The term solubility in chemistry has both general and specific meanings. In everyday situations, we might say that a salt is soluble, meaning that experimentally; we were able to dissolve a sample of the salt in a particular solvent. In a specific sense, however, the solubility of a salt refers to a definite numerical quantity. Typically, the solubility of a substance is indicated as the number of grams of the substance that will dissolve in 100 g of the solvent. More often than not the solvent is water. In that case the solubility could also be indicated as the number of grams of solute that dissolve in 100 mL (the density of water is close to 1.00 g/mL under ordinary conditions). Since solubility refers to a specific, experimentally determined amount of substance, it is not surprising that the various handbooks of chemical data contain extensive lists of solubility’s of various substances. In looking at such data in a handbook, you will notice that the temperature at which the solubility was measured is always given. Solubility changes with temperature. For example, if you like your tea extra sweet, you have undoubtedly noticed that it is easier to dissolve two teaspoons of sugar in hot tea than in iced tea. For many substances, the solubility increases with increasing temperature.

For a number of other substances, however, the solubility decreases with increasing temperature. For convenience, a graph of solubility’s are often used rather than lists of solubility data from a handbook. A graph of the solubility of a substance versus the temperature will clearly indicate whether or not the solubility increases or decreases as the temperature is raised. If the graph is carefully prepared, the specific numerical solubility may be read from the graph. It is important to distinguish experimentally between whether a substance is soluble in a given solvent and how fast or how easily the substance will dissolve. Sometimes an experimenter may wrongly 1 conclude that a salt is not soluble in a solvent, when actually the solute is merely dissolving at a very slow rate. The speed at which a solvent dissolves has nothing to do with the final maximum quantity of solute that can enter a given amount of solvent. In practice, we use various techniques to speed up the dissolving process, such as grinding the solute to a fine powder or stirring/shaking the mixture. Such techniques will not affect the final amount of solute that ultimately dissolves, however. The solubility of a salt in water represents the amount of solute necessary to reach a state of equilibrium between saturated solution and undissolved additional solute. This number is a constant for a given solute/solvent combination at a specific temperature.

A key factor affecting the solubility of a substance – how much solute can be dissolved in a solvent – is temperature. For most substances increasing temperature will increase solubility - more solute will be able to dissolve in the same volume of solvent.

A solubility curve illustrates how the solubility of a substance varies with temperature. By determining the mass of solute that can be dissolved in a volume of solvent under a variety of temperatures we can easily construct a solubility curve. . In this lab exercise I will create a solubility curve for an ionic compound, potassium nitrate, KNO3.

To calculate the solubility of a substance under a variety of temperatures.

To construct a solubility curve based on experimental data. To see the effect of temperature on solubility.

Use caution when using the hot water bath to avoid hot water and steam burns.

Balance Four test tubes Distilled water Test tube clamp Test tube rack Scoopula 10 mL graduated cylinder Solid potassium nitrate Ring stand Gas burner

1. Prepare a water bath by filling a large beaker approximately 2/3 full with water. Place the beaker on a ring stand above a gas burner and begin heating the water to just below boiling. While this is heating continue with Step 2. 2. Number your test tubes 1 through 4. 3. Accurately measure out the following masses of solid potassium nitrate, placing the salt in the appropriate test tube. It is not necessary that you measure out exactly the masses given below, but you must record the precise masses you actually use. Test tube 1: 2 grams of KNO3

Test tube 2: 4 grams of KNO3 Test tube 3: 6 grams of KNO3 Test tube 4: 8 grams of KNO3 4. Add exactly 5.0 mL of water to each of the test tubes. 5. Place each of the tubes into the water bath in order to dissolve the solid KNO3 in each test tube. You may find it necessary to use a stirring rod to help the dissolving process, particularly for tubes 3 and 4.

6. Remove test tube 1 from the hot water bath once the KNO3 has fully dissolved and place a thermometer in the tube. Watch the solution carefully. Record the temperature as soon as you see crystals forming within the test tube (you will need to wait awhile for crystals to form in this first test tube). 7. Repeat the Step for the other test tubes once the KNO3 dissolves.

NOTE;- Be prepared to act quickly for test tube 4 crystallization may occur very soon after you remove the test tube from the hot water bath. It may be necessary to return the test tube to the water bath to redissolve the salt and allowing it to recrystallize again.

Test Tubes Mass KNO3 (g)

Convert to g/100mL

Saturation Temperature

1 2.0g 40g/100ml 21C

2 4.0g 80g/100ml 38C

3 6.0g 120g/100ml 45C

4 8.0g 160g/100ml 65C

The mixtures we stirred in the test tubes before heating them did not dissolve, but in the hot water bath they did quite well. Test tube #1, which had 2.0 grams, began to crystallize after a few minutes after putting in cold water, unlike the other test tubes which reached crystallization faster by far even though they had a higher saturation temperature.

In conclusion my hypothesis says salt will dissolve faster as the temperature rises, and apparently my hypothesis was true. Now we can illustrate that it is better to determine the saturation temperature while the temperature is dropping instead of rising, and I am able to understand that stirring has a high affect on the rate at which a salt dissolves in water, however it doesn’t have an income on the solubility of salt in water.

Done by: Rashid Khalid AlneaimiClass: 11 -06Mr. Ibrahim jibren