Research Question: How does the solubility of potassium nitrate change with respect to the change in temperature by 20C keeping the pressure and the nature of the solute and solvent constant?

Solubility is a result of an interaction between polar water molecules and the ions which make up a crystal. Ionic compounds are soluble in water but are insoluble in organic solvents. Water is polar solvent and therefore has a high di-electric constant which weakens the strong electrostatic forces of attraction, resulting in the formation of free mobile ions and thus the dissolution of the compound. However, organic solvents are non-polar and therefore have a low di-electric constant which fails to weaken the strong electrostatic force of attraction due to which no free mobile ions are formed and thus the compound is insoluble in organic solvents.

Nitrates form ionic compounds and are therefore soluble in water. The nitrate anion has three equivalent Oxygen ions surrounding a central Nitrogen atom. This tends to spread the single negative charge and make it easier for water (using hydrogen bonds) to separate the ions in solution. Therefore, all nitrates are soluble in water. The change in temperature greatly affects the solubility of the nitrates. The solubility of a given solute in a given solvent typically depends on temperature of the substance.

Temperature greatly affects the solubility of a substance. In nitrates, an increase in the temperature will lead to an increase in solubility of the substance.

The temperature ranges from 20 to 100c and thus we can see that the solubility increases. The above diagrams show that an increase in temperature will lead to an increase in the solubility of potassium nitrate. The solubility of solutes is dependent on temperature. When a solid dissolves in a liquid, a change in the physical state of the solid takes place. Heat is required to break the bonds holding the molecules in the solid together. At the same time, heat is given off during the formation of new solute -- solvent bonds. If the heat given off in the dissolving reaction is less than the heat required to break apart the solid, the net dissolving reaction is endothermic (energy required). The addition of more heat facilitates the dissolving reaction by providing energy to break bonds in the solid. This is the most common situation where an increase in temperature produces an increase in solubility for solids. This is what will occur in the heating of the potassium nitrate and therefore the solubility will increase.

Variables

Independent:  Temperature  

Dependant: Solubility

Controlled:

1.  Pressure
2. Nature of Solute and Solvent ( i.e. Potassium Nitrate)  

Method

1. Set up a beaker on an iron ring and wire gauze. Add about 300 ml of tap water.
2. Weigh out the amount of potassium nitrate, KNO3 and transfer the solid to a large test tube.
3. Stir the mixture with a stirring wire to dissolve as much potassium nitrate as possible. Carefully insert a thermometer.
4. Heat the water in the beaker with the Bunsen burner. Clamp the test tube in the water.
5. Use a stirring wire to stir the mixture gently until all of the potassium nitrate has dissolved.
6. When all of the potassium nitrate has dissolved, loosen the clamp from the support stand and raise the tube out of the water bath. Turn off the burner. Re clamp the tube above and to one side of the beaker. Using the stirring wire, stir the solution gently and observe it as it cools. As soon as crystallization begins, note the temperature of the solution. It is easiest to see crystals by looking at the bottom of the test tube where they will collect as soon as they begin to form.
7. After the crystallization temperature has been recorded, put the test tube back into the water bath and warm the solution until all of the crystals have re dissolved. Repeat the cooling procedure to check the crystallization temperature. The two readings should be within 1 degree of each other. If the two readings do not agree within 1 degree, re warm the solution in the water bath, cool it, and continue until satisfactory agreement is obtained.
8. Record sample mass and then record the experimental temperature next to it.
9. Repeat the experiment two to three times to ensure accuracy.

Precautions:

• Handle hot equipment and solutions cautiously.
• The thermometer bulb is fragile; handle gently

Apparatus

1. 1 0.5 dm3 beaker
2. 1 iron ring
3. 1 wire gauze
4. 0.3dm3 tap water
5. 5g Potassium Nitrate
6. 2 Test tubes along with test tube holder
7. Stirring wire
8. Thermometer
9. Bunsen Burner
10. Water Bath

Data Table

Temperature       20

40

60

80

100