5 Ways To Teach Purification And Separation Of A Mixture

Separation and purification are two connected procedures in analytical chemistry; the fundamental distinction between them is that separation is the transformation of a mixture of substances into two or more distinct product mixtures, whereas purification is the removal of pollutants from an analyte sample. Analytical chemistry is a skill set that can be developed and as we proceed in this discourse, we will find out those techniques that a chemist should master to be a grounded professional in his field.

Figure 1: An image showing the analytical chemistry lab available in the purification and separation of a mixture simulation from Labster. The simulation is useful for High School and University/College courses.

Mixtures: The properties of the different kinds of mixtures reflect the properties of the compounds that make them up. As a result, from looking at the mixture alone we can start thinking of separation techniques. There are three main mixtures to be aware of during separation namely:

1. Suspensions contain solids or solid particles in a solvent. The particles are large enough so that they are insoluble in that solvent and can be separated using filtration.
2. Emulsions contain immiscible liquids, the liquids do not mix, which creates a liquid-liquid phase separation. When immiscible liquids settle, the phase separation forms a boundary layer, where the more buoyant or lighter liquid sits on top of the more dense liquid and can be separated using separating funnels.
3. Solutions are aqueous liquids that contain any number or amount of solutes uniformly dissolved in a solvent which can be separated by a range of techniques depending on the properties of the solute and solvent.

Figure 2: Image depicting suspensions, emulsions, and solutions

Read on for some thoughts on why this can be a difficult topic for teachers and students alike, five suggestions to help change that, and thoughts on why a virtual lab could make things easier.

Why purification and separation of a mixture can be complicated for students to grasp

There are three reasons, in particular, why purification and separation of a mixture can be difficult, even for the most active students. 

1. It feels abstract

Since the purification and separation of a mixture occur at the molecular level, learners can’t see or feel it. Not being able to visualize the processes and not seeing their relevance to the real world can make studying it demoralizing and make it hard for students to stay motivated.

2. It’s content-heavy

 Properties of compounds

It is important to know the properties of compounds in a mixture as each property can be linked to a purification technique.

1. Solubility: Some properties are visible to the naked eye, like insoluble compounds or solids in a mixture - known as a suspension. When a solute is soluble in a solvent then the resulting solution may change color. When a solute is soluble in water, we often show that in the chemical name.
2. Visibility: Another property visible to the naked eye is immiscibility - when two liquids do not mix and form a boundary layer. Oil and water do not mix, because the former is hydrophobic and the latter hydrophilic. While bodies of water can be immiscible because of differences in their density, temperature, and salinity.
3. Volatility/Boiling points: After simple tests or analyses, we can estimate the volatility or boiling point of a compound. Simple, organic compounds like alcohols mean they are highly volatile and so have low boiling points. This can be detected without any testing since we can smell alcohol at room temperature.
4. Other properties: More complex analyses, like NMR and mass spectrometry, can help reveal the chemical composition of compounds. From the chemical composition, we can estimate the molecular size and even predict the properties of the compounds. 

3. It's Complicated

Solid-liquid purification

Filtration is a solid-liquid purification technique used to separate solids from a liquid, in a suspension. They separate compounds based on one property, their (in)solubility in a solvent. There are two main kinds of filtration, gravity and vacuum filtration. Gravity filtration is the kind used to filter morning coffee, by pouring the coffee through the filter paper and waiting for the drink to drip and collect. Vacuum filtration is identical except the collecting flask is connected to a vacuum and your liquid is drawn far faster through the filter paper and into the flask. Vacuum filtration is generally used to speed up filtration when it is repeatedly used.

Figure 3: Image showing how to vacuum filter a suspension

Liquid-liquid purification consists of a range of techniques that separate compounds based on multiple properties, namely:

1. Visibility: Separating funnels separate organic and aqueous immiscible liquids at their boundary layer. 
2. Volatility: Simple distillation separates volatile compounds based on their boiling point. Simple distillation cannot separate compounds with similar boiling points well. 
3. Multiple properties: Chromatography is a powerful class of techniques that can separate compounds based on various properties. This includes separating by size, charge, and phase affinity. 
4. Other properties: Specialized techniques are used in a case-specific manner. For example, sublimation as a separation technique is only used for compounds that sublime (transition from solid directly to gas phase). 

5 ways to make purification and separation of a mixture a more approachable topic

With these points in mind, here are five things you can consider introducing into your purification and separation of a mixture class to make it more engaging, approachable, and enjoyable to teach for you and to learn for your students. 

1. Show the people behind the science

The career of analytical chemistry was established over a century ago. One of the first experts in chemical analysis is a Scientist from Sweden whose name is Torbern Bergman (1735-1784) he is recognized as the first to introduce an outstanding system of analysis to chemistry. Analytical chemistry and electrochemistry were propounded by German Chemists Wihelm Ostwald (1853-1932) and Walter Nernst (1864-1941). The father of modern analytical chemistry is Izaak Maurits Kolthoff whose research and teaching brought to the fore the techniques of separating, identifying, and quantifying chemical substances.

2. Relate it to the real world

 Reduction of Copper: Redox is the collective term used for reduction and oxidation. Copper can be neutral and have a 1+ or a 2+ charge. As copper reduces in charge, it becomes less polar and so less soluble in water. This is the premise behind how we precipitate copper from copper sulfate solution using precipitation and electrolysis reactions.

Precipitation: The electron configuration of copper means that it readily accepts electrons, and so is an oxidizing metal. Therefore, when copper ions are mixed with a reducing metal, they are reduced and neutralized into solid copper, precipitating it.

Figure 4: Precipitation of copper using the reducing metal, Zinc

3. Seeing is believing

Purification of solid copper

Copper forms two different types of solids. Black copper is impure, containing many metal and oxygen impurities. The shiny copper we are more familiar with is the other type. Pure metals are shiny because of the lack of impurities, meaning there is an optimal flow of electrons between the metal cations. Purification of copper from precipitation often forms black copper, because of the oxygen-rich environment and metal impurities in the zinc plate. Pure copper can be obtained using the following techniques:

• Recrystallization
• Electrolysis 
• Zone refining

4. Make it stick with word-play

Memory aids can be very helpful in understanding complex topics such as purification and separation of a mixture. Organic compounds go through a series of processes to get purified. These procedures are sublimation, crystallization, distinction, fractional distillation, vacuum distillation, differential extraction, and chromatography. Some skills needed to purify and separate the above-listed compounds are:

1. Analytical methods: this is used to determine the chemical or physical properties of a substance.
2. Procedures: these are conventional ways to perform a certain task.
3. Laboratory Equipment: a student needs to maintain laboratory equipment and instrumentation in good working order.

To commune to memory these stated skills, the aid of a pun will be needed; "AMPLE" means:

AM- Analytical Methods 

P - Procedures

LE- Laboratory Equipment.

5. Encourage the use of virtual lab simulations

A unique way to teach the purification and separation of a mixture is through a virtual laboratory simulation. At Labster, we’re dedicated to delivering fully interactive advanced laboratory simulations that utilize gamification elements like storytelling and scoring systems, inside an immersive and engaging 3Duniverse.

Check out the Labster purification and separation of a mixture simulation that allows students to learn how to use a range of techniques to separate compounds from a mixture. Using their individual properties, you will develop and execute an experimental procedure to separate each compound.