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Vapour pressure of liquids

الكلية كلية الهندسة     القسم  الهندسة البيئية     المرحلة 3
أستاذ المادة عدي عدنان جهاد الخيكاني       05/10/2012 11:38:29
Vapour pressure of liquids
A liquid in a closed container is subjected to partial vapour pressure due to the escaping molecules from the surface; it reaches a stage of equilibrium when this pressure reaches saturated vapour pressure. Since this depends upon molecular activity, which is a function of temperature, the vapour pressure of a fluid also depends upon its temperature and increases with it. If the pressure above a liquid reaches the vapour pressure of the liquid, boiling occurs; for example if the pressure is reduced sufficiently boiling may occur at room temperature. The saturated vapour pressure for water at 20c is 2.45*10^3 N/m2.



SURFACE TENSION
Surface tension arises from the forces between the molecules of a liquid and the forces (generally of a different magnitude) between the liquid molecules and those of any adjacent substance. The symbol for surface tension is ? and it has the dimensions [MT?2].
Water in contact with air has a surface tension of about 0.073 N•m?1 at usual ambient temperatures; most organic liquids have values between 0.020 and 0.030 N•m?1 and mercury about 0.48 N•m?1, the liquid in each case being in contact with air. For all liquids the surface tension decreases as the temperature rises. The surface tension of water may be considerably reduced by the addition of small quantities of organic solutes such as soapand detergents. Salts such as sodium chloride in solution raise the surface tension of water. That tension which exists in the surface separating two immiscible liquids is usually known as interfacial tension. As a consequence of surface tension effects a drop of liquid, free from all other forces, takes on a spherical form. The molecules of a liquid are bound to one another by forces of molecular attraction, and it is these forces that give rise to cohesion, that is, the tendency of the liquid to remain as one assemblage of particles rather than to behave as a gas and fill the entire space within which it is confined. Forces between the molecules of a fluid and the molecules of a solid boundary surface give rise to adhesion between the fluid and the boundary.
If the forces of adhesion between the molecules of a particular liquid and a particular solid are greater than the forces of cohesion among the liquid molecules themselves, the liquid molecules tend to crowd towards the solid surface, and the area of contact between liquid and solid tends to increase. Given the opportunity, the liquid then spreads over the solid surface and ‘wets’ it. Water will wet clean glass, but mercury will not. Water, however, will not wet wax or a greasy surface The interplay of these various forces explains the capillary rise or depression that occurs when a free liquid surface meets a solid boundary. Unless the attraction between molecules of the liquid exactly equals that between molecules of the liquid and molecules of the solid, the surface near the boundary becomes curved. Now if the surface of a liquid is curved the surface tension forces have a resultant towards the concave side. For equilibrium
this resultant must be balanced by a greater pressure at the concave
side of the surface. It may readily be shown that if the surface has radii of
curvature R1 and R2 in two perpendicular planes the pressure at the concave side of the surface is greater than that at the convex side by


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .