Question

The properties of gases, liquids, and solids differ in a number of respects. How would you use the kinetic molecular theory (see Section 10.6 ) to explain the following observations? (a) Ease of compressibility decreases from gas to liquid to solid. (b) Solids retain a definite shape, but gases and liquids do not. (c) For most substances, the volume of a given amount of material increases as it changes from solid to liquid to gas.

Short answer

The kinetic molecular theory explains that gases are most compressible due to spaced particles; solids retain a shape due to tightly packed particles; volume increases from solid to gas due to increased particle motion and separation.

Step-by-step solution

Understanding Kinetic Molecular Theory

The kinetic molecular theory explains the behavior of particles in different states of matter. It states that particles are in constant motion, and the energy and arrangement of these particles determine whether a substance is a solid, liquid, or gas.

Compressibility Explained

(a) According to kinetic molecular theory, gases are easily compressible because their particles are far apart with high kinetic energy, allowing space to be reduced with little resistance. Liquids have particles that are closer together, making them less compressible. In solids, particles are tightly packed in fixed positions, making compression extremely difficult.

Shape Retention Explained

(b) Solids retain a definite shape due to particles being tightly packed in a regular pattern, with only limited motion (vibration) in place. The energy in solids is low compared to liquids and gases. Liquids have particles that are close but can move past each other, leading to an indefinite shape that conforms to a container. Gases have particles in rapid, random motion, resulting in no fixed shape or volume.

Volume Change Explained

(c) As substances transition from solid to liquid to gas, their volume increases due to increased particle separation. In solids, particles have very little motion due to strong intermolecular forces, keeping volume minimal. Liquids have particles with more kinetic energy, partially overcoming intermolecular forces, causing expansion. In gases, particles have maximum energy, fully overcoming these forces and occupying more space.

Key concepts

Compressibility

Compressibility refers to a material's ability to decrease its volume under pressure. Within the context of the kinetic molecular theory, this property changes significantly across different states of matter. Gases are known for their high compressibility. This results from the vast spaces between their particles, which move freely and rapidly. Because the particles are so spread out, pushing them closer together is easy with minimal resistance.

In contrast, liquids demonstrate lower compressibility. Their particles are closer together compared to gases, making it harder to compress them into a smaller space. Finally, solids are the least compressible. This is because their particles are tightly packed in a fixed structure, leaving almost no room for further reduction in volume. This orderly arrangement in solids is why compressing them requires significant force.

States of Matter

Matter exists in three primary states: solid, liquid, and gas. The behavior and arrangement of particles in each state are central to their characteristics and are well-explained by the kinetic molecular theory.

Solids have a definite shape and volume because their particles are ordered in a fixed position, only vibrating slightly. This rigid structure means a solid keeps its shape regardless of the container. Liquids, however, have an indefinite shape but a definite volume. In this state, particles are closely packed but not in a fixed pattern. They can slide past each other, which allows liquids to flow and take the shape of their container.

Gases have neither a definite shape nor volume. Their particles move quickly and randomly, filling any available space. This random motion allows gases to expand and fill the entire volume of a container, no matter the shape or size.

Particle Motion

Particle motion varies dramatically between solids, liquids, and gases due to differences in the kinetic energy and arrangement of particles. In solids, particle motion is minimal. Particles vibrate in fixed positions giving solids their firmness and defined shape. Low kinetic energy in solids restricts other types of motion.

With liquids, particles have higher kinetic energy and are not held in a fixed position. They move relative to one another, allowing liquids to flow and change shape. This increased energy enables particles to partially overcome intermolecular forces, contributing to the fluidity of liquids.

In gases, particles have maximum kinetic energy, moving rapidly and freely in all directions. This extensive motion results in gases having no fixed shape or volume. The high energy allows gas particles to overcome nearly all intermolecular attractions, enabling them to fill any container entirely.

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together and significantly influence the properties of solids, liquids, and gases. These forces are strongest in solids, where the tightly packed particles experience considerable attraction to one another. This results in the fixed, orderly structure of solids, maintaining their definite shape and volume.

In liquids, intermolecular forces are less strong but still significant. They allow for a fairly dense arrangement of particles, though not as rigid as in solids. The reduced but present forces lead to a state where particles can move past each other, contributing to liquids' ability to take the shape of their container.

For gases, intermolecular forces are very weak, almost negligible. Gaseous particles are spread far apart and move independently, largely unaffected by these attractions. This is why gases can expand fully to fill a container, reflecting their high kinetic energy and substantial particle separation.