Solutions

Solutions What is a Solution? (5-1)

What is a solution? Definition: When a substance dissolves, it goes into solution. It seems to disappear. Definition: A solution is a mixture in which the molecules or ions of one substance mix evenly with the particles of another substance. Example: When salt is mixed with water, it dissolves and seems to disappear. Actually, the sodium and chlorine ions mix evenly with the H2O molecules.

What is a solution? There are three different types of solutions… Liquid solution Formed when solids, liquids, or gases dissolve in liquids Gaseous solution Formed when solids, liquids, or gases dissolve in gases Solid solution Formed when solids, liquids, or gases dissolve in solids

What is a solution? TYPES OF SOLUTIONS Substance Dissolved in Examples Liquid Antifreeze in water Gas Water droplets in air (fog & clouds) Solid Sulfur in rubber CO2 in water (club soda) O2, Argon, and other gases in N2 (air) Hydrogen in palladium Salt in water (the oceans) Iodine mist in air Zinc in copper (brass)

What is a Solution? Which of the following mixtures are solutions? Sugar & water Sea water Sand & water Soda Salt & pepper Air Brass Mud Cereal & milk

What are the parts of a solution? (5-2) Solutions What are the parts of a solution? (5-2)

What are the parts of a solution? All solutions are made when one substance dissolves in another substance. Thus there are two key ingredients to every solution: Definition: The solute is the substance in the solution that dissolves. Definition: The solvent is the substance in the solution in which a solute dissolves.

What are the parts of a solution? Identify the solute and the solvent for each of the following solutions. Solution Solute Solvent Soda Ocean water Fog

What are the parts of a solution? Definition: Some substances are more easily dissolved than other substances, thus they are more soluble. Definition: The solubility of a substance is the maximum mass of that substance that will dissolve in 100 grams of solvent at a given temperature. Example: Approx. 33 g of NaCl will dissolve in 100 g of H2O at 20oC. If you add any more NaCl to the solution, it won’t dissolve.

What are the parts of a solution? The following graph shows the solubility of a number of different solutes when water is the solvent. Note how the solubility of each solute changes as a function of the solvent’s temperature (the temperature of the water in this case).

What are the parts of a solution? Note that for substances like sodium chloride (NaCl) and potassium nitrate (KNO3), that more solute can be dissolved as the temperature of the solvent (H2O in this case) increases. Thus their solubility increases! For other substances like ammonia (NH3), more solute can be dissolved as the temperature of the solvent (again, H2O in this case) decreases. Thus their solubility decreases!

What are the parts of a solution? Definition: A substance that is not able to dissolve in a given solvent (e.g. water), is said to be insoluble in that solvent. Some solutes don’t dissolve as well as others, therefore they change the color of the solvent: for example, instant coffee in hot water.

Why is water a good solvent? (5-3) Solutions Why is water a good solvent? (5-3)

Why is water a good solvent? Water is sometimes referred to as the “universal solvent.” Many substances dissolve in water! Water is a particularly good solvent b/c its molecules are polar, meaning one side has a positive electric charge, and the other a negative electric charge.

Why is water a good solvent? Water is called a polar solvent. Polar solvents will dissolve ionic compounds, and compounds made up of other polar molecules. For example: Oils and fats are composed of nonpolar molecules, therefore do not dissolve in water.

How can you change the rate at which substances dissolve? (5-4) Solutions How can you change the rate at which substances dissolve? (5-4)

Rate at which substances dissolve There are several factors that affect the rate at which substances dissolve. Stirring Stirring & blending helps mix the solute evenly more quickly Temperature Solid solutes dissolve faster in liquid solvents as the temperature increases. Gaseous solutes dissolve faster in liquid solvents as the temperature decreases.

Rate at which substances dissolve There are several factors that affect the rate at which substances dissolve (cont’d). Surface area The smaller the size of the solute particle, the faster it dissolves. Crushing, grinding, chopping, will all reduce the solute particle size. Pressure The higher the pressure, the more gas solute can dissolve. Pressure has little to no effect on the rate at which solid or liquid solutes can dissolve.

Rate at which substances dissolve There are several factors that affect the rate at which substances dissolve (cont’d). Type of solvent Some solvents dissolve certain solutes faster than others. Polar solvents dissolve solutes with polar molecules and ionic compounds best. Nonpolar solvents dissolve solvents with nonpolar molecules best. In other words… “like dissolves like!”

How do solutes affect the freezing & boiling points? (5-6 & 5-7) Solutions How do solutes affect the freezing & boiling points? (5-6 & 5-7)

Freezing & Boiling Points *Recall from earlier in the course... as heat energy is added to or removed from a substance, it can undergo a change of phase. A physical change brought on by a change in the arrangement of the molecules. See the phase-change diagram on the next slide. Note that the temperature doesn’t change while the substance changes phase.

Freezing & Boiling Points Freezing Point

Freezing & Boiling Points Freezing Point – The temperature at which a liquid changes to a solid (or vice versa). Freezing Point Depression – Adding solute to a solvent lowers the freezing point of that solvent. The more solute, the lower the freezing point.

Freezing & Boiling Points Ice Melting Solutes Compound Freezing Pt. Advantages/Disad. Rock Salt (NaCl) -9oC Abundunt & inexpensive BUT pollutes streams, lakes, & corrodes metal Potassium Chloride (KCl) -7oC Less toxic than rock salt but too much can burn grass & plants Calcium magnesium acetate (CMA) Less toxic to plants than other salts & less corrosive to automobiles Magnesium Cloride (MgCl2) -15oC Less toxic to plants & less damage ot concrete than rock salts Calcium Chloride (CaCl2) -32oC Abundant & inexpensive BUT damages concrete, causes skin irritation

Freezing & Boiling Points Boiling Point – The temperature at which a liquid changes to a gas (or vice versa). Boiling Point Elevation– Adding solute to a solvent raises the boiling point of that solvent. The more solute, the higher the boiling point. Example: Automobile antifreeze causes BOTH freezing point depression AND boiling point elevation!

Freezing & Boiling Points Boiling Points of Different Solutions Substance Boiling Pt. Distilled water 100oC Sea water 103oC Vinegar (acetic acid) 118oC Ethyl alcohol 78.5oC Acetone 56oC