Question

Determine whether each of the following is a physical or chemical change, and explain your reasoning. (a) Iron rusts. (b) Kernels of corn are heated to make popcorn. (c) Molten copper is mixed with molten gold to make an alloy. (d) Heavy cream is churned to make butter. (e) Water vapor cools into water.

Short answer

(a) Chemical change - new substance iron oxide is formed. (b) Physical change - no new substances formed. (c) Chemical change - new substance (alloy) is created. (d) Physical change - reversible and no new substances. (e) Physical change - phase change without new substance.

Step-by-step solution

Identify Changes in Iron Rusting

To determine whether the rusting of iron is a physical or chemical change, we need to consider whether the identity of the substance changes. Rusting involves iron reacting with oxygen to form iron oxide, which is a new substance with different properties. Therefore, it's a chemical change.

Analyze the Popcorn Popping Process

Heating kernels of corn to make popcorn involves adding heat to cause the moisture inside the kernel to expand, creating pressure until the kernel pops. This process changes the physical appearance and texture of the corn but not its chemical composition. Therefore, it's a physical change.

Examine the Formation of an Alloy with Copper and Gold

Mixing molten copper with molten gold to form an alloy involves combining two different metals to create a new material with its own unique properties. The identities of the substances change at the atomic level, resulting in a new substance. This is a chemical change.

Consider the Process of Making Butter from Heavy Cream

Churning heavy cream to make butter is a process of agitation that causes the fats in the cream to clump together separating from the liquid. This change is reversible and does not involve formation of new substances. Thus, it's a physical change.

Determine the Nature of Water Vapor Cooling into Water

When water vapor cools into water, it's a phase change from gas to liquid. There are no new substances formed during this process, and the molecular structure of H2O remains unchanged, so this is a physical change.

Key concepts

Chemical Change

A chemical change, also known as a chemical reaction, occurs when substances interact to form one or more new substances with different chemical properties. This transformation results in the alteration of the chemical identity of the substance involved. For instance, when iron rusts, it reacts with oxygen in the environment to create iron oxide. Iron oxide has different properties and a distinct chemical structure from iron, indicating that a chemical change has occurred. Indicators of chemical changes can include but are not limited to color changes, temperature change, gas production, and the formation of precipitates.

Comparing to Iron Rusting

Looking at the rusting of iron example from the exercise, it's clear that this is a chemical change. The iron (Fe) bonds with oxygen (O2) from the air to form a new compound: iron oxide (Fe2O3). This process alters the chemical composition and properties of the original iron, making it a classic example of a chemical change.

Physical Change

Physical changes are quite different from chemical ones in that they involve a substance changing its form, state, or appearance without altering its chemical identity. Physical changes are typically reversible. When you pop corn kernels, you apply heat which turns water inside the kernels into steam. The pressure from the steam causes the kernels to explode into popcorn. The chemical composition of the corn is not changed; only its physical form is altered which is why popping corn is considered a physical change.

Butter and Water Examples

In the textbook example questions, churning cream into butter and cooling water vapor into liquid water are both physical changes. They alter the physical properties and appearance of the substance but do not result in the creation of a new substance, maintaining the same basic chemical structure.

Phase Change

Phase changes are a type of physical change where a substance moves from one state of matter to another: solid, liquid, or gas. These transitions occur without a change in the chemical structure of the substance. For instance, freezing water into ice or boiling water into steam are both examples of phase changes. They are reversible; ice can melt back into water just as water vapor can condense back into liquid water.

The Condensation of Water Vapor

As detailed in the textbook solution about water vapor cooling into water, this is a phase change because the water molecules are moving from a gaseous state back into a liquid state. There is no change in the molecular structure of H2O during this process, affirming that it is indeed a physical change.

Substance Identity

Substance identity refers to the chemical composition and properties that define a particular substance. These characteristics determine how a substance behaves under different conditions and how it reacts with other substances. When a chemical change occurs, the substance identity is altered, as new substances with new properties are formed. On the other hand, during a physical change or phase change, the substance identity remains constant as no new substances are created.

Alloy Formation and Substance Identity

An illustrative example from our exercise is the creation of an alloy by mixing molten copper with molten gold. Here, the metals blend on an atomic level, resulting in a new substance with characteristics distinct from those of the original metals. The substance identity of the copper and gold is forever changed, demonstrating a chemical change. Understanding substance identity is crucial when determining whether a process is a chemical or physical change.