The Polarity of the solute and solvent molecules will affect the solubility.
Generally polar solute molecules will dissolve in polar solvents and non-polar solute molecules will dissolve in non-polar solvents.
The polar solute molecules have a positive and a negative end to the molecule. If the solvent molecule is also polar, then positive ends of solvent molecules will attract negative ends of solute molecules.
This is a type of intermolecular force known as dipole-dipole interaction.
Chemistry 30S Unit 4 - Solutions
The Solution Process Chemistry 30S Unit 4 - Solutions
When a solute dissolves in a solvent, the individual particles of the solute separate from the other particles of the solute and move between the spaces of the solvent particles. The solvent particles collide with the solute particles and forces of attraction between solute and solvent particles "hold" the solute particles in the spaces.
For a solute to be dissolved in a solvent, the attractive forces between the solute and solvent molecules must be greater than the forces of attraction between the solute molecules.
The solvent particles must move apart to make room for solute particles. This process requires energy to overcome forces of attraction between solvent particles. The first step in the dissolving process is endothermic.
The solute particles must separate form the other solute particles. This process also requires energy to overcome the forces of attraction between the solute particles. The second step in the dissolving process is endothermic.
When the solute particles move between the solvent particles the forces of attraction between solute and solvent take hold and the particles "snap" back and move closer. This process releases energy. The final step in the dissolving process is exothermic.
The total heat change in the dissolving process is the sum of the three heat changes. If the sum of the heat absorbed in the first two steps of the dissolving process is greater than the heat released in the last step, the dissolving of that substance will be endothermic . If the dissolving process for a substance is endothermic, the container will feel cooler as the substance dissolves.
In some cases, the exothermic process is larger than the sum of the two endothermic processes. In these cases, as the solid dissolves large amounts of heat are evolved.
The reason that the solubility of solids is so variable is that much of the energy for solvation is required to separate the solid particles from each other in either the crystal lattice or from the molecular solid structure.
This energy absorption is not required for a gas because all the particles are already separated.
Consequently, the overall net process becomes exothermic with the result that solubility is inversely proportional to temperature. The higher kinetic energy of gas particles allows them to escape from a solution more readily. As a result, the solubility of gases decreases with an increase in temperature.
Increasing pressure increases the solubility of a gas in a liquid because increasing the pressure forces the gas particles into contact with the liquid. As the gas particles contact the liquid, forces of attraction from the liquid cause the gas to condense and dissolve.
Since the definition of boiling is the temperature at which the vapour pressure equals the pressure above the liquid, it can be readily seen that if the vapour pressure is lowered, it will require additional energy to raise the temperature to where the vapour pressure equals that of the pressure above the solution. Hence the boiling point elevation .
In order for a liquid to freeze, it must achieve a very ordered state that results in the formation of a crystal. If there are impurities in the liquid, i.e. solute, then the liquid in inherently less ordered. Therefore a solution is more difficult to freeze than the pure solvent and a lower temperature is required to freeze the solution.
Another way to explain this is to say, as a solution is cooled, solvent molecules lose average kinetic energy to enable them to settle into the crystal structure of the pure solvent. As the crystal grows, solute molecules interfere with the growth of the solvent crystals. To compensate, more kinetic energy must be taken from the solution thus depressing the freezing point.
It is important for chemists to know how much solute is in a certain volume of solution. For example, chemists provide the data for safe concentrations of various substances in drinking water. When chemists speak about concentration they are referring to the amount of solute per unit of solution, unlike solubility that is the amount of solute per unit of solvent.
The units of parts per million and billion are often used in describing concentrations of solutes in water and air quality. The percent solute is often used when describing alcoholic beverages (for example, 5% alcohol by volume). The other two units are often used in the medical field to describe the quantities of solutes, like cholesterol, in the blood.
The units most commonly used for concentration in chemistry is moles of solute in one Litre of solution, mol/L. This is also known as molar concentration, or molarity , M . The M for molarity should not be con fused with the M for molar mass.
The equation indicates that when aluminum chloride dissolves it dissociates into one aluminum ion and 3 chloride ions in solution. We can calculate the concentration of each kind of ion by using the stoichiometry of this equation.
Chemists will often store solutions in a more concentrated form to save on storage space and the ease of making new solutions. These types of solutions are called stock solutions. Virtually an infinite number of solutions of varying concentrations can be made from a single stock solution, simply by adding more solvent. To dilute a solution means to add more solvent without adding more solute. Since the number of moles of solute remains the same (since no more solute is added), calculations of new concentrations becomes easier.
The concentration calculations that we have done essentially involved preparing a solution from scratch. We started with separate solvent and solute and figured out how much of each you would need to use.
Quite often, however, solutions are prepared by diluting a more concentrated solution. For example, if you needed a one molar solution you could start with a six molar solution and dilute it.