Question

Sometimes, after clothes are washed, they are hung in the sun to dry. Describe the change or changes that occur in terms of the kinetic-molecular theory. Are the changes that occur physical or chemical changes?

Short answer

Drying involves a physical change with water evaporating due to increased kinetic energy.

Step-by-step solution

Understanding the Process

The process of clothes drying in the sun involves water molecules evaporating from the surface of the fabric. The sun provides heat energy that increases the kinetic energy of the water molecules.

Explaining Kinetic-Molecular Theory

According to the kinetic-molecular theory, molecules are in constant motion, and this motion increases with temperature. As water molecules receive energy from the sun, their kinetic energy increases, allowing them to overcome intermolecular forces and transition from the liquid phase to the gaseous phase, resulting in evaporation.

Determining the Type of Change

Assess whether the change involves altering the fundamental structure of the substance. Since water vaporizes but remains H2O, this is a physical change, not a chemical change, because no new substance is produced.

Key concepts

Evaporation Process

When clothes are hung out in the sun, the water molecules in the fabric begin to evaporate. This is an everyday example of the evaporation process, a key concept in the kinetic-molecular theory. Evaporation is the process where liquid water changes to vapor, a gas, at the surface of the liquid. This happens when molecules in a liquid move rapidly enough to break free from the surface.

Several factors influence evaporation:

• Temperature: As the sun heats the water in the fabric, the temperature boosts the kinetic energy of water molecules, making evaporation more likely.
• Surface Area: A larger surface area, like clothes spread out in the sun, allows more molecules to escape into vapor form.
• Air Movement: Wind or a breeze can carry away water vapor, speeding up the drying process.

The evaporation of water from clothes is a gradual process, which dries them as more and more water molecules escape into the air.

Physical Change

When considering what happens to wet clothes drying in the sun, it is important to distinguish between physical and chemical changes. A physical change is a type of change in which the form of matter is altered, but one substance is not transformed into another.

In the case of drying clothes, the water in the fabric evaporates. The water undergoes a state change—from liquid to gas—but remains water (\(H_2O\)). It retains its chemical structure even as its physical appearance changes. This is a classic case of a physical change because:

• No new substances are formed.
• The process is reversible, meaning the water vapor can condense back into a liquid.
• There is no alteration to the inherent molecular structure of water.

Understanding this helps clarify why drying clothes is a physical, not a chemical change.

Molecular Motion in Liquids

The molecular motion in liquids is a fundamental concept when discussing how liquids behave, especially within the context of the kinetic-molecular theory. In liquids, molecules are closely packed but are still free to move around each other, which is why they can flow and take the shape of their container.

When water is heated by the sun while clothes are drying, this motion increases. As temperature rises, the kinetic energy of the water molecules increases, allowing them to break free from their liquid constraints and move into the gaseous phase during evaporation. This increased motion is why heat leads to faster evaporation.

Key aspects of molecular motion in liquids include:

• Intermolecular Forces: The forces hold molecules together in a liquid but are weaker than the forces in a solid.
• Temperature-related Kinetics: The warmer the temperature, the more energetic and rapid the molecular motion becomes.
• Density and Volume: While changing phase, the density and volume of the substance change as molecules spread out in the air.

By understanding this, students can better grasp how the kinetic energy and movement of molecules play a pivotal role in everyday phenomena like drying clothes.