Crystallisation from Supersaturated Solution
A plastic ampoule filled with supersaturated solution of sodium acetate is prepared from a Beral pipet. By cutting the pipet stem an exothermic process of crystallisation can clearly be observed followed by volume reduction.
Introduction
One of the experiments that attracts students attention are experiments that present the process of crystallisation. Some time it can be show spontaneous process of formation of well defined crystals with amazing geometrical shape that excite the students and in another case the transformation of a liquid into solid followed by heat releasing can show. The performance of the last is mentioned in the literature as demonstration experiment that shows growing of a stalagmite. Namely, a supersaturated solution of sodium acetate is poured onto a watch glass that contains some sodium acetate crystals. When the solution hits the crystals it re-crystallises to form a column up.
Materials
Test tube (12 mm x 75 mm), 2 mL syringe, forceps, scissors, microburner, Beral pipet, permanent marker.
Chemicals
6 g of sodium acetate trihydrate
Fig. 1. Materials
Fig. 2. Preparation of the ampoule with the supersaturated solution
Experimental
Place 6 g of sodium acetate trihydrate in the test tube and then with the syringe add 0.5-1 mL distilled water (Fig. 2a). Heat the test tube with the flame of microburner (Fig. 2b) until a clear solution without any crystals is formed and heat a few minutes more. The temperature of the solution should be 10-20 ºC above the melting temperature of the sodium acetate trihydrate. Then, fill the Beral pipet with the solution. Do not allow any air to remain in the pipette. That could be achived by tilting the Beral pipet upside down (Fig. 2c), pressing the pipet bulb until the air is removed and releasing it with the stem immersed in the solution. The ampoule can be prepared by heating the stem of the pipet in the flame of the microburner relatively far from the flame to avoid the stem to catch fire and still close to provide enough heat to bring the stem to its melting temperature. When that state is reached the stem will bend and that is the time to use forceps to press the stem and close the ampoule (Fig. 2d). By pulling the end of the stemit will be removed from the prepared ampoule. The ampoule with the solution should be left to cool slowly. Do not force the cooling (with water or any kind of cooling spray). After the system-ampoule is cooled down, the crystallisation can be initiated by cutting the stem (Fig. 3.). The cutting should be done slowly to leave the solution in a short contact with the metal of the scissors. Using a thermometer the exothermic reaction can be demonstrated quantitatively (Fig. 4).
Fig. 3. Crystallisation
Fig. 4. Egzotermic process
Observation and discussions
The solutions can contain variable amount of dissolved substance and depending on it one can observe three cases:
a) An unsaturated solution is one that has not dissolved as much solute as possible at a given temperature. If you add more solute to an unsaturated solution it will dissolve.
b) A saturated solution is a solution that has dissolved as much solute as possible at a given temperature. If you add more solute to a saturated solution it will not dissolve.
c) A supersaturated solution contains more dissolved substance than a saturated solution. If you add more solute to a supersaturated solution you will provoke crystallisation. Supersaturated solutions are not stable and can easily be disturbed by mechanical stress or by adding a crystal of the solute.
The crystallisation in this experiment can be provoked by contact of the solution with metal (cutting or stabbing with a needle). In contrast of the well-known experiment this proposed experiment can find its place as a hands on experiment saving time by using previously prepared ampoules. The advantage of this experiment is first that the students can touch the ampoule and feel the liberated heat and second they can see deformation of the ampoule and can make a conclusions about volume contractions and seek for the reason which is a better package of the molecules and ions.
The proposed experiment can be simply performed by observing what happens when the crystallisation is initiated in an ampoule or could be performed in a more sophisticated way using a digital thermometer and two ampoules in which case the temperature probe indicates that the crystallisation is exothermic.