UNIT 4 Solutions and Solubility Chapter 8 Solutions and Concentration Types of Solutions The simplest solutions contain 2 substances: 1. SOLVENT o any substance that has another substance o dissolved in it. The substance present in the largest amount (by volume, mass, or moles) 2. SOLUTE o The substance dissolved in the solvent. When solute dissolves in a solvent, it is a physical interaction and the substances can be separated by physical means (melting or boiling points). Solutions are HOMOGENEOUS mixtures and can be gases, liquids, or solids. Steel Carbon + Iron Kool aid Sugar + Water Air Nitrogen + Oxygen + Carbon Dioxide Most common solutions are liquid The most common of these are AQUEOUS SOLUTIONS where WATER is the SOLVENT. Many substances dissolve easily in any proportion. These substances are said to be MISCIBLE with each other. Ex. Water and Ethanol
Some substances do not dissolve in each other. These are said to be IMMISCIBLE. Ex. Water and Oil SOLUBILITY is a measurement of the amount of solute that dissolves in a given amount of solvent, at a certain temperature. Ex. The solubility of NaCl in H 2 O at 20 o C is 36g per 100mL. A SATURATED SOLUTION is a solution that can not dissolve any more solute. If we try to add more than 36g of NaCl to 100mL of H 2 O, it will not dissolve. If you try to add 40 g... An UNSATURATED SOLUTION is a solution that has less than the maximum amount of solute dissolved in the solvent.
Solutes described as SOLUBLE have a solubility of greater than 1g/100mL Solutes described as INSOLUBLE have a solubility of less than 0.1g/100mL Solutes described as SPARINGLY or SLIGHTLY SOLUBLE have a solubility of between 0.1g 1g / 100mL Factors Affecting Rate of Dissolving The RATE OF DISSOLVING measures how fast a solute dissolves. This rate is dependent on many factors.
1. TEMPERATURE a. Solid solutes dissolve faster at Temp. b. temp means kinetic energy, collisions between molecules of solute and solvent. 2. Agitation a. Stirring or shaking increases the rate of dissolving b. Helps bring undissolved solute in contact with solvent 3. Surface Area a. surface area, rate of dissolving. b. surface area, # of collisions between solute and solvent.
The reason that one substance dissolves in another is because of THE FORCES OF ATTRACTION BETWEEN SOLVENT AND SOLUTE PARTICLES. The intermolecular forces include the attraction between: 1. SOLUTE and SOLUTE particles 2. SOLVENT and SOLVENT particles 3. SOLUTE and SOLVENT particles When the force of attraction between DIFFERENT particles is GREATER THAN the attraction between LIKE particles a solution is formed. The STRENGTH of attraction between particles determines solubility Recall the Thought Lab on pg. 288: Iodine was insoluble in water. This means that the attraction between the non polar iodine and the polar water was weak. Also the force of attraction between water and water (intermolecular forces) are strong. Iodine was soluble in kerosene. The force of attraction between non polar iodine and nonpolar kerosene is stronger than the attraction between iodine and iodine as well as between kerosene and kerosene.
INTERMOLECULAR FORCES In Unit 1 you learned the difference between polar and non polar molecules. Recall water: Water is polar because it appears to other molecules to have 2 oppositely charge ends or poles. Molecules like this are said to have a permanent DIPOLE. A DIPOLE consists of 2 opposite charges separated by a short distance. Remember how water sticks to water? This is a result of the interactions between opposite charges on different polar molecules. These are called DIPOLE DIPOLE ATTRACTIONS.
This is a type of INTERMOLECULAR FORCE. They act between molecules. These forces are about 1% as strong as an ionic or covalent bond (Intramolecular Force) A special type of dipole dipole attraction happens when hydrogen is chemically bonded to O, N, or F. Because these atoms are so electronegative, H gets a positive charge that is extra strong. This leads to HYDROGEN BONDING. When IONIC substances are dissolved in a polar solvent the cation and anions of the ionic compound are attracted to different ends of the polar compound. This is known as ION DIPOLE ATTRACTION. In an aqueous solution each cation and anion will be surrounded by water molecules. Each ion is said to be HYDRATED and hydrated ions can move freely in the solution making it an ELECTROLYTE (sol n with the ability to conduct electricity).
Most ionic compounds are soluble but NOT ALL. To predict solubility compare electronegativity: Large difference Polar/Ionic Bonds Should dissolve Small difference Covalent Bond Should be insoluble When covalent compounds are mixed with water, most are insoluble. This is because many are non polar. Exceptions are alcohols and sugars. Both of these form H bonds with water. However, non polar covalent compounds are soluble in non polar solvents. In general LIKE dissolve LIKE. Ionic and polar covalent solutes dissolve in polar solvents. Some substances behave as both polar and nonpolar substances. Acetic acid, small alcohols, and soap are examples Non polar solutes dissolve in non polar solvents.
Factors Affecting Solubility (Don t these get confused with rate of dissolving) 1. Molecule Size Small molecules are more soluble than large. Think of different alcohols 2. Temperature Solubility varies with temperature When a solid dissolves energy is required to break bonds between particles. At increased Temp, more energy available,solubility increases. This is true for solids. For liquids energy is not required so temp has little effect on solubility. When gases dissolve in liquids they actually lose energy, so Temp adds energy back and solubility. 3. Pressure Changes in pressure have little/no effect on solid and liquid solubility The solubility of a gas is directly proportional to the pressure of the gas above the liquid. For example in a 2L pop bottle the pressure of CO 2 is high (400 kpa). Solubility of CO 2 in the liquid is high. When you open the bottle, the pressure of CO 2 above the liquid drops to 0.03 kpa, solubility and CO 2 gas comes out of solution!
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