Crystallization of Sodium Acetate from a Supersaturated Solution
by Oliver Seely
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An aqueous solution can be rendered supersaturated by first dissolving the solute in water at an elevated temperature using enough to give a concentration just under its solubility at that temperature. After the last of the solute crystals have dissolved the solution is cooled. The cooled solution has a concentration above the saturation point and is said to be supersaturated. Crystals do not form unless the cooled solution is disturbed in some way, most often by allowing it to come into contact with a small crystalline fragment of the substance. The model used to describe this phenomenon is that once a template of the crystalline form of the substance is made available to the supersaturated solution, spontaneous crystallization begins immediately.
Here is a supersaturated solution of sodium acetate in water..
The stopper is removed and replaced with a stopper holding a thermometer.
Often, that change in environment is enough to start crystallization, though in this case it did not. The stopper is loosened and a few seed crystals are introduced on a spatula to the top of the supersaturated solution. In this solution there is a top layer which is not supersaturated owing to some distilled water which was introduced to wash down the inner wall just prior to heating. Note that the crystals begin to grow immediately from the spot where the seed crystals fell to the point of supersaturation.
Within a few seconds, the crystals have grown almost to the bulb of the theromometer.
The crystals grow past the bulb.
Finally, the crystallization process is complete and has changed the appearance of the contents from a transparent solution to an inhomogeneous and opaque mixture.
The final temperature is more than 25 OC, above the starting point; the room temperature was close to 20 OC and the final temperature is close to 47 OC
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The term saturated solution is used in chemistry to define a solution in which no more solute can be dissolved in the solvent. It is understood that saturation of the solution has been achieved when any additional substance that is added results in a solid precipitate or is let off as a gas.
Understanding Saturated Solutions
There are many different factors that can affect whether something is a saturated solution. For example, saturation is affected by:
- The solution's temperature
- The solution's pressure
- Chemical makeup of substances involved
Ways to make a saturated solution include:
- Add solute to liquid until dissolving stops.
- Evaporate a solvent from a solution until the solute begins to crystallize or precipitate.
- Add seed crystals to a solution that is supersaturated.
Everyday Examples of Saturated
- Carbonated water is saturated with carbon, hence it gives off carbon through bubbles.
- Adding sugar to water until it no longer dissolves creates a saturated solution.
- Continuing to dissolve salt in water until it will no longer dissolve creates a saturated solution.
- The Earth's soil is saturated with nitrogen.
- Mixing powdered soap into water until it will not dissolve creates a saturated solution.
- In beer or sparkling juices there is a saturation of carbon dioxide that is let off as a gas.
- Coffee powder added to water can create a saturated solution.
- Salt added to vinegar can create a saturated solution when the salt no longer dissolves.
- Chocolate powder added to milk can create saturation at the point that no more powder can be added.
- Sugar dissolved into vinegar until it will no longer do so creates a saturate solution.
- Water can be saturated with juice powder to create a beverage.
- Milk can be saturated with flour at which point no more flour can be added to the milk.
- Melted butter can be saturated with salt when the salt will no longer dissolve.
- Bathing salts can saturate water when there is no more ability to dissolve them.
- Sugar can be added to milk to the point of saturation.
- Processed tea powders can be added to water to saturate the water.
- Protein powder could be used to create a saturated solution with milk, tea, or water.
- Laxative powders could saturate juice or water with which they are mixed.
- Cocoa powder could be mixed into water to the point of saturation.
- Sugar could be mixed into tea to the point that the tea is saturated.
- Coffee can be saturated with sugar when no more will mix in to the coffee.
Things that are insoluble in water cannot create the saturated solutions. For example, pepper and sand can not be dissolved in water and therefore cannot create a saturated solution.
However, now you have seen some examples of different ways that a saturated solution can be created and different kinds of saturated solutions.