Problem 29 - Chemistry Principles and Practice

Question

Which of the following processes describe physical changes, and which describe chemical changes? (a) Milk souring (b) Water evaporating (c) The forming of copper wire from a bar of copper (d) An egg frying

Step-by-Step Solution

Answer

(a) Chemical, (b) Physical, (c) Physical, (d) Chemical

Step 1: Understand Physical vs Chemical Changes

A physical change affects the form of a substance, but not its chemical composition. Chemical changes result in the formation of one or more new substances with new chemical properties.

Step 2: Analyze Option (a) Milk Souring

Milk souring is a chemical change because it involves the fermentation process, in which lactose is converted into lactic acid, changing the chemical composition of the milk.

Step 3: Analyze Option (b) Water Evaporating

Water evaporating is a physical change because it involves a change of state from liquid to gas without changing the chemical structure of H₂O.

Step 4: Analyze Option (c) Forming Copper Wire

Forming copper wire from a bar of copper is a physical change because it involves changing the physical shape of copper without altering its chemical composition.

Step 5: Analyze Option (d) Egg Frying

Egg frying is a chemical change because the heat causes proteins in the egg to denature and recombine, forming new substances with different chemical properties.

Key Concepts

Physical Change

A physical change occurs when a substance changes its form or state, such as from solid to liquid or liquid to gas, without altering its chemical composition. This means that the substance before and after the change is the same on a molecular level. Some common indicators of a physical change include changes in shape, size, texture, or even phase (like melting ice or evaporating water).
In this context, when water evaporates, it changes from a liquid to a gaseous state, yet it remains H₂O, the water molecule. Similarly, transforming a bar of copper into a copper wire alters its shape but not what it's chemically made of. Copper remains copper throughout the process. Physical changes are typically reversible; for example, evaporated water can be condensed back into liquid water.

• Change in state (liquid to gas, solid to liquid)
• Change in shape or form
• No change in chemical composition
• Often reversible

Chemical Change

Chemical changes, unlike physical changes, involve transforming a substance into one or more different substances with new chemical properties. This means the original substance's chemical composition changes, leading to new molecular structures and formations.
A good example is when milk sours; this process is chemical because the lactose in milk is converted into lactic acid through fermentation. This changes the milk's chemical structure entirely. Likewise, when you fry an egg, the heat restructures the proteins in the egg, resulting in new substances with different chemical properties compared to raw egg,
often making chemical changes irreversible. These transformations might involve changes in color, energy, the formation of a precipitate, or the production of gas.

• Formation of new substances
• Change in chemical composition
• Often irreversible
• Might involve energy changes

Chemical Composition

Chemical composition refers to the identity and ratio of the elements that make up a compound or substance. It determines the chemical properties and behavior of the substance, as it defines what molecules are present and how they are bonded together. The chemical composition remains unchanged in physical changes but is altered in chemical changes.
For instance, in the making of copper wire, the chemical composition isn't altered; copper is still composed of copper atoms. However, when milk sours or an egg fries, the chemical composition changes as new molecules are formed. Understanding the chemical composition helps predict how a substance will interact under different conditions, and why certain processes lead to physical or chemical changes.

• Identifies elements and their ratios in substances
• Key to understanding substance properties
• Remains unchanged in physical changes
• Alters in chemical changes