Question

Indicate whether each statement is true or false: (a) The liquid crystal state is another phase of matter, just like solid, liquid, and gas. (b) Liquid crystalline molecules are generally spherical in shape. (c) Molecules that exhibit a liquid crystalline phase do so at well-defined temperatures and pressures. (d) Molecules that exhibit a liquid crystalline phase show weaker-than- expected intermolecular forces. (e) Molecules containing only carbon and hydrogen are likely to form liquid crystalline phases. (f) Molecules can exhibit more than one liquid crystalline phase.

Short answer

(a) True - The liquid crystal state is an intermediate phase between solid and liquid, exhibiting properties of both solids and liquids. (b) False - Liquid crystalline molecules typically have a rod-like or disc-like shape, not spherical. (c) True - Liquid crystalline phases occur under specific temperature and pressure conditions. (d) False - The intermolecular forces in liquid crystalline materials are weaker than solids but not weaker than expected. (e) False - Molecules containing only carbon and hydrogen usually do not form liquid crystalline phases. (f) True - Molecules can exhibit multiple liquid crystalline phases (mesophases) depending on the conditions.

Step-by-step solution

Statement (a)

The liquid crystal state is another phase of matter, just like solid, liquid, and gas. This statement is true. The liquid crystal state is an intermediate phase between solid and liquid, which exhibits properties of both solids (crystalline order) and liquids (flow, viscosity).

Statement (b)

Liquid crystalline molecules are generally spherical in shape. This statement is false. Liquid crystalline molecules typically are long and rigid with a rod-like or disc-like shape that helps them align and pack in intermediate arrangements between a solid and a liquid.

Statement (c)

Molecules that exhibit a liquid crystalline phase do so at well-defined temperatures and pressures. This statement is true. The formation of the liquid crystalline phase occurs under specific temperature and pressure conditions, which depend on the properties of the individual molecules and their interactions.

Statement (d)

Molecules that exhibit a liquid crystalline phase show weaker-than-expected intermolecular forces. This statement is actually false. The intermolecular forces in liquid crystalline materials are usually strong enough to maintain long-range order but weaker than the forces in solid crystals. So, the intermolecular forces are weaker than solids but not weaker than expected.

Statement (e)

Molecules containing only carbon and hydrogen are likely to form liquid crystalline phases. This statement is false. Most liquid crystalline molecules have a combination of polar and nonpolar groups (e.g., aromatic rings, ester links, and alkyl chains). Molecules consisting solely of carbon and hydrogen usually do not have the required structural features necessary for the formation of liquid crystalline phases.

Statement (f)

Molecules can exhibit more than one liquid crystalline phase. This statement is true. There are different types of liquid crystalline phases called mesophases, and molecules can exhibit multiple mesophases depending on the temperature and other conditions (e.g., nematic, smectic, or cholesteric phases).

Key concepts

Phases of Matter

Matter commonly exists in three primary phases: solid, liquid, and gas. Each of these phases has distinct characteristics.
Within these phases, solids have strong intermolecular forces giving them a definite shape, liquids have weaker forces so that they can flow while still maintaining a fixed volume, and gases have very weak or negligible forces allowing them to fill any container completely. Among these typical phases, there is a special state called the liquid crystal state.
This state is unique because it displays characteristics of both liquids and solids.
For example, liquid crystals can flow like liquids but also have ordered structures like solids.
This intermediate phase plays a crucial role in many technologies, such as display screens in electronic devices.

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together. They are crucial for determining the phase of matter of a substance.
These forces vary in strength depending on the distance between molecules and the nature of their interactions.

• In solids: The forces are strong, maintaining a fixed structure.
• In liquids: These forces are weaker but still strong enough to hold molecules together in a fluid form.
• In gases: The forces are very weak, allowing molecules to move freely.

Molecules that exhibit liquid crystalline phases have intermolecular forces that are strong enough to maintain order over long-range but not as strong as in solids.
This balance provides the structure and fluidity unique to the liquid crystal state.

Temperature and Pressure Conditions

The phase of matter that a substance exists in can be significantly affected by temperature and pressure conditions.
These factors dictate how molecules interact, impacting the energy needed for a change in phase.
Liquid crystals are particularly sensitive to these conditions because they form only within specific temperature ranges and pressure levels.
Under exactly the right conditions, molecules align themselves in the necessary order to exhibit liquid crystalline properties.
Changes in temperature and pressure can cause a material to switch from one liquid crystalline phase to another, such as from a nematic phase to a smectic phase.
Understanding how temperature and pressure affect phase behavior is crucial for utilizing liquid crystals in practical applications like thermometers and electronic displays.