure. This pressure causes the kernel to explode, turning the kernel inside out, which is both a physical burst and a chemical transformation.
The chemistry of popcorn includes:

• Moisture inside the kernel, which is crucial for the popping process.
• Starch that gelatinizes and expands when heated, forming the fluffy structure.
• A chemical reaction inside the kernel altering the starch.

This combination of chemical and physical dynamics illustrates the complexity and beauty of everyday chemistry.