Why Substances Are Liquid
- The state of a substance can be explained in terms of the forces that exist between its atoms or molecules. These forces are strongest in solids, where particles are held in a rigid and often ordered configuration. In the gaseous state, these intermolecular forces are very weak in some cases negligible, which is why gases are so disordered. Liquids exists somewhere between the solid and gas states. The intermolecular forces are strong enough to prevent the free movement of particles, but not strong enough to hold these particles in firm position. Liquids will spread out to form the most energetically favorable state; however, unlike gases, liquids are affected by gravity. This is is why water stays in the bottom of a drinking glass, for example.
The Cause of Surface Tension
- The intermolecular forces acting in a liquid pull its particles together and these forces result in a liquid contracting to have a minimum surface area. These intermolecular forces act in all directions, except at the boundary of the liquid. Here, the forces act towards the center of the liquid to create a film on the surface of the liquid. This is why drops of water will have approximately spherical dimensions, as spheres have the smallest surface area for a given volume of matter.
Capillary Rise Method
- Surface tension can be calculated by using the capillary rise method. Inserting a glass tube or capillary into a liquid causes the fluid to travel up that liquid. The higher the liquid rises; the greater the surface tension. Surface tension is also proportional to the density of the liquid and the height to which the liquid rises depends on the diameter of the capillary. However, both of these quantities are easily measured. Density is the mass per unit volume so it is very straightforward to measure out and then weigh an exact volume of liquid. Similarly, the capillary diameter can be accurately measured using a microscope. To simplify calculation, the capillary must be inserted into liquid at right angle to its surface.
- Jaeger's method is an alternative means of finding surface tension and works by measuring the excess pressure needed to blow an air bubble into the liquid being tested. Surface tension is calculated by measuring the density of the liquid and the pressure at which the bubble bursts.