Solubility and Temperature

Solubility and Average Kinetic Energy The temperature of a solvent can have a big effect on the solubility of a solute. Many ionic compounds are known as endothermic salts: an increase in the temperature leads to an increase in solubility. For example, sodium chloride is a slightly endothermic salt:

Endothermic Salts Looking on Table G, sodium chloride’s solubility slightly increases with temperature.

NH 4 Cl and KNO 3 Compare sodium chloride to these two other endothermic salts found on Table I:

Changes in Solubility Vary Note that potassium nitrate’s solubility increases dramatically with temperature compared to NaCl

According to Reference Table G, how does a decrease in temperature from 40°C to 20°C affect the solubility of NH 3 and KCl? The solubility of NH 3 decreases, and the solubility of KCl decreases. The solubility of NH 3 decreases, and the solubility of KCl increases. The solubility of NH 3 increases, and the solubility of KCl decreases. The solubility of NH 3 increases, and the solubility of KCl increases.

Exothermic Salts There are no exothermic salts on Table G. However, there is one on Table I: The solubility of sodium hydroxide decreases with increasing temperature.

Gas Solubility The solubility of gases decreases with increasing temperature. Increased kinetic energy makes it more likely that gas molecules can escape the solvent.

Under which conditions are gases most soluble in water? high pressure and high temperature high pressure and low temperature low pressure and high temperature low pressure and low temperature

Where is the best place to go fishing? Colder water has higher dissolved O 2 levels than warm water does; When nutrients are present and oxygen levels are high – lots of fish!

Pressure and Gas Solubility Solubility of gases is strongly impacted by pressure. High pressure: higher solubility (Henry’s Law)

Saturated Solutions A saturated solution contains the maximum amount of solute possible for that volume at a given temperature – the lines indicate this.

Saturated Solutions When a solution is saturated, rate of dissolving = rate of crystallizing No apparent change in the amount of solute dissolves occurs, yet dissolving and crystallization continues. Further added solute will accumulate at the bottom of the container.

Unsaturated Solutions A solution that contains less than the maximum amount of solute at that temperature is said to be unsaturated.

Saturated or Unsaturated? 100 ml of water contains 30 g of NaCl at 30°C. How much more salt is needed to saturate the solution?

Which statement is true for a saturated solution? It must be a concentrated solution. It must be a diluted solution. Neither dissolving nor crystallizing is occurring. The rate of dissolving equals the rate of crystallizing.

At which temperature do NaNO 3 and KNO 3 have the same solubility? 100 g of water saturated with KClO 3 is cooled from 50°C to 30°C. How much solid crystallizes? How much NH 4 Cl is needed to saturate 50 g of water at 35°C? Which substance on the graph shows the smallest increase in solubility over the range 80°C to 100°C? Which of the substances on the graph have approximately the same solubility over the range 20°C to 25°C?

321 g of KNO 3 are used to saturate water at 60°C. What is the mass of water that is used? When a saturated solution of KClO 3 at 24°C is evaporated to dryness, the mass decreases 200 g. How much solid remains? What is the smallest mass of water necessary to dissolve 40 g of NH 3 completely at 4°C? Which of the substances on the graph has a solubility that is relatively unaffected by changes in temperature? Which substances on the graph have solubilities that decrease with increases in temperature?

30 g of KI are dissolved in 300 g of water at 10°C. How much additional KI is necessary to saturate the solution? 500 g of water is saturated with KCl at 10°C. If the temperature is raised to 60°C, how much additional KCl is needed to resaturate the solution? What is the average rate of increase in solubility (in g per 100 g H 2 O per °C) for NaNO 3 in the range 10°C - 20°C? Which substance shows the largest increase in solubility in the range 30°C - 70°C? Which substance is most soluble at 50°C? Which substance is least soluble at 50°C? 100 g of water is saturated with KClO 3 at 70°C. To what temperature must the solution be cooled in order for 10 g of solid to crystallize? Assuming that all of the following can form supersaturated solutions, indicate whether the following solutions are saturated, unsaturated, or supersaturated. – 40 g of KCl in 100 g of H 2 O at 80°C – 120 g of KNO 3 in 100 g of H 2 O at 60°C – 80 g of NaNO 3 in 100 g of H 2 O at 10°C

Based on Reference Table G, which salt solution could contain 42 grams of solute per 100 grams of water at 40°C? a saturated solution of KClO 3 a saturated solution of KCl an unsaturated solution of NaCl an unsaturated solution of NH 4 Cl

Lab 12: Solubility of Potassium Nitrate First prepared by Hasan al-Rammah (Syria) in 1270 Known as saltpeter, “Chinese snow,” or “Chinese salt” Isolated from bat guano, among other sources

Objective: to examine the change in solubility of a salt at different temperatures and construct a solubility curve for that salt. Materials 400 ml beaker, ring stand, wire gauze, ring clamp, four 6’ test tubes, KNO 3, thermometer, stirring rods.

Procedure Number four test tubes 1-4 with a wax pencil. Weigh out the following amounts of KNO 3 and place them in corresponding test tubes: Test Tube 1: 2.0 g Test Tube 3: 6.0 g Test Tube 2: 4.0 g Test Tube 4: 7.0 g Prepare a water bath using a 400 ml beaker that is around 2/3 full. Heat the water to near boiling (90-95  C) but do not let it boil. Add 5.0 ml of water to each of the four test tubes. Place test tubes in the water bath; stir each solution to dissolve the salt. Wash and dry the stirring rod before using it in the next solution.

When the salt is dissolved in all of the test tubes, put the thermometer in test tube #4. Remove the other test tubes and place them in a test tube rack to cool. Record the temperature at which crystallization first appears in test tube #4. Then wipe the thermometer and place it in test tube #3 and record the temperature of crystallization. Repeat this with test tubes 2 and 1. You may want to hasten the cooling of test tube 1 with a beaker of cold tap water. Record your data in the table. Calculate the amount of solute that is soluble in 100 g of water at each temperature. Plot your points on a graph; temperature on the x axis, g/100 g water on the y-axis. To convert your results to g/100 g, you multiply the mass of potassium nitrate in the tube by 20. Make sure your graph has an appropriate title. Using another color, plot the table G values on the same graph. Be sure to include a key on your graph.

Data Table Test Tube g KNO 3 /5 g waterSaturation Temperature (ºC) g KNO 3 /100g water Table G accepted value

Questions How does temperature affect the solubility of potassium nitrate? Compare your results to Table G for each of your four temperatures. Calculate % errors for each measurement. The formula for % error is in your reference tables. What are some of the possible sources of error in this experiment?