Chemistry

### DEFINITION

•color{green} ("Surface chemistry") : Surface chemistry is that branch of chemistry which deals with the study of the bulk phenomena occurring at the surface or interface,i.e., at the boundary separating two bulk phenomena.

•color{green} ("Adsorption") :The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.

•color{green} ("Adsorbate") : The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate.

•color{green} ("Adsorbent"): The material on the surface of which the adsorption takes place is called adsorbent.

•color{green} ("Desorption") :The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.

•color{green} ("Sorption") :Both adsorption and absorption can take place simultaneously also. The term sorption is used to describe both the processes.

•color{green} ("Physisorption") :If accumulation of gas on the surface of a solid occurs on account of weak van der Waals’ forces, the adsorption is termed as physical adsorption or physisorption.

•color{green} ("Chemisorption") : When the gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption.

•color{green} ("Adsorption isotherm") : The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.

•color{green} ("Lyophilic sols") : Substances like gum,gelatine,rubber etc. which when mixed with a suitable liquid as the dispersion medium directly form the colloidal sol are called lyophilic and the sols thus obtained are called lyophilic sols.

•color{green} ("Adsorption isotherm") : Substances like metals, their sulphides etc. when simply mixed with a the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared only by special methods. Such substances are called lyophobic and the sols formed are called lyophobic sols.

•color{green} ("Catalysis") : Substances, which alter the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction, are known as catalysts, and the phenomenon is known as catalysis.

•color{green} ("Promoters and poisons") : Promoters are substances that enhance the activity of a catalyst while poisons decrease the activity of a catalyst.

•color{green} ("Homogeneous catalysis") : When the reactants and the catalyst are in the same phase (i.e., liquid or gas), the process is said to be homogeneous catalysis.

•color{green} ("Heterogeneous catalysis") :The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.

•color{green} ("Shape selective catalyst") :The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis.

•color{green} ("Optimum temperature") :The rate of an enzyme reaction becomes maximum at a definite temperature, called the optimum temperature.

•color{green} ("Optimum pH") : The rate of an enzyme-catalysed reaction is maximum at a particular pH called optimum pH, which is between pH values 5-7.

•color{green} ("Colloid") : A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium.

•color{green} ("Multimolecular Colloid") :On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size in the colloidal range (diameter < 1 nm). The species thus formed are called multimolecular colloids.

•color{green} ("Macromolecular Colloid") :Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects.

•color{green} ("Associated Colloid") :There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids.

•color{green} ("CMC") :The formation of micelles takes place only above a particular temperature called Kraft temperature (T_k) and above a particular concentration called critical micelle concentration (CMC).

•color{green} ("Soap") :It is sodium or potassium salt of a higher fatty acid and may be represented as RCOO^(-)Na^+ (e.g., sodium stearate CH_3(CH_2)_(16)COO^(–) Na^+, which is a major component of many bar soaps).

•color{green} ("Purification of colloidal solution") :The process of reducing the impurities of the electrolytes to the minimum required level is known as purification of the colloidal solution.

•color{green} ("Dialysis") :It is the process of separating the particles of colloids from those of crystalloids by diffusion of the mixture through a parchment or an animal membrane is called dialysis.

•color{green} ("Ultrafiltration") : Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by specially prepared filters, which are permeable to all substances except the colloidal particles.

•color{green} ("Tyndall effect") :Tyndall effect may be defined as the scattering of light by colloidal particles present in a colloidal solution.

•color{green} ("Brownian movement") :Brownian movement may be described as continuous zig-zag movement of the colloidal particles in a colloidal sol.

•color{green} ("Coagulating value") : The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of a sol in two hours is called coagulating value.

•color{green} ("Protective colloids") :When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around lyophobic particles and thus protect the latter from electrolytes. Lyophilic colloids used for this purpose are called protective colloids.

•color{green} ("Emulsion") :An emulsion is a colloidal dispersion in which both the dispersed phase and dispersion medium are liquids.