Chapter Nineteen: Solutions 19.1 Water 19.2 Solutions 19.3 Acids, Bases, and pH

19.1 Water We live on a watery planet. All life on Earth depends on this combination of hydrogen and oxygen atoms. What are the properties of water that make it so valuable?

19.1 The shape of a water molecule When two hydrogen atoms each share one electron with an oxygen atom, a neutral molecule is formed.

19.1 The shape of a water molecule Because negative charges repel, the electrons pairs around the oxygen atom are located where they are the farthest apart. This results in a geometric shape called a tetrahedron.

19.1 Water is a polar molecule A water molecule has a negative end (pole) and a positive end. A molecule (like water) with a charge separation is called a polar molecule.

19.1 Another polar molecule Ammonia, NH3, is another polar molecule. With one lone pair and three bonding pairs of electrons. This gives the ammonia molecule a pyramid shape.

19.1 Water is a polar molecule Methane, CH4, is a nonpolar molecule. Since there are no lone pairs of electrons, the electrons are shared equally between atoms.

19.1 Hydrogen bonds A hydrogen bond is a bond between the hydrogen on one molecule to another atom on another molecule. Hydrogen bonds are relatively weak so they constantly break and re-form.

19.1 Ice and hydrogen bonds Frozen water (or ice) has an organized structure that resembles a honeycomb because each water molecule can form hydrogen bonds with four other water molecules.

19.1 Properties of water Water has a high specific heat value because of hydrogen bonds. In order for water to boil, enough energy must be added to separate the hydrogen bonds.

19.1 Plants and hydrogen bonds The attraction between water molecules helps water travel from roots to stems and leaves.

19.1 Water as a solvent Water dissolves sodium chloride (salt) to form a solution of sodium (+) and chlorine (-) ions.

19.1 Water as a solvent In general, like dissolves like: water dissolves polar substances non-polar solvents dissolve non-polar substances

19.2 Water and solutions A solution is a mixture of two or more substances that is uniform at the molecular level.

19.2 Water as a mixture Muddy water not a solution. Muddy water is heterogeneous because it contains larger particles of soil or plant debris.

19.2 Water and solutions Although we often think of solutions as mixtures of solids in liquids, solutions exist in every phase; solid, liquid, or gas. Solutions of two or more solids are called alloys. Steel is an alloy (solution) of iron and carbon.

19.2 Suspensions In a mixture called a suspension the particles can range widely in size. Muddy water, a suspension, will settle when it is left still for a period of time.

19.2 Colloids Colloids are mixtures, and look like solutions, but their particles are too small to settle to the bottom of their container over time. Examples of colloids are mayonnaise, egg whites, and gelatin.

19.2 Tyndall effect Tyndall effect is occurring if you shine a flashlight through a jar of liquid and see the light beam.

First, try filtering it, then look for the Tyndall effect. 19.2 Types of mixtures How can you tell the difference between a solution, a colloid and a suspension? First, try filtering it, then look for the Tyndall effect.

19.2 Water and solutions A solution contains at least two components: a solvent, and a solute. The solvent is the part of a mixture that is present in the greatest amount. Which of these is the solvent?

19.2 Water and solutions When the solute particles are evenly distributed throughout the solvent, we say that the solute has dissolved.

On the molecular level, dissolving of a solid (like sugar) occurs when molecules of solvent interact with and separate molecules of solute

19.2 Solubility The term solubility means the amount of solute (if any) that can be dissolved in a volume of solvent. Is there a limit for how much seltzer (solute) can dissolve in water (solvent)?

19.2 Solubility Chalk and talc do not have solubility values. These substances are insoluble in water because they do not dissolve in water.

19.2 Solubility A solution is saturated if it contains as much solute as the solvent can dissolve. Any solute added in excess of the substance’s solubility will not dissolve. What will happen to any un-dissolved gas when you unscrew the cap of seltzer water?

Seawater is a solution of water, salt, and other minerals. Solving Problems Seawater is a solution of water, salt, and other minerals. How much salt can dissolve in 200 mL of water at 25 °C? Looking for: …grams of solute Given … v = 200 mL; T = 25 C

Solving Problems Looking for: Given Relationships: Solution …grams of solute Given … v = 200 mL water; T = 25 C Relationships: Solubility table for reference 37.7 g of salts in 100 mL water at 25 °C Solution …if there are 37.7 g salt/100 mL water, then “x” g/ 200 mL water = 75.4 g salts are need in 200 mL of water

19.2 Solubility A solution is saturated if it contains as much solute as the solvent can hold. An examples of a saturated solution is air. Air can be saturated with water. (We call it humidity!)

19.2 Solubility graphs Solubility values for three solutes are plotted in this temperature-solubility graph.

19.2 Solubility For something to dissolve in water, the water molecules need to break the bonds between the solute molecules. Water dissolves each substance differently because the chemical bond strengths between atoms found in different solutes are not the same.

19.2 Concentration In chemistry, it is important to know the exact concentration of a solution—that is the exact amount of solute dissolved in a given amount of solvent.

19.2 Concentration Two other common ways of expressing the concentration of a solution are molarity and mass percent.

19.2 Concentration Molarity is equal to the moles of solute per liter of solution.

19.2 Concentration The mass percent of a solution is equal to the mass of the solute divided by the total mass of the solution multiplied by 100%.

What is the molarity of this solution? Solving Problems How many grams of salt (NaCl) do you need to make 500 grams of a solution with a mass percent of 5% salt? The formula mass of NaCl is 58.4 g/mol. What is the molarity of this solution? Looking for: …grams of solute Given … mass solvent = 500 g; concentration = 5% …formula mass = 58.4 g/mol

Relationships: Solution Solving Problems Mass percent = mass of solute x 100% total mass of solution Molarity = moles of solution liter of solution Solution … 5% = (mass of salt ÷ 500 g) × 100% …0.05 × 500 g = 25 g # of moles = 25 g = .4 moles 58.4 g/mol Molarity = .4 mol/.5 L = .8 M solution

19.2 Equilibrium When a solute like sugar is mixed with a solvent like water, two processes are actually going on continuously. Molecules of solute dissolve and go into solution. Molecules of solute come out of solution and become “un-dissolved.” When the rate of dissolving equals the rate of coming out of solution, we say equilibrium has been reached.

19.2 Equilibrium When a solute like sugar is mixed with a solvent like water, two processes are actually going on continuously. Molecules of solute dissolve and go into solution. Molecules of solute come out of solution and become “un-dissolved.” When the rate of dissolving equals the rate of coming out of solution, we say equilibrium has been reached.

19.2 Equilibrium When a solution is unsaturated its concentration is lower than the maximum solubility. A supersaturated solution means there is more dissolved solute than the maximum solubility.

19.2 Solubility of gases in liquids Some solutions have a gas as the solute. When you drink carbonated soda, the fizz comes from dissolved carbon dioxide gas (CO2).

19.2 Solubility of gases in liquids When temperature increases, the solubility of gases in liquid decreases.

19.2 Solubility of gases in liquids The variety and no. or organisms is controlled somewhat by the relationship between dissolved oxygen and temperature.

19.2 Solubility of gases in liquids Oil and vinegar salad dressing separates because oil is not soluble in water. Liquids that are not soluble in water may be soluble in other solvents.

19.2 Solubility rules A set of solubility rules helps predict when an ionic compound is soluble or insoluble.

19.3 What are acids? An acid is a compound that dissolves in water to make a particular kind of solution. Chemically, an acid is any substance that produces hydronium ions (H3O+) when dissolved in water.

19.3 What are acids? Some properties of acids are: Acids create the sour taste in food, like lemons. Acids react with metals to produce hydrogen (H2) gas. Acids change the color of blue litmus paper to red. Acids can be very corrosive, destroying metals and burning skin through chemical action.

19.3 Bases A base is any substance that dissolves in water and produces hydroxide ions (OH-).

19.3 What are bases? Some properties of bases are: Bases create a bitter taste. Bases have a slippery feel, like soap. Bases change the color of red litmus paper to blue. Bases can be very corrosive, destroying metals and burning skin through chemical action.

19.3 Acids and bases One of the most important properties of water is its ability to act as both a weak acid or as a weak base. In the presence of an acid, water acts as a base. In the presence of a base, water acts as an acid.

19.3 The pH scale The pH scale ranges from 0 to 14. Acids have a pH less than 7. A base has a pH greater than 7. Pure water has a pH equal to 7.

19.3 Determining pH pH is an abbreviation for “the power of hydrogen”. The pH for a solution equals the negative of the exponent of the hydronium ion (H3O+) concentration.

Solving Problems A solution contains a hydronium ion concentration of 10-4.5 M. What is the pH value of the solution? Is this solution acidic or basic? Looking for: …pH and whether acid or base Given: … H3O+ conc. = 10-4.5 M. Relationships: …pH = neg exponent of H3O+ concentration Solution: …pH = 4.5

19.3 pH in the environment The pH of soil directly affects the availability of nutrients for plants. Blueberries grow best in what pH soil?

19.3 pH in the environment The pH of water directly affects aquatic life. How are frogs and amphibians sensitive to pH changes?

19.3 The pH scale Red and blue litmus paper are pH indicators that test for acids or bases.

19.3 Acids and bases in your body Many reactions, such as the ones that occur in your body, work best at specific pH values.

19.3 pH and blood The pH of your blood is normally within the range of 7.3–7.5. Holding your breath causes blood pH to drop. High blood pH can be caused by hyperventilating.