Summary. Mixtures of solids may be separated by differing solubilities of the solids. Components of a solution composed of a non-volatile solid solute and a liquid solvent can be separated by distillation. Mixtures of liquids with reasonably different boiling points can also be separated by distillation.

Explanation: Filtration works best when the solute isn’t dissolve in the solvent. For instance, sand and water can be seperate through filtration as both compounds do not dissolve with each other. However, sugar and water would not be seperated through filtration as they dissolve with each other.

Separating Mixtures

• Filtration. One of the simplest methods used to separate mixtures is filtration.
• Distillation. When one compound is dissolved in another, or when two liquids are mixed together, the most commonly used method to separate them is distillation.
• Chromatography.
• Extraction.

Crystallization is primarily employed as a separation technique in order to obtain pure crystals of a substance from an impure mixture. Another important application of crystallization is its use to obtain pure salt from seawater. Crystallization can also be used to obtain pure alum crystals from an impure alum.

Summary of Recrystallization Steps

1. Add a small quantity of appropriate solvent to an impure solid.
2. Apply heat to dissolve the solid.
3. Cool the solution to crystallize the product.
4. Use vacuum filtration to isolate and dry the purified solid.

The proper way to wash the crystals is to SHUT OFF the vacuum, add a minimum amount of cold solvent so that the crystals are barely sitting in solvent for about 5 Page 4 seconds (the solvent will not drip through quickly) and then apply the vacuum. The solvent will be sucked into the filtrate.

Recrystallization is often used as a final step after other separation methods such as extraction, or column chromatography. Recrystallization may also be used to separate two compounds with very different solubility properties.

Recrystallization. Recrystallization is the most important method of purifying nonvolatile organic solids. Recrystallization involves dissolving the material to be purified (the solute) in an appropriate hot solvent. Recrystallization does not involve breaking any chemical bonds.

Impurities can be easily removed if they are either much more soluble or much less soluble in the solvent than the compound of interest. The insoluble material is then filtered while the solution is kept hot (called “hot filtration”), and then the desired compound is crystallized and collected by suction filtration.

Why is it necessary to use only a minimum amount of the required solvent for recrystallization? Using the minimum amount minimizes the amount of material lost by retention in the solvent. Soluble impurities will dissolve in a solvent, leaving behind crystals of a pure compound.

The criteria used to choose an appropriate recrystallization solvent includes: a.) finding a solvent with a high temperature coefficient. The solvent must not dissolve the compound at low temperatures (that includes room temperature), but must dissolve the compound at high temperatures.

What makes a solvent pair too good? A solvent which is “too good” will not allow recovery of much of the compound. On the other hand, if the solvent is “too poor,” an excessively large volume of solvent would be needed.

• target compound dissolves at high temperatures, but not room temp.
• impurities must be soluble at room temp OR insoluble at high temp.
• cant react with either the target compound or the impurities.
• must be suitably volatile.

For a two-solvent recrystallization, you should have one solvent (solvent #1) in which your desired compound is soluble at the boiling point. The second solvent (solvent #2) should induce crystallization when added to a saturated solution of your compound in the primary solvent.