8.2 Solubility and Concentration

Solubility Sugar is more soluble than salt maximum grams of solute that will dissolve in 100 g of solvent at a given temperature Varies with temperature Sugar is more soluble than salt

Solubility Gases are more soluble at... low temperatures & Solids are more soluble at... high temperatures. Gases are more soluble at... low temperatures & high pressures (Henry’s Law).

Classifying Solutions Solutions are classified based on how much solute is present. Three types of solutions: Saturated Unsaturated Supersaturated

Saturated Solutions Contains maximum amount of solute that solvent can hold at a given temperature. “filled” with solute If more solute is added to saturated solution, it will not dissolve.

Unsaturated Solution Solution with less than the maximum amount of solute that can be dissolved.

Supersaturated Solution Solution with more solute than it can normally hold at a given temperature. Very unstable Solubility is given at specific temperature-if solvent is heated above that temperature, more solute may dissolve, which allows solution to be supersaturated. Ex: carbonated water

SUPERSATURATED SOLUTION Solubility UNSATURATED SOLUTION more solute dissolves SATURATED SOLUTION no more solute dissolves SUPERSATURATED SOLUTION becomes unstable, crystals form increasing concentration

Solubility Curve Solubility vs. Temperature for Solids 140 KI 130 Graph that shows the dependence of solubility on temperature gases solids 120 NaNO3 110 100 KNO3 saturated: solution has “just right” amt. of solute; on line unsaturated: solution could hold more solute; below line supersaturated: solution has “too much” solute dissolved in it; above the line 90 80 HCl NH4Cl 70 Solubility (grams of solute/100 g H2O) 60 NH3 KCl 50 “Solubility Curves for Selected Solutes”   Description: This slide is a graph of solubility curves for 10 solutes. It shows the number of grams of solute that will dissolve in 100 grams of water over a temperature range of 0cC to 10 cC. Basic Concepts The maximum amount of solute that will dissolve at a given temperature in 100 grams of water is given by the solubility curve for that substance. When the temperature of a saturated solution decreases, a precipitate forms. Most solids become more soluble in water as temperature increases, whereas gases become less soluble as temperature increases. Teaching Suggestions Use this slide to teach students how to use solubility curves to determine the solubilities of various substances at different temperatures. Direct their attention to the dashed lines; these can be used to find the solubility of KClO3 at 50 cC (about 21 g per 100 g of H2O). Make sure students understand that a point on a solubility curve represents the maximum quantity of a particular solute that can be dissolved in a specified quantity of solvent or solution at a particular temperature. Point out that the solubility curve for a particular solute does not depend on whether other solutes also are present in the solution (unless there is a common-ion effect; this subject usually is covered at a later stage in a chemistry course). Questions Determine the solubilities (in water) of the following substance at the indicated temperatures: NH3 at 50 oC; KCl at 90 oC; and NaNO3 at 0 oC. Which of the substances shown on the graph is most soluble in water at 20 oC? Which is lease soluble at that temperature? For which substance is the solubility lease affected by changes in temperature? Why do you think solubilities are only shown between 0 oC and 100 oC? In a flask, you heat a mixture of 120 grams of KClO3 and 300 grams of water until all of the KClO3 has just been dissolved. At what temperature does this occur? You then allow the flask to cool. When you examine it later, the temperature is 64 oC and you notice a white powder in the solution. What has happened? What is the mass of the white powder? Compare the solubility curves for the gases HCl, NH3, and SO2) with the solubility curves for the solid solutes. What generalizations(s) can you make about the relationship between solubility and temperature? According to an article in an engineering journal, there is a salt whose solubility in water increases as the water temperature increases from 0 oC to 65 oC. The salt’s solubility then decreases at temperatures above 65 oC, the article states. In your opinion, is such a salt likely to exist? Explain your answer. What could you do to verify the claims of the article? 40 30 NaCl KClO3 20 10 SO2 0 10 20 30 40 50 60 70 80 90 100

Solubility of Sodium Acetate 150 Supersaturated solution Saturated 100 Solubility(g/100 g H2O) A single crystal of sodium acetate, NaC2H3O2, is dropped into a supersatureated solution Unsaturated solution Add 100 g sodium acetate into 100 g of water at 55oC. Allow to cool to 20oC. Only 82 g of sodium acetate should remain in solution at this temperature. Place a seed crystal in the supersaturated solution and ~18 g of sodium acetate will precipitate out of solution. The resulting solution will be saturated. Include a table of data for students to graph. 50 The small crystal causes extensive crystallization, and eventually the solute forms a solid mass of NaC2H3O2. 25 50 75 100 Temperature (oC) Charles H.Corwin, Introductory Chemistry 2005, page 378

Solubility  how much solute dissolves in a given amt. of solvent at a given temp. SOLUBILITY CURVE Temp. (oC) Solubility (g/100 g H2O) KNO3 (s) KCl (s) HCl (g) unsaturated: solution could hold more solute; below line saturated: solution has “just right” amt. of solute; on line supersaturated: solution has “too much” solute dissolved in it; above the line

Reading Solubility Curves There are a few types of questions you need to be prepared to answer by reading a solubility curve. One question type has to do with temperature. *Question: At what temperature will 60 grams of potassium nitrate (KNO3) in 100 grams of water form a saturated solution? What are we looking for? temperature Step 1 – find 60 grams on the y-axis. Grams are always on the y-axis of a solubility curve. Step 2 – Move horizontally until you hit the solubility curve of the substance Step 3 – Move straight down unit you find the temperature value on the x-axis In this example, the answer would be about 38 ⁰C.

Reading Solubility Curves Another type of question you need to be prepared to answer by reading a solubility curve is one that deals with solubility level (saturated, unsaturated or supersaturated). *Question: What type of solution is formed when 80 grams of potassium nitrate (KNO3) is dissolved at 40⁰C? What are we looking for? Is the solution saturated, unsaturated or supersaturated. Step 1 – plot the point given on the graph. Step 2 – Where is the point in relation to the solubility curve? Answer: The point is above the line so that means this point would show a supersaturated solution

Reading Solubility Curves Another type of question you need to be prepared to answer by reading a solubility curve is one that deals with two substances having the same solubility. *Question: At what temperature will KNO3 and KBr have the same solubility? What are we looking for? At what temperature will the solubility curves for KNO3 and KBr cross? Step 1 – find the point where the lines for the substances in the question cross. Step 2 – Move down to temperature on the x-axis to find the value. Answer: These points cross at the value in between 40 ⁰C and 60 ⁰C which would be 50 ⁰C

Factors Affecting Solubility Polarity of the Solvent Temperature Pressure

Polarity of the Solvent “like dissolves like” Solutions are more likely to form when solute and solvent are either both polar or both nonpolar. Oil does not dissolve in water because oil is nonpolar and water is polar.

Temperature The solubility of solids increases as the temperature of the solvent increases Gases become less soluble as temperature of solvent increases

Increasing the pressure on a gas increases its solubility in a liquid.

Concentration of Solutions Concentration of a solution is the amount of solute dissolved in specified amount of solution. Concentration can be expressed as: Percent by volume Percent by mass Molarity