1.3  Compounds and Mixtures

Understanding the difference between compounds and mixtures is very important in understanding how to separate things.

Compounds

In the previous chapter we learnt about elements and how they are made up of a single kind of atom. What if these different elements react together? They chemically combine to form a new substance known as a compound. These compounds have different properties from their parent elements because the new composition is completely different. For example: a Carbon atom and  2 Oxygen atoms react together to form carbon dioxide, the byproduct of respiration.

 

Now O (oxygen) is an element, but what about O2  ? Is it an element or a compound? It is neither. It's just a molecule. It is not an element because it can still be broken down to get two oxygen atoms. It is not a compound because it doesn't consist of two or more different kinds of atoms. Therefore, all compounds are molecules;however, all molecules are not compounds.

O

O

H

Water Molecule

C

C

C

C

C

Diamond Molecule

The above molecule is water. A hydrogen atom bonded together to two oxygen atoms. It is a compound You may notice that the atoms are not really touching each other. This is because even though the atoms are attracted to each other their electron clouds are repelling the atoms. This is will be explained in 'Types of Bonding'.

Next to it is a molecule of diamond, one of the strongest substances found naturally on earth. However, this not exactly a compound since it is made up of only one kind of atom. It is not an element either since the structure has 5 atoms.

In compounds the ratio of atoms in each molecule is fixed. Water will always have one hydrogen atom and two oxygen atoms. Since these are combined chemically, separating them using physical methods is next to impossible. Chemical methods which can break strong chemical bonds will have to be used to separate the components of compound molecules. These will be discussed later on.However, for mixtures, separations are very simple.

Mixtures

The components in mixtures, unlike the atoms or molecules in compounds, are not chemically combined. In fact they are made from elements or compounds that undergo a physical change. For example mixing water (a liquid compound) with sugar (a solid compound) will give you a solution which is a simple mixture.

What is a physical change? This is a change that only affects the appearance of the element or compound in the mixture. For example, ice turning to water and water turning to water vapor are physical changes. Although the state of matter is changing, the chemical composition of the compound,water, is still the same. Physical changes only affect the physical properties of the substance; its mass, shape, solubility, state of matter, etc.

There are different types of mixtures but the main one we will talk about is solution. A solution is a type of mixture that consists of a solvent and solute. Let refer to the example above. The solvent is the medium in which you are going to dissolve a substance, in this case water. Solute is the substance you want to dissolve, in this case sugar.

 

Assume they are water molecules

Assume they are sugar molecules

Now, lets us try to understand how the water molecules and sugar molecules form a solution. 

1

2

3

4

One sugar grain

In reality each tiny sugar grain contains many millions of molecules of sugar bonded together.

1. Let's assume the cluster of 10 black molecules represents one sugar grain.

 

 

 

2. When you add it into water, the attractive forces between the sugar molecules weaken because they are more attracted to water molecules than each other.

3. Gradually the sugar molecules separate from each other and are surrounded by water molecules. 

4.When they uniformly distribute themselves in the solvent you have a solution that is clear since the grain of sugar that was once visible have separated into individual molecules that are invisible to the naked eye.

What if you keep adding sugar? Will the sugar molecules continue breaking away to form bonds with water. Well, there is a limit to how much sugar you can dissolve in water.

5

6

7

5. As you keep adding sugar, it will dissolve continuously by uniformly spreading its molecules in the solvent.

 


 

6. However, as sugar molecules start crowding in the solution, attraction between sugar molecule and sugar molecule is increasingly preferred over attraction between water molecule and sugar molecule. Therefore, more and more sugar molecules slowly join to form clusters.

7. When the clusters become large enough it becomes visible and you can see sugar settling on the bottom.This point where solute no longer dissolves in the solvent is known as the saturation point.

Increasing temperature can increase the saturation point. In other words, you can dissolve more sugar much easily in a cup of tea than in a cold glass of juice. Therefore, saturation point changes with temperature.

As stated earlier there are different types of mixtures. Solutions, suspensions and colloids. I only focused on solutions because that is the scope of IGCSE Chemistry. However, I will explain briefly so that you don't confuse them.

Solution as you know by now is clear, the solute particles are small and the components stay mixed.

A suspension has a solute particles that are larger and they don't dissolve; therefore, these mixtures are not clear and the solute eventually settles at the bottom because its heavy and big. An example is sand in water.

A colloid is actually between a solution and a suspension. It has solute particles that are larger than in solution but smaller that suspensions. Therefore, colloids are also not clear; however, unlike suspensions the solute doesn't settle because it dissolves. An example is milk.

Impurities

As stated earlier, mixtures are much easier to separate than compounds. Since a mixture is a result of a physical change, you can separate them by using changes that affect their physical properties.

Physical properties like boiling and melting points and solubility are very important when talking about separation. Let's briefly discuss how they are affected. Consider a pure substance. A pure substance is a substance that has no impurities; only consists of a one type of a compound or element.

 

Glass of pure water only has water molecules without any impurities. But is it possible to have water that is a 100% pure? Its actually very difficult to reach 100 % purity. Distilling water can help you reach a higher purity but it is still not 100% pure. Besides, drinking 100 % pure water is not very healthy because some minerals that are dissolved in water are very beneficial to our health. However, when dealing with pharmaceuticals for example purity is very important. Impurities could lead to chemical reactions that could potentially create toxic compounds in the body.

 

So now that we know what purity is, how does it affect the boiling and melting point of substances. We all know that water boils at 100 C and ice melts at 0 C. What happens if we dissolve salt, an impurity, in water?

 

The boiling point rises because salt molecules form bonds with water. These bonds require more energy to be broken for water to boil. Melting point decreases because you are going to have to take away more energy to freeze it solid. This why salt is dispersed on sidewalks and roads to reduce the formation of ice during winter. The more salt you add the higher the boiling point and lower the melting point. 

As you can see, the physical properties of one of the components is affected by the other and vice versa. If we focus on physical properties of one of the components and introduce a stimulus that only affects that, we can make the components in the mixture to separate as shown in the next section.

Separation Methods

Separating impurities from mixtures is relatively easy. It can be demonstrated even in a school lab.

The following are the separation methods that will be discussed:

  1. Filtration

  2. Crystallization

  3. Evaporation

  4. Simple Distillation

  5. Fractional Distillation

  6. Paper Chromatography

Note that they all use mechanism that take advantage of each of the components'  physical properties.

Filtration
 

This is a separation methods used to separate a solids from a liquid. This is a simple method where you can just use a filter paper to separate the solids from the liquid. When separating fine solids filter that has finer pores can be used.

However, can you separate sugar from water using this method? No because the water molecules and sugar molecules are attracted to each other (dissolved); therefore the solution will just pass right through. 

This method can only be used to separate suspensions like sand and water or chalk and water where the solids have not dissolved. The solids left behind in the process is known as residue while the filtered liquid is called the filtrate.

Filtrate

Residue

Crystallization

This method can be used to separate the solute from solvent in a solution. It takes advantage of the saturation point of a solution.

 

Lets take a sugar solution. If we heat it in an evaporating basin or beaker, water will gradually evaporate. This means that the amount of solvent reduces while the amount of solute stays is the same. Therefore, eventually, it will reach saturation point. This can be tested for by dipping a glass rod in the hot solution. If crystals form on the rod when it cools the solution is at saturation point. Then ,if we let the solution at saturation point cool, crystals will start to form. The crystals can then be separated from the remaining solution by filtration​.

Evaporation

This method is similar to the crystallization method; however, in this case you let all the solvent evaporate to leave behind the solute. This is useful in situations where the saturation point of the solution does not change noticeably when increasing temperature. The only yeild in this method is the solute.

Simple Distillation

This method is similar to evaporation but in evaporation, you are not able to retain the solvent. In this case all the components can be collected. When you heat the boiling flask containing the solution it will eventually reach its boiling point. The solvent will turn to gas and as it evaporates it will be cooled down by the condenser and collected in a beaker. The solute will be left over in the boiling flask.

Fractional Distillation

This method is similar to simple distillation. In this case you are separating a mixture of liquids which relatively difficult compared to the previous cases . To separate them you would have to know their respective boiling points.

 

Let's take a mixture of water and ethanol as an example. Water's boiling point is 100 C; whereas, ethanol boils at 78 C. With the help of a thermometer, if we can maintain the fractionating column at a temperature of 78 C, we can boil off the ethanol. As the sample in the flask is heated, higher concentration of ethanol vapor along with a lower concentration water vapor will start rising up the column. As water vapor touches the glass beads it will condense because the beads are cooler (less than 100 C) and further away from the heat source. But the ethanol vapor will continue rising because the beads are at a temperature of about 78 C, its boiling point. Eventually it will be condensed in the condenser and flow into the beaker. The remaining liquid in the flask will be a high concentration of water.

Fractional distillation is widely used in the oil industry to separate crude oil into its various components

Paper Chromatography

Paper chromatography is dependent on solubility of the components in the mixture. 

 

It uses a solvent  to dissolve the sample being analyzed. The solubility of each component in the mixture is different; therefore, some of the components will dissolve and form stronger attractions with the solvent than the others.

 

First, drops of the sample is placed on a chromatography paper. The paper is then placed in a beaker containing the solvent making sure that the solvent level is below the level of drops of sample. As the solvent travels up along the paper it will come in contact with the sample. The sample will dissolve in the solvent. As the solvent continues travelling, the dissolved components will move up together. Components with higher solubility will travel further with the solvent than those with lower solubility; thereby separating from each other. The paper that is now displaying the separated components is called a chromatogram.  

Paper chromatography is used as teaching tool to explain how solubility can be used to separate substances. Thin layer chromatography is a more practical derivative of paper chromatography. It works like paper chromatography but instead of paper, a rigid medium like metal or glass coated in an adsorbent substance is used. This gives better resolutions and separations.

Chromatography is used when mapping DNA, in forensics and clinical diagnoses, and in the food industry to detect impurities and maintain quality. It is also used to analyze natural dyes. 

Solvent Front

Sample Spot Level

After carrying out the experiment and getting a chromatogram, how do we quantify the results?

 

For this purpose the retardation factor, Rf, is used. R = Distance moved by the component / Distance moved by the solvent front. Solvent front is the level to where the solvent has moved up to at the end of the experiment.The distance should be measured from where the original sample spot was placed on the paper.

 

These Rf values can now be assigned to each of the components and compared.

End

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