Problem 22 - Introductory Chemistry Atoms First

Question

Why does a gas expand to fill the container it is in, but a liquid and a solid do not?

Step-by-Step Solution

Answer

Gases expand to fill their containers due to the constant, random motion and high kinetic energy of their widely spaced molecules, along with their weak intermolecular forces (van der Waals forces), which cannot hold them together in a specific shape or volume. In contrast, liquids and solids have stronger intermolecular forces and closer molecular arrangement, giving them a definite volume while only allowing liquids to flow and fill the shape of a container. Solids maintain both a definite volume and shape due to the strongest intermolecular forces among the three states of matter.

Step 1: Describe the molecular behavior of gases

In a gas, the molecules are in constant, random motion and are widely spaced apart. They have high kinetic energy, which means they move quickly and collide with each other and with the walls of the container. These collisions cause the gas to expand and occupy the entire volume of its container.

Step 2: Describe the intermolecular forces in gases

Gases have weak intermolecular forces (the forces between molecules), known as van der Waals forces. These forces are not strong enough to hold the gas molecules together in a specific shape or volume. As a result, gas molecules spread out and occupy the entire container.

Step 3: Describe the molecular behavior of liquids

In a liquid, the molecules are closer together than in a gas, with less space between them. They still move freely but with less energy, colliding occasionally but staying relatively close to their neighboring molecules.

Step 4: Describe the intermolecular forces in liquids

Intermolecular forces in liquids are stronger than those in gases, often including hydrogen bonding, dipole-dipole forces, and van der Waals forces. These forces are strong enough to hold liquid molecules together, giving liquids a definite volume, but not strong enough to hold them in a rigid shape.

Step 5: Describe the molecular behavior of solids

In a solid, the molecules are even closer together than in a liquid, with very little space between them. They vibrate in place but do not move around. This arrangement results in a fixed and rigid structure.

Step 6: Describe the intermolecular forces in solids

The intermolecular forces in solids are the strongest of the three states of matter. They may include ionic or covalent bonding, as well as other forces such as hydrogen bonding, dipole-dipole forces, and van der Waals forces. These forces are strong enough to hold solid molecules together in a specific shape and volume.

Step 7: Compare molecular behavior and intermolecular forces between gases, liquids, and solids

Gases expand to fill their containers because their molecules are in constant motion, widely spaced apart, and have weak intermolecular forces. On the other hand, liquids and solids do not expand to fill their containers because their molecules are closer together and held in place by stronger intermolecular forces. In liquids, these forces are strong enough to maintain a definite volume but not a fixed shape, while in solids, they are strong enough to maintain both a definite volume and shape.