Problem 69 - Chemistry The Central Science

Question

For a given substance, the liquid crystalline phase tends to be more viscous than the liquid phase. Why?

Step-by-Step Solution

Answer

The liquid crystalline phase tends to be more viscous than the liquid phase because of the increased molecular order in the liquid crystalline phase. In this phase, the molecules exhibit some degree of long-range order, typically forming layered or aligned structures, which restricts their freedom of movement and makes it harder for them to slide past each other. This results in stronger intermolecular forces and reduced mobility, leading to higher viscosity compared to the liquid phase, where molecules are randomly arranged and can easily slide past one another.

Step 1: Definition of Liquid and Liquid Crystalline Phase

A liquid is a state of matter characterized by its ability to flow and adapt to the shape of its container, with short-range molecular order but no long-range order. On the other hand, a liquid crystalline phase is an intermediate phase between a liquid and a crystal, where the molecules exhibit some degree of long-range order while still maintaining fluidity.

Step 2: Definition of Viscosity

Viscosity (\(\eta\)) is a measure of a fluid's resistance to flow, or deformation by shear stress or tensile stress. It refers to the internal friction between adjacent layers of fluid that are moving at different velocities. Higher viscosity means that a fluid has more resistance to flow and appears thicker. Conversely, lower viscosity indicates a thinner fluid that flows more easily.

Step 3: Molecular Structure in Liquid and Liquid Crystalline Phase

In the liquid phase, the molecules are randomly arranged and can easily slide past one another, which allows the liquid to flow easily. In the liquid crystalline phase, the molecules are more ordered, typically forming layered or aligned structures. These ordered structures reduce the freedom of movement of the molecules, making it harder for them to slide past each other.

Step 4: Comparing Viscosity in Liquid and Liquid Crystalline Phase

Due to the increased molecular order in the liquid crystalline phase, the molecules experience more resistance to flow. This results in a higher viscosity in the liquid crystalline phase compared to the liquid phase. The ordered molecular arrangements in the liquid crystalline phase create stronger intermolecular forces and reduce the mobility of the molecules, thus increasing the viscosity.