Problem 44 - General Chemistry

Question

For each of the following, decide whether a physical or a chemical change is involved. a. dissolving of sugar in water b.rusting of iron c. burning of wood d. evaporation of alcohol

Step-by-Step Solution

Answer

a. Physical change; b. Chemical change; c. Chemical change; d. Physical change.

Step 1: Understanding Physical Changes

A physical change involves a change in the state or appearance of matter that does not change its composition. Common examples include dissolving, melting, boiling, and evaporating.

Step 2: Understanding Chemical Changes

A chemical change involves a transformation that alters the chemical composition of a substance, resulting in new substances being formed. Common examples include rusting, burning, and reactions with acids.

Step 3: Assessing the Dissolving of Sugar in Water

When sugar dissolves in water, the sugar molecules disperse in the water but do not change their chemical structure, indicating a physical change.

Step 4: Assessing the Rusting of Iron

Rusting involves the reaction of iron with oxygen in the presence of moisture to form iron oxide, a new substance. This is a chemical change.

Step 5: Assessing the Burning of Wood

Burning wood results in the production of ash, carbon dioxide, and water vapor as new substances are formed. This is a clear example of a chemical change.

Step 6: Assessing the Evaporation of Alcohol

When alcohol evaporates, it changes from a liquid to a gas but retains its chemical composition, which makes this a physical change.

Key Concepts

Chemical Change

A chemical change occurs when a substance undergoes a transformation that alters its chemical structure, resulting in the creation of new substances with different properties. These changes often involve breaking and forming chemical bonds. During a chemical change, substances may change color, produce gas, emit light, or release or absorb heat. Importantly, the process is typically irreversible without another chemical reaction.

For example, when iron rusts, it combines with oxygen and water to form iron oxide, a completely new compound. This process is a chemical change because the original substance, iron, has changed composition and cannot easily return to its original form. Other classic examples include the burning of wood or paper, where the material reacts with oxygen to produce ash, carbon dioxide and water vapor, all new substances resulting from the reaction.

Physical Change

Physical change refers to a change in the physical form or properties of a substance without a change in its chemical composition. This means that while the state or appearance of a substance may alter, its essential structure remains unchanged.

Common examples of physical changes include transformations like melting, freezing, boiling, dissolving, and evaporating. These changes are generally reversible. For instance, when ice melts into water, or when water evaporates into steam, the molecular structure of the substance remains as H₂O throughout the process. A crucial aspect of physical changes is that no new substances are formed. Understanding this is key to distinguishing physical changes from chemical ones in everyday observations.

Examples of Physical Change

Physical changes are all around us and are frequently encountered in daily life. Here are some examples:

• Dissolving of Sugar in Water: When sugar dissolves in water, its molecules spread out in the water but maintain their chemical makeup. Thus, it remains a physical change.
• Evaporation of Alcohol: Alcohol evaporating from a surface involves changing from liquid to gas. This process retains the chemical identity of alcohol, clearly marking it as a physical change.
• Melting of Ice: Ice changing to water involves a change of state but not a change in chemical identity, making it a physical change.
• Boiling of Water: The transition from liquid water to steam involves no change in chemical structure. It is purely physical.

Each example demonstrates how physical changes alter appearance or state without altering chemical structure.

Examples of Chemical Change

Chemical changes often signify the transformation of substances into entirely new materials.

• Rusting of Iron: Iron reacts with oxygen, usually when moisture is present, to form iron oxide, a completely new substance. This is a hallmark chemical change.
• Burning of Wood: When wood burns, it reacts with oxygen to form ash, carbon dioxide, and water vapor. All of these are different substances from the original wood, highlighting a chemical change.
• Cooking an Egg: The proteins in an egg change structure when heated, resulting in a new texture and composition, signaling a chemical change.
• Souring of Milk: As milk ages, bacteria convert lactose into lactic acid, changing its composition and making it sour—this is a chemical change.

These examples illustrate how chemical changes involve creating new products and altering the original compounds' nature.