UNDERSTANDING SOLUTIONS PART 1 SOLUTIONS & SUSPENSIONS • A suspension is a mixture in which particles: – Can be seen with the eye – Settle to the bottom – Can be filtered out Example: pepper in water. • A solution has the same properties throughout. Its particles… – Can not be seen – Will not settle to the bottom – Can not be filtered out. Example: Salt in water SOLVENTS & SOLUTES • SOLVENT- The part of the solution present in the largest amount. The substance that dissolves the other substance. • SOLUTE- The part of the solution present in the smaller amount. The substance that is dissolved. Question: In salt water which is the solute? WATER AS A SOLVENT • Water is the solvent in many common solutions. • Life depends on water solutions: – Sap in trees – Nutrients in rainwater through roots – Saliva, Tears & even Blood! • Water has a simple molecular structure. It is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. • Water is a "polar" molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge (- ) near the oxygen atom due to the unshared pairs of electrons, and partial positive charges (+ ) near the hydrogen atoms. SOLUTIONS WITHOUT WATER • Many solutions are made without water: – Gasoline – Acetone (Nail Polish) – Hexane (Spot Remover) – Ethanol (Perfume) PARTICLES IN A SOLUTION • Whenever a solution forms, particles of the solute leave each other and become surrounded by particles of the solvent. IONIC SOLIDS IN WATER… • The positive and negative ions are attracted to polar water molecules. Water molecules surround each ion as it leaves the surface of the crystal. As each layer of the solid is exposed, more ions can dissolve. • BREAK DOWN INTO INDIVIDUAL IONS MOLECULAR SOLIDS IN WATER… • Molecular solids– don’t break into ions in water. – Break up into individual neutral molecules. (like ions they are completely surrounded by the water) Example: Sugar in water, •BREAK DOWN INTO MOLECULES! SOLUTIONS & CONDUCTIVITY • A solution of ionic compounds in water will conduct electricity. • A solution of molecular compounds in water may not. NiCl2(aq) + KMnO4(aq) 7 of 19 COLLOIDS • A colloid is a mixture with small undissolved particles that do not settle out. • It’s NOT a suspension or a solution! Examples: Jello, Mayonnaise, Fog, Shaving Cream, Whipped Cream. A colloid contains particles large enough to scatter a light beam. EFFECTS OF SOLUTES ON SOLUTIONS • Solutes lower the freezing point of a solvent. – Example: Salt melts ice because the salt molecule get in the way of ice crystals forming • Solutes raise the boiling point of solvent. – Example: antifreeze keeps cars from overheating. CHECK FOR UNDERSTANDING 1 1. List 3 ways to tell the difference between a solution & a suspension. 2. Describe what happens to the molecules of a solid, such as a sugar cube, when it dissolves in water. How does the process differ for an ionic compound, such as table salt? 3. Why does salt sprinkled on icy roads cause the ice to melt? 4. Why does ocean water freeze at a lower temperature than fresh water? 1. The particles of a solution will not settle out, are not visible to the eye and will pass through a filter. The particles of a suspension will settle out, are visible to the eye, and will not pass through a filter. 2. The molecules of sugar separate from each other and become surrounded by water molecules, but the molecules do not separate into ions or atoms. When an ionic compound dissolves, it breaks up into ions, which are surrounded by water molecules. 3. The salt is a solute, which lowers the freezing point of the water as the two substances begin to mix. 4. Ocean water is a solution of salt and water. The salt lowers the freezing point of water below that of fresh water. UNDERSTANDING SOLUTIONS PART 2 CONCENTRATION & SOLUBILITY CONCENTRATION • The amount of one material dissolved in a given amount of another. – Think: how “strong” a solution is. Dilute: Only a little solute is dissolved in the solvent. Concentrated: A lot of solute is dissolved in the solvent. MEASURING CONCENTRATION • To measure concentration you compare the amount of solute to the amount of solvent. -OR• You compare the amount of solute to the total amount of solution. SAMPLE PROBLEM • SportMax is a mixture of fruit juice & water. The concentration of a 500 ml sample is 20% juice by volume. Find the volume of juice in the solution Think: What number is 20% of 500? SAMPLE PROBLEM 2 • FruityPoo BBQ is a mixture of fruit juice & vinegar. The concentration of a 300 ml sample is 40% juice by volume. Find the volume of juice in the solution Think: What number is 40% of 300? SOLUBILITY • Solubility is a measure of how well a solute can dissolve in a solvent at a given temperature. Think: is there a limit to how sweet you could make a glass of iced tea by adding sugar? WHAT AFFECTS SOLUBILITY? • TEMPERATURE • TYPE of SOLVENT And Pressure Saturated and Unsaturated Solutions • A saturated solution contains as much dissolved solute as possible at a given temperature. • An unsaturated solution does not yet hold as much of a solute as is possible at the given temperature. SUPERSATURATED SOLUTIONS • If you allow a heated saturated solution to cool slowly, sometimes the extra solute will remain dissolved. • You’ve created a supersaturated solution: a solution that has more dissolved solute that you would predict based on its temperature. WORKING WITH SOLUBILITY • Solubility is a physical property that helps us identify substance. CHECK FOR UNDERSTANDING 2 1. What quantities are compared when the concentration of a solution is calculated? 2. Why would an ionic compound be more likely to dissolve in water than oil? 3. How does a saturated solution differ from an unsaturated solution? 4. When you heat tap water on the stove, you can see tiny bubbles of oxygen form. They rise to the surface long before the water begins to boil. Explain what causes these bubbles to appear. 1. The amount of solute is compared to the amount of solvent or to the total amount of solution. 2. Polar water molecules attract the positive and negative ions. Nonpolar oil molecules do not. 3. A saturated solution contains as much solute as can dissolve in a solvent at a given temperature. An unsaturated solution does not (more solute could be dissolved in it without heating it). 4. Oxygen dissolved in the tap water becomes less soluble as the temperature goes up. The bubbles form when the gas comes out of the solution.