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Physical Pharmacy SURFACE TENSION

Physical Pharmacy SURFACE TENSION. By Abdul- Wahhab Khedr Demonstrator of Pharmaceutics and Industrial Pharmacy. Surface tension of liquids. Why the free drops of water form spherical droplets?. Surface Tension:.

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Physical Pharmacy SURFACE TENSION

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  1. Physical PharmacySURFACE TENSION By Abdul-WahhabKhedr Demonstrator of Pharmaceutics and Industrial Pharmacy

  2. Surface tension of liquids • Why the free drops of water form spherical droplets?

  3. Surface Tension: • Equals the work (W) or energy necessary to increase the surface area (A) by 1 cm2 • Units: • Is defined as “ The force in (dynes) acting along the surface of a liquid at right angle to any line 1 cm in length”.

  4. Experimental Determination of Surface Tension • Drop weight and Drop number method: (Using Stalagmometer) • In this method the surface tension of a liquid (γ1) can be determined in relation to another liquid of known surface tension (γ2). a b

  5. Experiment • By using Stalagmometer, determine the relative surface tension of the given liquids with reference to distilled water (S.T=72.8 dyne/cm at 20⁰C) • Procedure: • Wash the stalagmometer several times with water before use. • Introduce the flat tip into small beaker half-filled with water, suck water till its level becomes above the upper mark (avoid entrance of air bubbles). • Adjust the level of water to the upper mark and remove any suspended drop. • Count the number of drops falling until the level of water reaches the lower mark (n1). • Repeat the procedure twice again and determine (n2) & (n3). • Repeat the procedure for the liquid to be measured after washing the apparatus.

  6. Results

  7. Comment • The drop of a mass (m) gets released when its weight G=mg is equal or greater than the surface force at the end of tube

  8. Comment • There are two line marks on the stalagmometer: top line above the wide part and bottom line bellow it. The volume between these two lines is (V), and liquid with density (ρ) contained in this volume has a mass (M) • Such a volume V corresponds to n drops, which are released from the stalgmometer upon the decrease of liquid level from top to bottom line mark. • Here, the average mass (m) of one drop is

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