Solutions, Acids, and Bases Section 2 Dissolving and Solubility PHYSICAL SCIENCE Solutions, Acids, and Bases Section 2 Dissolving and Solubility

Dissolving and Solubility Objectives Describe how a substance dissolves in terms of its solubility, molecular motion, and solute-solvent interactions. Identify several factors that affect the rate at which a substance dissolves. Relate the structure of water to its ability to dissolve many different substances. Distinguish between saturated, unsaturated, and supersaturated solutions.

The Process of Dissolving Solutions with a larger surface area dissolve faster. A given volume of solute will dissolve much faster if it is in smaller pieces as opposed to larger pieces When a solid is whole, most molecules are buried within it The broken (or crushed) solid has more surface area which leads to more solute-solvent collisions – hence it dissolves quicker

Surface Area & Solubility

The Process of Dissolving Stirring or shaking a solution helps the solute dissolve faster. A solute can diffuse (spread out) in a solution Stirring or shaking the solution moves the dissolved solute away from the portion of the solute that has not come into contact with the solvent

The Process of Dissolving Solutes dissolve faster when the solvent is hot. Solutes dissolve faster in hot water than they do in cold water If a substance is heated, its particles move faster, creating the potential for more collisions between particles The collisions between particles also transfer energy

Heat and Stirring Heating and stirring a solution can increase the solubility of solutes in solvents. Instruments can be purchased that do both in the lab to allow for solutions to be prepared.

The Process of Dissolving Not every substance dissolves. Substances that dissolve in a solvent are said to be soluble in that solvent Substances that do not dissolve in a particular solvent are said to be insoluble in that solvent It is also possible for substances to be only partly soluble in a solvent

Water: A Common Solvent Water is sometimes called the universal solvent because so many substances can dissolve in water. Water molecules are polar because the oxygen atom attracts the shared electrons more strongly than the hydrogen atoms in a water molecule. The result is a polarized molecule in which the oxygen atom has a partial negative charge and the hydrogen atoms have a partial positive charge.

Water: A Common Solvent The lower case Greek letter delta () is used to show that a charge is partial. The polarity of water molecules is the key to hydrogen bonding which influences many of the characteristics that we observe in water.

Water: A Common Solvent Substances made of molecules that dissolve in water also have partial charges that interact with water molecules. Like dissolves like means that non-polar solutes can be dissolved in non-polar solvents and polar solutes can be dissolved in polar solvents.

Concentration Concentration is the quantity of solute dissolved in a given quantity of solution. If a small quantity of solute is dissolved in a large volume of solvent, the resulting solution is said to be dilute A concentrated solution has a large quantity of dissolved solute An unsaturated solution is a solution that is able to dissolve more solute.

Concentration A saturated solution is a solution that cannot dissolve any more solute at the given conditions. A saturated solution is in equilibrium A saturated solution contains the greatest quantity of solute that will dissolve in a given quantity of solvent at a particular temperature Solubility is the greatest quantity of a solute that will dissolve in a given quantity of solvent to produce a saturated solution.

Concentration Heating a saturated solution allows you to dissolve even more solute. The solubility of many solutes increases as the temperature of the solution increases A supersaturated solution is a solution holding more dissolved solute than is specified by its solubility at a given temperature A supersaturated solution is created by heating a solution and adding more solute than specified by its solubility at a lower temperature, then dropping the temperature of the solution

Concentration Heating a saturated solution allows you to dissolve even more solute. Cont’d Adding a small crystal of solute to a supersaturated solution usually results in the excess solute crystallizing out of the solution until the solution is once again saturated at the lower temperature

Concentration Measuring concentration precisely can be done a couple of ways. One can express a solution as grams of solute per 100 g of solvent Concentration can also be expressed as a mass percent, or grams of solute per 100 g of solution Concentration may be expressed in units of molarity Molarity = moles of solute/liters of solution or M=mol/L For example, a 1.0 M (one molar) solution of sodium chloride contains 1.0 mol of dissolved NaCl for every 1.0 L or solution