When water is poured on clean glass, it tends to spread, forming a thin, uniform film over the glasses surface. This is because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass, thus avoiding the repulsion between like molecules.
When a liquid is placed on a smooth surface, the relative strengths of the cohesive and adhesive forces acting on that liquid determine the shape it will take and whether or not it will wet the surface.
If the adhesive forces between a liquid and a surface are stronger, they will pull the liquid down, causing it to wet the surface. However, if the cohesive forces among the liquid itself are stronger, they will resist such adhesion and cause the liquid to retain a spherical shape and bead the surface. The meniscus is the curvature of a liquid's surface within a container, such as a graduated cylinder.
However, before we explain why some liquid have a concave up meniscus while others share a concave down meniscus, we have to understand the adhesive forces at work of surface tension.
Water, for example, is a polar molecule that consists of a partial positive charge on the hydrogens and a partial negative charge on the oxygen.
Thus, within liquid water, each molecule's partial positive charge is attracted to its neighbor's partial negative charge. This is the origin of the cohesive forces within the water. Water molecules buried inside the liquid is then being pulled and pushed evenly in every direction, producing no net pull. Meanwhile, the molecules on the surface of the liquid, lacking pulling forces in the upward direction, thus encompass a net downward pull.
How does this cohesive force create both a concave up and concave down surface then? The answer is in its relationship to the adhesive forces between the water molecules and the container's surface. When the cohesive force of the liquid is stronger than the adhesive force of the liquid to the wall, the liquid concaves down in order to reduce contact with the surface of the wall. When the adhesive force of the liquid to the wall is stronger than the cohesive force of the liquid, the liquid is more attracted to the wall than its neighbors, causing the upward concavity.
In agitated glasses of wine, droplets of wine seemingly "float" above the meniscus of the liquid and form "tears. Alcohol is more volatile than water. As a result, "evaporation of alcohol produces a surface tension gradient driving a thin film up along the side of a wine glass" Adamson.
They'll travel up the glass as far as water's cohesive forces will allow them, until gravity prevents them from going further. Cohesion is an intermolecular attraction between like molecules other water molecules in this case. Few people take the time to consider the importance of water menisci in their lives. But, imagine this chilling scenario:. In your high-school chemistry final exam you mistakenly read a meniscus as Your GPA falls from 4.
Consequently, you don't get that prestigious engineering job, where, 20 years later, you would have invented a new water-based chemical to allow rubber to grip better. Sadly, 10 years later, a mother and her adorable 4-year old daughter are leaving the ice cream store and the little girl, whose shoes don't have your un-invented coating, slips on a napkin and drops her ice cream cone.
She cries at her loss The moral of this fictional tale is that it is important to read the measurement correctly, and yes, in the picture above, the true volume in the graduated cylinder is at the bottom of the water level— As this picture shows, a meniscus can go up or down.
It all depends on if the molecules of the liquid are more attracted to the outside material or to themselves. A concave meniscus, which is what you normally will see, occurs when the molecules of the liquid are attracted to those of the container. This occurs with water and a glass tube. A convex meniscus occurs when the molecules have a stronger attraction to each other than to the container, as with mercury and glass.
A flat meniscus occurs with water in some types of plastic tubes; tubes made out of material that water does not stick to. In any case, you get the true volume of the liquid by reading the center of the liquid in the tube, which will the lowest vertical point of the liquid. Looking at water, you might think that it's the most simple thing around. Pure water is practically colorless, odorless, and tasteless. But it's not at all simple and plain and it is vital for all life on Earth.
Where there is water there is life, and where water is scarce, life has to struggle or just "throw in the towel. Surface tension in water might be good at performing tricks, such as being able to float a paper clip on its surface, but surface tension performs many more duties that are vitally important to the environment and people. Find out all about surface tension and water here.
It occurs when the intermolecular attractive forces between the liquid and the solid surrounding surfaces adhesive forces are stronger than the cohesive forces within the liquid. If the diameter of the tube is sufficiently small, then the combination of surface tension which is caused by cohesion within the liquid and adhesive forces between the liquid and container act together to lift the liquid. The height h of a liquid column is given by:.
Notice that the height to which the liquid is lifted is inversely proportional to the radius of the tube, which explains why the phenomenon is more pronounced for smaller tubes. Consider a water-filled glass tube with a radius of 2 cm 0. For these values, the height of the water column is 0.
By comparison, for a 4 m diameter glass tube in the lab conditions given above radius 2 m, or 6. For a 0. A common apparatus used to demonstrate capillary action is the capillary tube. When the lower end of a vertical glass tube is placed in a liquid, a concave meniscus forms. Adhesion forces between the fluid and the solid inner wall pull the liquid column up until there is a sufficient mass of liquid for gravitational forces to counteract these forces. The meniscus is the curve caused by surface tension in the upper surface of a liquid.
It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other cohesion than to the material of the container adhesion , causing the surface of the liquid to cave downward.
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