11.5
Oberflächenspannung, Kapillarwirkung und Viskosität
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Q1: Why does water form a sphere when it drops?
Water minimizes its surface area by forming a sphere because surface molecules experience unbalanced attractive forces. Interior molecules are pulled equally in all directions by cohesive forces, but surface molecules are attracted only downward. This creates surface tension, causing the liquid to contract into the shape with the lowest surface-area-to-volume ratio, which is a sphere.
Q2: What causes water to climb up the inside of a glass tube?
Water climbs glass tubes through capillary action, driven by adhesive forces between water and glass that exceed cohesive forces within water. Water molecules spread along the tube's surfaces via adhesion, drawing the rest of the liquid upward by cohesion. This continues until gravity balances the adhesive and cohesive forces, with narrower tubes allowing water to rise higher.
Q3: Why does mercury form a different meniscus shape than water in a tube?
Mercury forms a convex meniscus because its cohesive forces between mercury atoms far exceed adhesive forces with glass. In contrast, water forms a concave meniscus because adhesive forces between water and glass are stronger than cohesive forces within water. The relative strength of these intermolecular forces determines whether the liquid wets or beads on a surface.
Q4: Why is glycerol more viscous than methanol?
Glycerol is more viscous than methanol because it has three hydroxyl groups that form more hydrogen bonds per molecule, creating stronger intermolecular attractions. Although both liquids form hydrogen bonds, glycerol's additional bonding sites result in greater resistance to flow. Stronger intermolecular forces make molecules move less freely, increasing viscosity.
Q5: How does temperature affect a liquid's viscosity?
Increased temperature decreases viscosity because thermal energy weakens intermolecular forces, allowing molecules to move more freely. As kinetic energy increases, molecules can overcome the attractive forces holding them together more easily. This enables liquids to flow faster at higher temperatures, reducing their resistance to flow.
Q6: Why do longer hydrocarbon molecules have higher viscosity?
Longer hydrocarbon molecules have higher viscosity because they interact over greater surface areas and entangle more frequently, strengthening dispersion forces between them. These increased intermolecular interactions create greater resistance to flow. Hydrocarbons with increasing molar masses and lengths experience stronger attractions, making them more viscous than shorter chain hydrocarbons.
Q7: How do cohesive and adhesive forces differ in their effects on liquids?
Cohesive forces are attractions between identical molecules within a liquid, while adhesive forces are attractions between different molecules, such as water and glass. Cohesive forces cause liquids to minimize surface area and resist spreading. Adhesive forces determine whether a liquid wets a surface or beads up, depending on their relative strength compared to cohesive forces.