Lesson Explainer: Filtration and Crystallization Chemistry • First Year of Secondary School

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.

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.

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.

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.

Once the resulting crystals and remaining solution have cooled down, they 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.

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.