UNIT SEVEN: Earth’s Water  Chapter 21 Water and Solutions  Chapter 22 Water Systems  Chapter 23 How Water Shapes the Land

Chapter Twenty-One: Water and Solutions  21.1 Water  21.2 Solutions  21.3 Acids, Bases, and pH

Chapter 21.2 Learning Goals  Explain how solutions are formed.  Define solubility and interpret solubility graphs.  Describe factors that affect the concentration of solutions.  Compare and contrast solubility of solid, liquid, and gaseous matter.

Investigation 21B  Key Question: What is a solubility curve? Solubility Curve of KNO 3

21.2 Water and solutions  A solution is a mixture of two or more substances that is homogeneous at the molecular level.  Homogeneous means the particles are evenly distributed.

21.2 Water as a mixture  Muddy water is heterogeneous because it contains larger particles of soil or plant debris.

21.2 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.

21.2 Tyndall effect  Tyndall effect is occurring if you shine a flashlight through a jar of liquid and see the light beam.  This distinguishes a colloid from a solution.

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

21.2 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.

21.2 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?

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

21.2 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)?

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

21.2 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?

Solving Problems 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

 Relationships:  Solubility table for reference  35.9 g of salts in 100 mL water at 25 °C  Solution  …if there are 35.9 g salt/100 mL water, then  “x” g/ 200 mL water  = 71.8 g salts are need in 200 mL of water Solving Problems

21.2 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!)

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

21.2 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.

21.2 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.

21.2 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.

21.2 Solutions of gases and liquids  Some solutions have a gas as the solute.  In carbonated soda, the fizz comes from dissolved carbon dioxide gas (CO 2 ).

21.2 Solutions of gases and liquids  When temperature increases, the solubility of gases in liquid decreases.

21.2 Solutions of gases and liquids  The variety and no. or organisms is controlled somewhat by the relationship between dissolved oxygen and temperature.

21.2 Solutions of gases and liquids  The concentration of dissolved oxygen in water is important for fish and aquatic life.

21.2 Solutions of gases and liquids  Oil and vinegar salad dressing separates because oil is not soluble in vinegar (mostly water).  Liquids that are not soluble in water may be soluble in other solvents.

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