Lab Study Guide - Lowest highest boiling point BA...

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A B Lowest highest boiling point Crystallization Crystallization: is a slow selective formation of the crystal framework resulting in a pure compound o Precipitation: rapid formation of a solid with several impurities trapped within the solid’s crystal framework o main difference: unable to separate the impurities from the pure compound and the crystal lattice will not form (precipitation is much faster) recrystallization relies on the property that as the temperature of a solvent increases, the solubility of the compound in that solvent also increases Steps of Recrystallization: 1. Find a suitable solvent for the recrystallization 2. Dissolve the impure solid in a minimum volume of hot solvent 3. Remove any insoluble impurities by filtration 4. Slowly cool the hot solution to crystallize the desired compound from the solution 5. Filter the solution to isolate the purified solid compound 4 Properties of Choosing a Good Solvent: 1. the compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature – very soluble at high temps. and low solubility at low temp. [will crystallize] (impurities are exactly the opposite) 2. the unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent 3. the solvent should not react with the compound being purified
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4. the solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized - more volatile Methods to Induce Crystallization: o Put some of the pure compound into the mixture as a seed crystal o Scratch the glass with a glass stirring rod Melting Point Solid → liquid (use a Meltemp) 1. Can be used to identify a compound – it is a characteristic property 2. Able to establish the purity of a compound o Small range (less than 3 ° = melting range) o Increase in melting point Effect of impurities: o Decrease melting point o Broadens the melting point range Mixed melting point: 50:50 (pure compound: your sample) mixture and take the melting point o The melting point should be no higher or lower – will give a sharp range → indentify the compound used Melting range: the first point is the temperature at which the first drop of the liquid phase appears in the sample and the second point is the temperature at which the last bit of solid melts and the sample becomes a clear liquid Distillation
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Distillation: separation of components of a mixture based on boiling points Simple vs. Fractional Distillation
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Lab Study Guide - Lowest highest boiling point BA...

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