Book cover for Living by Chemistry

Living by Chemistry

Angelica M. Stacy

ISBN #9781464142314

2nd Edition

940 Questions

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129,188 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This section highlights the dramatic differences in density between solids, liquids, and gases, explaining that as substances change phase through processes like sublimation and evaporation, their molecular spacing increases significantly, leading to large increases in volume and decreases in density. The molecular view helps us understand not only why gases expand to fill their containers but also how they exert pressure on surfaces, with practical demonstrations such as inflated tires, submerged paper, and the balloon-in-a-bottle experiment reinforcing these concepts.

Learning Objectives

1

Explain how the density of gases differs from that of solids and liquids, and describe the methods used to measure gas density.

2

Understand the processes of sublimation and evaporation, and recognize the molecular behavior behind these phase changes.

3

Compare the densities and volumes of different phases of water (ice, liquid water, and water vapor) and relate these changes to real-world phenomena.

4

Describe how the behavior of gas molecules leads to observable air pressure and explain demonstrations that provide evidence of air pressure.

Key Concepts

CONCEPT

DEFINITION

Gas Density

The mass per unit volume of a gas, which is typically much lower than that of liquids or solids because the particles are far apart.

Sublimation

A phase change in which a solid turns directly into a gas without passing through the liquid state, as seen with carbon dioxide (dry ice).

Evaporation

The process where a liquid turns into a gas, typically at the surface of the liquid, with an associated large increase in volume and decrease in density.

Molecular View

A model that explains the behavior of gas molecules, including how they spread apart, move rapidly, and exert pressure by colliding with surfaces.

Air Pressure

The force that air exerts on a surface, defined as the force per unit area, resulting from collisions of air molecules with that surface.

Example Problems

Example 1

How does the density of a gas compare with the density of a solid?

Example 2

Draw a molecular view of carbon dioxide gas.

Example 3

In your own words, define sublimation and evaporation.

Example 4

Describe three ways to show that gases exist

Example 5

When water freezes, the water molecules move apart very slightly. What evidence can you provide to support this claim?
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Step-by-Step Explanations

QUESTION

How can you determine the density of a gas if it is difficult to measure its mass directly?

STEP-BY-STEP ANSWER:

Step 1: Start with a known mass of the substance in its solid or liquid form.
Step 2: Convert the substance into a gas phase without any loss of mass by allowing it to evaporate or sublime.
Step 3: Measure the volume occupied by the gas.
Step 4: Calculate density by dividing the original mass by the measured volume of the gas.
Final Answer: Density (g/mL) = Mass (g) / Volume (mL) of the gas.

Measuring Gas Density

QUESTION

Why does the volume of CO2 increase dramatically during sublimation from a solid to a gas?

STEP-BY-STEP ANSWER:

Step 1: Recognize that in the solid state, CO2 molecules are closely packed, occupying a small volume.
Step 2: When dry ice sublimates, the CO2 molecules separate and spread out due to high molecular motion.
Step 3: This increased spacing results in a much larger volume for the same mass—a nearly 800-fold increase.
Final Answer: The transition from closely packed molecules in the solid to widely spaced molecules in the gas causes a drastic increase in volume and a corresponding drop in density.

Sublimation of Dry Ice (CO2)

QUESTION

How do everyday experiments like the balloon in a bottle and submerged paper demonstrate air pressure?

STEP-BY-STEP ANSWER:

Step 1: In the balloon-in-a-bottle experiment, the trapped air inside resists the expansion of the balloon because the air molecules exert pressure in all directions.
Step 2: In the submerged paper experiment, the trapped air inside an upside-down cup pushes against the water, keeping the paper dry.
Step 3: These examples illustrate that gas molecules exert force on surfaces, manifesting as pressure.
Final Answer: Both experiments show that even though gases are low in density, their molecular collisions generate measurable pressure.

Demonstrating Air Pressure

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Common Mistakes

  • Confusing sublimation with evaporation, even though sublimation involves a direct solid-to-gas transition without a liquid phase.
  • Assuming that gas molecules are large or clumped together, rather than spread out and moving rapidly.
  • Overlooking the fact that the mass remains constant during phase changes, despite dramatic changes in volume and density.
  • Misinterpreting experimental demonstrations by not recognizing that gas pressure is exerted uniformly in all directions.