Physical Change Versus Chemical Change

Gasoline burning is a chemical change. When gasoline burns, it undergoes a chemical reaction with oxygen in the air, resulting in the production of carbon dioxide, water vapor, and heat. This change is irreversible, as the original gasoline molecules are transformed into new substances. Therefore, it is considered a chemical change rather than a physical change, which refers to a change in the physical properties of a substance without altering its chemical composition.

During a physical change, the substance undergoes a transformation in its physical properties, such as appearance, smell, or any other outer change. This means that the substance may change its state of matter, shape, size, or other observable characteristics, but its internal structure and composition remain the same.

During a chemical change, the substance undergoes a transformation in its composition. This means that the arrangement and bonding of atoms in the substance are altered, resulting in the formation of new substances with different properties. This change is often accompanied by other observable effects such as a change in color, smell, or the dissolution of the substance. However, the key characteristic of a chemical change is the alteration of the substance's composition.

Whipping egg whites is a physical change because it does not involve the formation of any new substances. When egg whites are whipped, air is incorporated into them, causing the proteins in the egg whites to denature and form a foam-like structure. However, the chemical composition of the egg whites remains the same. Therefore, this process does not result in the formation of new substances and is considered a physical change.

Bread rising is a chemical change. During the process of bread rising, yeast in the dough undergoes fermentation, converting sugars into carbon dioxide gas. This gas gets trapped in the dough, causing it to expand and rise. The chemical composition of the dough is altered as a result of this reaction, making it a chemical change rather than a physical one.

Dissolving sugar into water is a physical change because it does not involve a chemical reaction. In this process, the sugar molecules are dispersed throughout the water, forming a homogeneous mixture. The chemical composition of both the sugar and water remains the same, only the physical state of the sugar changes from solid to dissolved. No new substances are formed, indicating that it is a physical change rather than a chemical change.

A change in state, such as from solid to liquid or liquid to gas, is considered a physical change because it does not involve a change in the chemical composition of the substance. In a physical change, the molecules or atoms rearrange themselves, but their fundamental structure remains the same. This is different from a chemical change where new substances with different chemical properties are formed. Therefore, a change in state is classified as a physical change.

When a substance changes in color, texture, or temperature, it is considered a physical change. This is because these changes do not alter the chemical composition or the identity of the substance. Physical changes only affect the physical properties of the substance, such as its appearance or state of matter. In this case, the change in color, texture, or temperature is reversible and does not involve a chemical reaction.