Is "sharpening a pencil" more of a physical change than souring of milk, fermenting of grape juice or rusting of iron?
Thanks anyone!
definitely. Sharpening a pencil leaves the lead, the wood, and the eraser in their original formulations. The others you mention changes the composition of the milk, grape juice or iron into something else.
Thanks, Dr. Bob. So are new substances formed in a chemical reaction?
yes
To determine whether "sharpening a pencil" is more of a physical change than the other examples given (souring of milk, fermenting of grape juice, and rusting of iron), we need to understand the nature of physical and chemical changes.
A physical change refers to a change in the physical properties of a substance without altering its chemical composition. Examples of physical changes include changes in state (such as melting or freezing), changes in shape or size, and changes in appearance. Typically, physical changes are reversible.
On the other hand, a chemical change, also known as a chemical reaction, involves a transformation of substances into different substances with new properties. It typically involves a change in the chemical composition of the materials, resulting in the formation of new substances. Chemical changes are usually irreversible.
Now, let's analyze the examples given:
1. Sharpening a pencil: This process involves physically shaving or removing a portion of the wood and graphite from the pencil, resulting in a pointed tip. Since the composition of the pencil remains the same (wood and graphite), it is considered a physical change.
2. Souring of milk: This is a chemical change. Milk contains various proteins, fats, and sugars. When exposed to certain bacteria, primarily lactic acid bacteria, these microorganisms ferment the lactose (sugar) in the milk into lactic acid. As a result, the pH of the milk decreases, giving it a sour taste and causing it to curdle. This change in composition is a chemical reaction.
3. Fermenting of grape juice: Similar to the souring of milk, fermenting grape juice is a chemical change. The natural sugars present in grape juice are transformed into alcohol and carbon dioxide through the actions of yeast. This process is called fermentation.
4. Rusting of iron: Rusting is a chemical change that occurs when iron reacts with oxygen and moisture in the air, forming iron oxide (rust) on the surface of the metal. This process alters the composition of the iron and is irreversible.
In conclusion, while sharpening a pencil is indeed a physical change, both the souring of milk, fermenting of grape juice, and rusting of iron are chemical changes.