Wired Chemist

Here blue solvent molecules are shown attracting the purple solute molecules at the surface and drawing them into solution. During this process, the solute-solute attraction and solvent-solvent attractions are overcome by the solute-solvent attraction.

The first part of this animation is the same as that of the previous animation and shows the solute being pulled into solution by the solvent. This animation shows the remainder of the equilibrium process that occurs in a saturated solution. After solute molecules enter the solution, some are attracted back into the solute lattice. At the same time other solute molecules are entering the solution. This process continues and at some point equilibrium is reached. At this point the concentration of the solute in solution remains constant, BUT, solute molecules are constantly moving back and forth between the solid and solution phases.

The process of preparing a saturated solution is shown here. As the first several portions of a solid are added to the solvent they enter the solution (they seem to dissapear) easily and no solid remains in the flask. As subsequent portions are added, they enter the solution with more difficulty; that is, it takes longer for them to dissolve. Finally, only a small amount of the last portion dissolves and solid remains, which bring the total weight of solute added to 7.61 grams. In order to just achieve saturation in this solution, more than 7.32, but less than 7.61 grams of the solute are required. While the solid is being added the solution must be stirred because the rate of dissolution is greatly affected by stirring.

At the beginning of the animation we see the hydrogen bonding between the water molecules. Sodium chloride is then added and falls to the bottom of the beaker as a solid. Sodium ions and chloride ions are then attracted to the water molecules through the charge -dipole interaction. This interaction just compensates for the decrease in charge-charge interactions and hydrogen-bonding in solution. The enthalpy of the system therefore remains relatively constant during the dissolution process.

Like the previous animation, this portrays the hydrogen-bonding between water molecules. When solid iodine is added, a small number of iodine molecules enter solution and break up some of the hydrogen-bonding. In solution the forces between the iodine molecules is also not as strong as it is in the solid state. Consequently, energy is required to break the solute-solute interaction and the hydrogen-bonding interaction, and the enthalpy change for the dissolution process is positive.

The beaker on the right contains a solution of sugar, the beaker on the left contains pure water. When the beakers are covered, the levels of the solution are the same. However, as time goes on, more molecules escape from the beaker containing the water because the vapor pressure of the water is greater. Some water molecules also escape from the sugar solution, but overall the volume of the sugar solution increases and the volume of the water decreases because of a net transfer of water molecules to the sugar solution.