JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II Year PHYSICAL PHARMACEUTICS-I Previous Question Paper (March-2021 - R17)

Latent heat is the quantity of heat absorbed or released by a substance during a change of state (phase transition) without changing its temperature. It is categorized into latent heat of fusion (solid to liquid) and latent heat of vaporization (liquid to gas). The energy is used to overcome intermolecular forces rather than increasing kinetic energy.

Vapour pressure is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phase (solid or liquid) at a given temperature in a closed system. As temperature increases, the kinetic energy of molecules increases, leading to a higher rate of evaporation and consequently higher vapour pressure.

Eutectic mixtures are solid mixtures of two or more compounds that solidify at a lower temperature than any of the individual components. The eutectic point represents the specific composition and temperature where the mixture melts or freezes as a single substance, behaving like a pure compound. This is significant in pharmacy for preparing solid dispersions to enhance the solubility of poorly water-soluble drugs.

Relative humidity is the ratio of the partial pressure of water vapour in a gas mixture to the saturated vapour pressure of water at the same temperature, expressed as a percentage. It indicates the current state of atmospheric moisture relative to the maximum amount the air can hold at that temperature. It is a critical parameter in pharmaceutical stability testing and the storage of hygroscopic dosage forms.

The refractive index (n) is a fundamental physical property defined as the ratio of the velocity of light in a vacuum to the velocity of light in a given medium. It is a measure of how much a substance bends light as it passes from one medium to another, governed by Snell's Law, which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media.

The determination of refractive index is typically performed using an Abbe Refractometer. The principle relies on the phenomenon of critical angle refraction. When light passes from a medium of higher refractive index (the prism) to a medium of lower refractive index (the sample), the angle of refraction increases until it reaches 90 degrees, known as the critical angle. Beyond this, total internal reflection occurs.

The method of determination involves the following steps:

1. Clean the prism surfaces thoroughly with ethanol or a suitable solvent and allow them to dry.
2. Place a few drops of the liquid sample on the lower prism and close the upper prism to create a thin, uniform film.
3. Adjust the light source and the mirror to illuminate the prism assembly evenly.
4. Rotate the prism adjustment knob while looking through the eyepiece until the interface between the light and dark fields (the shadow line) aligns exactly with the center of the crosshairs.
5. Read the refractive index value directly from the scale provided in the instrument.
6. Ensure the temperature is controlled, as refractive index is highly temperature-dependent, and use a circulating water bath if necessary to maintain a constant temperature (usually 25°C).

The refractive index is a valuable parameter in pharmaceutical analysis for identifying pure substances, checking the purity of oils and fats, and determining the concentration of solutions.