Wired Chemist

The Melting Of A Solid

Here we see a solid spread on a surface warm enough to cause it to melt. As it begins to melt the outer edges begin to soften and round. Eventually, the entire solid turns into a liquid. The melting range extends from the temperature at which the edges begin to soften to the temperature at which the entire mass becomes liquid. For pure solids, this temperature range is generally one degree or less. The more impure the solid, the greater the melting range.

The Dependence Of Viscosity On Intermolecular Forces

Two liquids are added dropwise to the inside of an inclined tube. A small amount of a dye has been added to the liquids to make them easier to see. The rate of descent of the liquid is inversely proportional to its viscosity. The viscosity, in turn, is a function of the strength of the intermolecular forces in the liquid. The first liquid is hexane, which has only van der Waals forces, and, therefore, flows rapidly. The second liquid, glycerol, has three hydroxyl groups per molecule and therefore has a large amount of hydrogen-bonding. Consequently, the glycerol has very strong intermolecular forces, a very high viscosity, and flows very slowly.

The Change In Energy During Evaporation

The thermocouple is surrounded by a ball of cotton. The temperature of the surroundings is first measured and then acetone is poured onto the cotton ball. As the acetone evaporates it removes heat from its surroundings and the temperature of the cotton ball decreases.