Download Rashid`s Lab Report

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 Convert to
(g)
g/100mL
Saturation
Temperature
1
2.0g
21C
40g/100ml
2
4.0g
80g/100ml
38C
3
6.0g
120g/100ml
45C
4
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.
8.0g
160g/100ml
65C
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
Alneaimi
Class: 11 -06
Mr. Ibrahim jibren