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Separation Of The Components Of A Mixture ( Practicum Lab )

Objective

To become familiar with the methods of separating substances from one another using decantation, extraction, and sublimation techniques.


Apparatus and chemicals 

-Balance
-Burner and hose
-Clay triangles
-Evaporating dishes
-Glass stirring rod
-50 or 100 ml graduated cylinder
-Iron rings
-Ring stands
-Tongs
-Watch glass
-Unknown mixture of sodium chloride, ammonium chloride, and silicon dioxide

Theory

Materials that are not uniform in composition are said to be impure or heterogenous and are called mixtures. Most of the materials we encounter in everyday life , such as cement, wood. and soil, are mixtures. When two or more substances that do not react chemically are combined , a mixture results. Mixtures are characterized by two fundamental properties : First,each of the substances in the mixture retains its chemical integrity ; second , mixtures are separable into these components by physical means. If one of the substances in a mixture is preponderant that is, if its amount far exceeds the amount of the other substances in the mixture-then we usually call this mixtures an impure substance and speak of the other substances in the mixture as impurities.

The preparation of compounds usually involves their separation or isolation from reactants or other impurities . thus, the separation of mixtures into their components and the purification of impure substances are frequently encountered problems. you are probably aware of everyday problems of this sort. for example, our drinking water usually begins as a mixture of silt , sand, dissolved salts, and water. Since water is by far the largest component in this mixture, we usually call this impure water. How do we purify it ? The separation of the components of mixtures is based upon the fact that each component has different physical properties. The components of mixtures are always pure substances , either compound or elements, and each pure substance possesses a unique set of properties. The properties of every sample of a pure substance are identical under the same conditions of temperature and pressure. This means that once we have determined that a sample of sodium chloride ( NaCl ) is water soluble and a sample of silicon dioxide ( SiO2 ) is not , we realize that all samples of sodium chloride are water soluble and all sample of silicon dioxide are not.

Likewise, every crystal of a pure substance boils at a specific temperature. and at a given pressure , every pure substance boils at specific temperature.

While there are numerous physical properties that can be used to identify a particular substance , we will be concerned in this experiment merely with the separation of the components and not with their identification. The methods we will use for the separation depend upon differences in physical properties, and they include the following :

1. Decantation. This is process of separation of a liquid from a solid ( sediment ) by gently pouring the liquid from the solid so as not to disturb the solid.

2. Filtration. This is the process of separating a solid from a liquid by means of a porous substance , a filter , which allows the liquid to pass through but not the solid. Common filter materials are paper, layers of charcoal , and sand. Silt and sand can be removed from our drinking water by this process.

3. Extraction . This is separation of a substance from a mixture by preferentially dissolving that substance in a suitable solvent. by this process compound is usually separated from an insoluble compound.

4. Sublimation. This is the process in which a solid passes directly to the gaseous state and back to the solid state without the appearance of the liquid state. Not all substances possess the ability to be sublimed , Iodine , naphthalane, and ammonium chloride are common substances that easily sublime.

The mixture that you will separate contains three components : NaCl, NH4Cl, and SiO2. Their separation will be accomplished by heating the mixture to sublime the NH4Cl , extracing the NaCl with water , and finally drying the remaining SiO2.

Procedure

Carefully weigh a clean , dry evaporating dish to the nearest 0,01 g. Then obtain from your instructor a 2-3 g sample of the unknown mixture in the evaporating dish. Weigh the evaporating dish containing the sample and calculate the sample weight.

Place the evaporating dish containing the mixture on a clay triangle , ring, and ring stand assembly . Heat the evaporating dish with a burner until white fumes are no longer formed. Heat carefully to avoid spattering , especially when liquid is present. After the first 10 min remove the flame and gently stir the mixture with your glass stirring rod, then apply the heat again.

Allow the evaporating dish to cool until it reaches room temperature and then weigh the evaporating dish with the contained solid. Never weigh hot or warm objects ! The loss in weigh represent the amount of NH4Cl in your mixture , calculate this.

Add 25 ml of water to the solid in this evaporating dish and stir gently for 5 min. Then weigh another clean, dry evaporating dish and watch glass . Decant the liquid carefully into the second evaporating dish , which you have weighed , being careful not to transfer any of the solid into the second evaporating dish as before. Repeat with still another 10 ml of water. This process extracts the soluble NaCl from the sand. You now have two water. the remaining problem is to recover these two substances so that they can be weighed.

Place the evaporating dish containing the sodium chloride solution carefully on the clay triangle on the ring stand. Begin gently heating the solution to evaporate the water. Take care to avoid boiling or spattering . while the water is evaporating you may proceed to dry the SiO2 in the second evaporating dish as explained in the next paragraph, if you have another bunsen burner available. when you have dried the sodium chloride completely , no more water will condense on the watch glass , and it , too , wil be dry. Let the evaporating dish and watch glass cool to room temperature and weigh them. The difference between this weigh and the weigh of the empty evaporating dish and watch glass is the weigh of NaCl. Calculate this weight.

Place the evaporating dish containing the wet sand on the clay triangle on the ring stand and cover the evaporating with a clean dry watch glass . Heat slowly at first until the lumps break up and the sand appears. Then heat the evaporating dish to dull redness and maintain this heat for 10 min. Take care not heat and let the dish cool to room temperature . weigh the dish after it has cooled to room temperature . The difference between this weight and the weight of the empty dish is the weigh . Calculate this weigh.

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