Viscosity is the measure of a fluid's internal resistance to flow. It's often described as the "thickness" of a liquid (e.g., honey is more viscous than water).
This resistance arises from the intermolecular forces within the liquid. Stronger forces lead to higher viscosity.
The coefficient of viscosity (η) is defined as the force per unit area required to maintain a unit velocity gradient between two parallel layers of liquid.
F = η · A · (dv/dx)
Where: F is the force, A is the area, and (dv/dx) is the velocity gradient.
Principle (using Ostwald Viscometer): Viscosity is measured *relatively* by comparing the time (t) it takes for a fixed volume of liquid to flow through a capillary tube, compared to a reference liquid (usually water) at the same temperature. This is based on Poiseuille's equation.
Working Formula:
η_1 / η_2 = (ρ_1 · t_1) / (ρ_2 · t_2)
Where:
Viscosity decreases as temperature increases.
Reason: At higher temperatures, the molecules gain kinetic energy, which allows them to overcome the intermolecular forces of attraction more easily, thus reducing the resistance to flow.
Surface tension is the tendency of a liquid to shrink into the minimum possible surface area (which is why drops are spherical). It's a property of the liquid's surface that causes it to behave like a stretched elastic membrane.
Reason: A molecule in the *bulk* of the liquid is attracted equally in all directions by its neighbors. A molecule at the *surface* has no neighbors above it, so it feels a net inward pull. This creates tension at the surface.
The coefficient of surface tension (γ) is defined as the force acting per unit length of a line drawn on the liquid's surface.
Principle: This method compares the number of drops (n) formed by a fixed volume (V) of a test liquid versus a reference liquid (water). The weight of a single drop (mg) is balanced by the surface tension (γ) acting on the circumference (2πr) of the capillary tip.
mg = kγ
Since mass (m) = volume of one drop × density (ρ) = (V/n) × ρ
(V/n) · ρ · g = kγ
Working Formula:
γ_1 / γ_2 = (ρ_1 / n_1) / (ρ_2 / n_2)
γ_1 = γ_2 · (ρ_1 / ρ_2) · (n_2 / n_1)
Where 1 is the unknown liquid and 2 is the reference (water).
Surface tension decreases as temperature increases.
Reason: At higher temperatures, molecules have more kinetic energy, which weakens the intermolecular forces. The net inward pull on the surface molecules is reduced, so the surface tension drops.
Solids are characterized by a definite shape and volume. They can be classified based on the arrangement of their constituent particles.
| Property | Crystalline Solids | Amorphous Solids |
|---|---|---|
| Arrangement | Particles (atoms, ions, molecules) are arranged in a long-range, ordered, repeating 3D pattern. | Particles have only a short-range order, with no regular repeating pattern. |
| Melting Point | Have a sharp, definite melting point. | Melt over a range of temperatures (soften gradually). |
| Cleavage | When cut, they show a clean, flat cleavage plane. | When cut, they show an irregular, curved fracture. |
| Anisotropy | Anisotropic (physical properties like refractive index or electrical conductivity are different in different directions). | Isotropic (physical properties are the same in all directions). |
| Example | NaCl, Diamond, Quartz | Glass, Rubber, Plastic |
The simplest unit cells are cubic. There are three main types:
The spacing between the layers of atoms in a crystal is too small to be seen with visible light. W.H. Bragg and W.L. Bragg showed that X-rays, which have wavelengths (λ) similar to the atomic spacing (d), can be diffracted by these layers.
When X-rays hit the crystal, they are reflected by successive layers of atoms. Constructive interference (a bright spot) occurs only when the path difference between the waves reflected from adjacent layers is an integer multiple (n) of the wavelength.
Bragg's Law:
nλ = 2d sinθ
Where:
By measuring the angles (θ) at which diffraction occurs, we can use Bragg's Law to calculate the distance (d) between the atomic layers, thus determining the crystal structure.
An ideal crystal is a perfect, repeating arrangement. However, real crystals always contain imperfections or defects. These defects are crucial as they control many properties (e.g., conductivity, color, mechanical strength).
Point Defects (Stoichiometric): These are imperfections at a single lattice point. They do *not* change the overall stoichiometry (ratio of ions) of the solid.