Sodium acetate supersaturated solution

Figure 13 Crystallization of sodium acetate supersaturated solution...

When we cool saturated solutions of certain solutes, such as sodium acetate, the solute often has difficulty forming the first crystal, resulting in an unstable solution. More solute is in the solvent than is stable at the lower temperature. This type of solution is called a supersaturated solution. If a supersaturated solution is shaken or the inner surface of the container is scratched with a glass rod, the excess solute may crystallize out. A more certain way to get a supersaturated solution to crystallize is to add to it a tiny crystal of the solid solute, on which the excess solute can crystallize (Fignre 15.3). 

The hand-warmer contains a supersaturated solution of sodium acetate. (Supersaturation is very similar to supercooling, except that it is the solute, rather than the solvent, that crystallizes when nucleation occurs.) When the heat is required, you click a disc on the hand warmer, which causes nucleation, initiating crystallization of sodium acetate. This releases the latent heat and warms your hands. The hand warmer is reusable because you can re-dissolve all the crystals by heating it, e.g. by placing it in boiling water. As it cools down to room temperature, the solution becomes supersaturated again and, provided no crystals remain, it can be kept in this state until it is needed again. 

EXAMPLE 16.1. A solution at 0°C contains 119g of sodium acetate per lOOg of water. If more sodium acetate is added, it does not dissolve, and no sodium acetate crystallizes from solution either. Describe the following solutions as saturated, unsaturated, or supersaturated. (a) lOOg sodium acetate in lOOg water at 0°C. (b) 150g sodium acetate in lOOg water at 0°C. (c) 11.9g sodium acetate in lO.Og water at 0°C. 

The 150 g of sodium acetate is mixed with 100 g of water and heated to nearly 100°C, where 170g of solute would dissolve. The mixture is stirred until solution is complete, and then the solution is cooled until it gets to 0°C. The solute would crystallize if it could, but this particular solute has difficulty doing so, and thus a supersaturated solution is formed. Adding a crystal of solid sodium acetate allows the excess solute to crystallize around the solid added, and the excess solute precipitates out of the solution, leaving the solution saturated. 

Figure 7.3(A) shows a supersaturated solution of sodium acetate. It was prepared by adding sodium acetate to a saturated aqueous solution. The mixture was heated to dissolve the added crystals. Finally, the solution was left to return slowly to room temperature. In Figure 7.3(B), a single crystal of sodium acetate has been added to the supersaturated solution. As you can see, solute ions rapidly leave the solution, and solid forms.

These photographs show a supersaturated solution of sodium acetate and the effect of adding a crystal to it. Is either of these systems at equilibrium ... 

In some cases a solution may become supersaturated. A good example of this is sodium acetate, CH3COONa. When a solution containing sufficient sodium acetate is heated until the solid dissolves and allowed to cool slowly with minimal disturbance, the sodium acetate does not crystallize out as the solution cools. For the sodium acetate to settle out, a small seed crystal must be added to the solution to initiate the crystallization process. 

JL Lee R. Summerlin, Christie. L. WBorgford, and Julie. B. Ealy, "Supersaturation," Chemical Demonstrations, A Sourcebook for Teachers, Vol. 2 (American Chemical Society, Washington, DC, 1988), pp. 121-122. Excess solute suddenly precipitates from a supersaturated solution of sodium acetate trihydrate in this demonstration. 

VTj Jamil Ahmad, "Crystallization from a Supersaturated Solution of Sodium Acetate," /. Chem. Educ., Vol. 77, 2000,1446. 

Note that this definition requires a saturated solution to be at equilibrium with undissolved solid. Substances that are more soluble at high temperature than at low temperature can sometimes form what are called supersaturated solutions, which contain a greater-than-equilibrium amount of solute. For example, when a saturated solution of sodium acetate is prepared at high temperature and then cooled slowly, a supersaturated solution results, as shown in Figure 11.5. Such a solution is unstable, however, and precipitation occurs when a tiny seed crystal of sodium acetate is added to initiate crystallization. 

A. Prepare unsaturated, saturated, and supersaturated solutions of sodium acetate ... 

Prepare a supersaturated solution of sodium acetate. In a large test tube, add 7.0 g NaC2H302 to 10.0 ml distilled water and gradually heat the contents of the test tube over a Bunsen burner or alcohol lamp until all the sodium acetate is dissolved. Carefully place the test tube in a test tube rack and let it cool without being disturbed. All the sodium acetate will remain in solution. 

Supersaturated Solutions.—When a solution of a solid in water, already saturated at a given temperature, is heated up with more of the solid until the whole of the latter has passed into solution, crystallisation or precipitation of the excess of solute does not always take place upon cooling the system to the original temperature. It is clear that the solution must now hold a greater quantity of substance than corresponds to the ordinary solubility and is said to be supersaturated. Such solutions can readily be prepared by heating up sodium thiosulphate, sodium acetate, or sodium sulphate with water, and allowing to cool without agitation. 

In this investigation, you will prepare a supersaturated solution of sodium acetate. (A supersaturated solution contains more dissolved solute at a specific temperature than is normally possible.) Then you will use the solution to prepare your own ionic crystal. 

A tiny crystal of solid sodium acetate is added to three aqueous solutions of sodium acetate. Classify each original solution as saturated, unsaturated, or supersaturated. 

To one-half of the diaminonaphthol dihydrochloride solution saved from Section 1 add 3 mL of acetic anhydride, stir vigorously, and add a solution of 3 g of sodium acetate (anhydrous) and about 100 mg of sodium hydrosulfite in 20-30 mL of water. The diacetate may precipitate as a white powder or it may separate as an oil that solidifies when chilled in ice and rubbed with a rod. Collect the product and, to hydrolyze any triacetate present, dissolve it in 5 mL of 10% sodium hydroxide and 50 mL of water by stirring at room temperature. If the solution is colored, a pinch of sodium hydrosulfite may bleach it. Filter by suction and acidify by gradual addition of well-diluted hydrochloric acid (2 mL of concentrated acid). The diacetate tends to remain in supersaturated solution hence, either to initiate crystallization or to ensure maximum separation, it is advisable to... 

Sodium acetate (NaC2H302) is commonly used in the preparation of a supersaturated solution. When a seed crystal is added the excess sodium acetate quickly cmtallizes out of the solution, and Q. 

Sodium acetate crystals (NaC2H302) form quickly in a supersaturated solution when a small speck of solute is added. 

Heat packs are used for first aid purposes. There is a supersaturated solution of sodium acetate (NaCH3COO) in a packet. 

Can it be possible to dissolve more solute in an already saturated solution You may consider it impossible. Cooling a saturated solution usually causes a solid to crystallize out of the solution. Sometimes, however, this does not happen. The excess solute stays in the solution upon cooling. This type of solution is known as supersaturated. For example, 161 g of sodium acetate (NaCH3COO) can be dissolved in 100 g of water at 90°C. When this saturated solution is carefully cooled to 20°C, at which temperature the solubility of sodium acetate is 123.5 g/100 g of H20, all the solute may remain in the solution. The cooled solution contains more solute than it normally would. This is referred to as a supersaturated solution. Such a solution is unstable, and the excess dissolved solute (161 - 123.5 = 37.5 g) may crystallize by the addition of a seed crystal of the solute to the supersaturated solution (Figure 12) or of supersaturated solution onto the seed crystal of the... 

Figure 12 When a small seed crystal of sodium acetate is added to this supersaturated solution, the excess salt quickly crystallizes.

The formation and growth of precipitates and crystals are very important in analytical chemistry and in other areas of science. Shown in the photo is the growth of sodium acetate crystals from a supersaturated solution. Because supersaturation leads to small particles that are difficult to filter, it is desirable in gravimetric analysis to minimize the supersaturation and thus increase the particle size of the solid that is formed. 

Color plate 5 Crystallization of sodium acetate from a supersaturated solution (Section 12A-2). A tiny seed crystal is dropped into the center of a petri dish containing a supersaturated solution of the compound. The time sequence of photos taken about once per second shows the growth of the beautiful crystals of sodium acetate. 

Figure 14-S Another method of seeding a supersaturated solution is by pouring it very slowly onto a seed crystal. A supersaturated sodium acetate solution was used in these photographs.

A tiny crystal of sodium acetate, NaCHjCOO, was added to a clear, colorless, supersaturated solution of NaCH3COO. This photo shows solid NaCH3COO just beginning to crystallize in a very rapid process. 

FIGURE 12.1 In a supersaturated sodium acetate solution (left), sodium acetate crystals rapidly form when a small seed crystal is added. 

Crystallization of sodium acetate from a supersaturated solution occurs spontaneously (see p. 468). What can you deduce about the signs of AS and AHl... 

Figure 13.8 Sodium acetate crystallizing from a supersaturated solution.

When a seed crystal of sodium acetate is added to a supersaturated solution of the compound (A), solute begins to crystallize out of solution (B) and continues to do so until the remaining solution is saturated (C). 

The formation of solid sodium acetate from supersaturated sodium acetate solutions is slow by comparison and the supersaturated solutions can exist for long periods of time without a precipitate forming. Many collisions between the spherical Na ion and the linear CH3COO" (acetate) ion occur that do not result in reaction because the only collision that can result in reaction is between Na" " and the oxygen end of CH3COO" (Fig. 2-1). A collision between Na and any other part of the acetate ion cannot form a sodium acetate precipitate. 

Under suitable conditions it is possible to form solutions that contain a greater amount of solute than needed to form a saturated solution. Such solutions are supersaturated. For example, when a saturated solution of sodium acetate is made at a high temperature and then slowly cooled, all of the solute may remain dissolved even though its Solubility decreases as the temperature decreases. Because the solute in a supersaturated solution is present in a concentration higher than the equilibrium concentration, supersaturated solutions are unstable. For crystallization to occur, however, the solute particles must arrange themselves properly to form crystals. The addition of a small crystal of the solute (a seed crystal) provides a template for crystallization of the excess solute, leading to a saturated solution in contact with excess solid ( FIGURE 13.9). 

Figure 13.7 Precipitation from a supersaturated sodium acetate solution. The solution on the left wtas formed by dissolving about 170 g of the salt in 100 ml of water at 100°C and then slowly cooling it to 20°C. Because the solubility of sodium acetate in water at 20°C is 46 g per 100 ml of water, the solution is supersaturated. Addition of a sodium acetate crystal causes the excess solute to crystallize from solution. 

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