Lesson Explainer: Filtration and Crystallization Chemistry

In this explainer, we will learn how to separate mixtures using filtration and crystallization, decide the apparatus needed, and determine when each should be used.

We will frequently need to separate a mixture or purify a substance in the chemistry laboratory. There are a variety of techniques that we can use. Some techniques are suitable for separating certain mixtures and purifying certain substances. Other techniques are more suitable for separating altogether different types of mixtures and purifying altogether different types of substances. The filtration technique is ideal for separating mixtures that contain both an insoluble solid and a liquid. The filtration technique can be used to separate a mixture of sand and water.

Filtration is performed by pouring a mixture over a porous piece of filter paper. The porous piece of filter paper has to be placed in a funnel. The filtration technique separates solid and liquid substances based on the size of their particles. Liquid molecules and any dissolved ions are very small and they can pass through the filter paper while the solid particles cannot. The liquid that passes through the filter paper is called the filtrate, and the solid that remains in the filter paper is called the residue.

There are two filtration methods. Each has its own set of advantages and disadvantages. The first type is gravity filtration, a method that tends to be used when there is a large amount of mixture to separate. The gravity filtration method also tends to be used when we want to collect the filtrate and when the mixture needs to be filtered while it is hot.

Gravity filtration experiments can be performed by holding a funnel above an Erlenmeyer flask with either an iron ring or a clamp.

We should use a stemless funnel if we are filtering a hot mixture because crystals can form in the stem and this can prevent filtration processes.

Pieces of filter paper are always placed into the funnel before any mixtures are filtered. The filter paper can either be folded into a cone or it can be fluted before it is placed in a funnel.

There are some obvious differences between conical and fluted pieces of filter paper. Chemists tend to use fluted pieces of filter paper for gravity filtration experiments because they provide a larger surface area that the solvent can seep through. Fluted pieces of filter paper help to speed up filtration processes.

Once the filter paper has been added to the funnel, the mixture should be swirled so that the solid particulates are dispersed throughout the liquid. This will allow the solid to be poured more easily and it will help prevent solid particulates from being stuck at the bottom of the glassware.

We can then pour the mixture into the filter paper. We should pour slowly to avoid splashing and/or overflowing the filter paper. It is important that the mixture is poured into the center of fluted pieces of filter paper. The mixture should not be poured in between the folded edges and the funnel.

The solid residue will remain on the filter paper and the filtrate will pass right through.

How To: Performing Gravity Filtration

  1. Position a funnel above an Erlenmeyer flask.
  2. Fold a piece of filter paper and place it in the funnel.
  3. Swirl the flask containing the mixture to be filtered.
  4. Slowly pour the mixture into the folded filter paper.
  5. Allow the filtrate to drain and repeat until the entire mixture has been separated.

Example 1: Recognizing the Terms Given to the Solid and Liquid during Filtration

The given diagram shows the experimental apparatus for filtration.

  1. What is the name of the solid that remains in the filter paper?
  2. What is the name of the liquid that passes through the filter paper?

Answer

Part 1

Filtration is the technique of separating an insoluble solid and a liquid by pouring the mixture into a filter paper. The solid that remains in the filter paper is called the residue.

Part 2

The liquid that passes through the filter paper is called the filtrate.

The second type of filtration is vacuum filtration, a method used when we want to collect all of the solid and when there is a relatively small amount of mixture to separate. Büchner funnels are used to perform the vacuum filtration technique. A Büchner funnel serves the same purpose as a stem funnel. The main differences are that a Büchner funnel tends to be made of ceramic instead of glass and it also has a flat plate with many holes on the inside.

A circular piece of filter paper should be placed on the flat plate to cover up all the holes. The filter paper should have the same diameter as the Büchner funnel. The filter paper should be flat and it should not be bunched up at any point across the funnel surface. We can ensure that the filter paper lies completely flat over the holes by wetting the paper with a little solvent.

We will also need a Büchner flask, sometimes called a side-arm flask.

Heavy-walled tubing can be attached to the arm of the flask. The other end of the tubing is attached to a vacuum line or aspirator. The Büchner funnel should be attached to the Büchner flask using a rubber stopper, ring, or vacuum seal.

When the seal is tight and the vacuum is turned on, a strong suction will be applied to the Büchner funnel. With the vacuum on, we can pour our swirled mixture over the filter paper. The suction will pull the liquid into the Büchner flask, leaving the solid material on the filter paper. This method is much faster than the gravity filtration technique for separating mixtures because it relies on very strong suction forces instead of weaker gravity forces.

When performing vacuum filtration, we should ensure that the apparatus is secure by clamping the neck of the Büchner flask. We also need to take care not to overflow the Büchner flask. If the liquid in the Büchner flask reaches the arm, the liquid will be pulled into the vacuum line.

How To: Performing Vacuum Filtration

  1. Place a vacuum seal and Büchner funnel into a Büchner flask.
  2. Place a piece of filter paper the same diameter as the funnel into the Büchner funnel.
  3. Attach the arm of the Büchner flask to a vacuum line or aspirator with heavy-walled tubing.
  4. Turn on the vacuum.
  5. Wet the filter paper with a little solvent, ideally the same liquid as is in the mixture.
  6. Verify that the vacuum seal is good by listening for a sucking noise as the solvent is pulled into the flask.
  7. Swirl the flask containing the mixture to be filtered.
  8. Slowly pour the mixture over the filter paper until the entire mixture has been separated.

Example 2: Recalling a Method for Recovering Sand from Water

A student must separate a sample of sand from a water suspension. Which technique should the student use?

  1. Filtration
  2. Distillation
  3. Crystallization
  4. Evaporation
  5. Centrifugation

Answer

Crystallization and evaporation are separation techniques that can be used to separate a dissolved solid from a liquid. Distillation is a separation technique that can be used to separate two miscible liquids that have different boiling points. Centrifugation is a separation technique commonly used to separate very fine suspended particles from a solution. Filtration is a separation technique that can be used to separate an insoluble solid from a liquid.

Sand is insoluble in water. The particles are quite large, so centrifugation will not be the best method for separation. The technique that the student should use is filtration, answer choice A.

When a mixture contains a soluble solid and a liquid, for example, salt water, we can separate the two components by allowing the soluble solid to crystallize. In this method of separation, the liquid, also called the solvent, will be driven off by applying heat to the mixture. This means that only the soluble solid can be collected. This soluble solid can also be called the solute.

There are two methods for producing crystals. Each method has its own set of advantages and disadvantages. The first method is the evaporation technique, which is used if the solution is known to only contain one solute. The evaporation technique can also be used if the solution does not contain any impurities and we do not need to grow large regular crystals.

Evaporation is performed by first putting the test solution in an evaporating dish or evaporating basin. The solution can then be placed out in the open to allow the solvent to evaporate naturally. The solution can also be placed over a heat source to speed up the evaporation process.

The solution should never be heated too rapidly. Gentle heating helps to prevent the rapid boiling and bumping that cause some of the solution to be expelled from the dish. The solvent will begin to evaporate as the heat is applied and the solution will become more and more concentrated.

Eventually a point is reached when the solution becomes supersaturated and it can no longer hold all of the solute. At this point, small crystals will begin to form on the surface of the liquid or on the edges of the evaporating dish.

We should continue to heat the solution until the majority of the solvent has evaporated. The crystals should not be heated until they are completely dry as some crystals readily decompose upon further heating. Small crystals will line the dish, and they can be separated from the remaining solution using the filtration technique. As these crystals are formed rapidly, they will be small. In addition, if there were any impurities in the solution, they will also crystallize or become trapped within the solute’s crystal structure.

How To: Performing Evaporation

  1. Place the solution to be separated in an evaporating dish.
  2. Set the evaporating dish over a heat source.
  3. Slowly heat the evaporating dish until the majority of the solvent has evaporated.
  4. Use a filtration technique to filter the resulting crystals from any remaining solution.

Example 3: Recalling a Method for Recovering Sodium Chloride from Solution

A student dissolves some sodium chloride in water and wants to recover it. Which technique should they use?

  1. Crystallization
  2. Filtration
  3. Centrifugation
  4. Evaporation
  5. Distillation

Answer

Distillation is a separation technique that can be used to separate two miscible liquids that have different boiling points. Centrifugation is a separation technique commonly used to separate very fine suspended particles from a solution. Filtration is a separation technique that can be used to separate an insoluble solid from a liquid. Crystallization and evaporation are separation techniques that can be used to separate a dissolved solid from a liquid.

In this question, the sodium chloride is dissolved in water. The student could use either crystallization or evaporation to recover the sodium chloride. However, since there are no other solutes or impurities in the solution, evaporation will be the preferred technique as it is much faster than crystallization. The student should use evaporation, answer choice D.

The second method, the crystallization technique, is used if the solution is known to contain two or more solutes that have different solubilities. The crystallization technique should also be used if the solution contains impurities or if we want to grow large regular crystals.

The process of crystallization begins the same as evaporation. However, the evaporating dish is removed from the heat source once small crystals begin to form on the surface of the liquid or on the edges of the evaporating dish. We should then set the evaporating dish aside and allow it to continue to cool.

Less and less solute will be able to remain in solution as it cools down. The excess solute will precipitate out as large regular crystals. We can cause even more crystals to form by placing the evaporating dish in an ice bath. The image below shows some copper sulfate crystals that have been isolated from a copper sulfate solution through the crystallization technique.

Copper Sulfate Crystal

The above image shows the resulting crystals and remaining solution once they have cooled down. The crystals and solution can be separated using the filtration technique. Crystallization is an excellent way to purify the solution if it contains any impurities. When the crystals form slowly, a regular crystal structure can be achieved without trapping the impurities. The impurities will remain in the solution and they can then be filtered off.

If more than one soluble solute was present in the original solution, the less-soluble solute will precipitate out as crystals first. Multiple crystallizations may be needed to completely separate the solutes.

How To: Performing Crystallization

  1. Place the solution to be separated in an evaporating dish.
  2. Set the evaporating dish over a heat source.
  3. Slowly heat the evaporating dish until crystals just begin to form.
  4. Remove the evaporating dish from the heat and allow to cool.
  5. Use a filtration technique to filter the resulting crystals from the remaining solution.

Example 4: Recalling the Procedure for Performing Crystallization

The given diagram shows the setup for a crystallization process. When should the heat be removed from the evaporating dish?

Answer

When performing crystallization, the goal is to supersaturate a solution and then allow it to cool slowly to form large regular crystals. We heat the solution to drive off some of the solvent. We know that a supersaturated solution has been achieved when crystals just begin to form on the surface of the liquid or on the edges of the dish. We should stop heating when crystals start to form in the dish.

Example 5: Identifying a Series of Methods to Separate Sodium Chloride from Rock Salt

A mixture of sand and sodium chloride is commonly known as rock salt. Which of the following shows the correct order needed to separate sodium chloride from rock salt?

  1. Dissolution in water filtration crystallization
  2. Filtration dissolution in water distillation
  3. Filtration crystallization dissolution in water
  4. Crystallization dissolution in water filtration
  5. Dissolution in water crystallization filtration

Answer

Rock salt is a mixture of two solids: sand and sodium chloride. To separate them, we must first add water to the mixture. Sodium chloride is soluble in water while sand is not. The sodium chloride will dissociate into Na+ and Cl ions in water. We now have a mixture of sand and aqueous sodium chloride.

This mixture can be separated using filtration. In this method, the mixture is poured through a piece of filter paper. The large sand particles will be stopped by the filter paper while the water molecules and ions can travel through the filter paper.

To separate the solute, sodium chloride, from the solvent, water, we can perform evaporation or crystallization. In both of these methods, the solution is heated. The solvent (water) is evaporated off to leave behind the sodium chloride crystals.

The answer choice that shows the correct order needed to separate sodium chloride from rock salt is answer choice A, dissolution in water filtration crystallization.

Key Points

  • Insoluble solids can be separated from a liquid using filtration.
  • Gravity filtration is preferred when the filtrate is desired after separation, there is a lot of mixture to separate, or the mixture needs to be filtered while hot.
  • Vacuum filtration is preferred when the solid is desired after separation or when there is a small amount of mixture to separate.
  • Soluble solids can be separated from a liquid by crystallizing the solid.
  • Evaporation is preferred when the solution is known to contain only one solute, the solution does not contain any impurities, and we do not need to grow large regular crystals.
  • Crystallization is preferred when the solution is known to contain two or more solutes that have different solubilities, when the solution contains impurities, or if we want to grow large regular crystals.

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