Evaporation, filtration and crystallisation

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David Paterson discusses ideas for enhancing practical work on separating mixtures by filtration, evaporation and crystallisation

The physical processes of separating mixtures by filtration, evaporation and crystallisation are key to many industrial and research purposes. For example, water purification, edible salt production and pharmaceutical formulation require aspects of these techniques. Research chemists preparing new substances will frequently use these processes. Substances require purification to allow for further synthetic steps and for structural analysis.

Four images in one, clockwise from top left: metallic, pharmaceutical equipment; piles of salt in a watery field; a rotary evaporator; a scientist performing a test

Source: Clockwise from top left: Dmitry Kalinovsky; Surasit Rojtrairat; Mr. 1; Dmitry Kalinovsky / Shutterstock.com

Industrial and research uses of evaporation, filtration and crystallisation. Clockwise from top left: filtration in water purification; evaporation in salt production; a rotary evaporator in a chemical research laboratory; pharmaceutical formation

Underlying principles

The science of separating substances is based on the principles of purity, physical properties and solutions. For example, chromatography is a separation process based on solubility¹ and distillation is based on boiling point.² This article looks at separation by particle size (filtration) and removal of volatile solvents (evaporation and crystallisation).

Underlying all separation techniques are the concepts of purity and the difference between mixtures and compounds. You can discuss purity using everyday drinks as a context. Students could analyse the labels of bottled water or juices that claim to be ‘100% pure’ to show they contain various substances. Among other components, mineral waters contain varying levels of dissolved salts, and orange juices contain sugars, protein and vitamin C.

Flow diagram showing the progression of concepts involved in separation by evaporation, filtration and crystallisation

Source: Royal Society of Chemistry

The key concepts involved in separation by evaporation, filtration and crystallisation

Practical activities to help students distinguish mixtures from compounds include the classic iron sulfide synthesis reaction.³ The reaction of aluminium with iodine also provides a memorable demonstration.⁴ Students could repeat the simple separations of mixtures they have carried out at primary or 11–14 level. For example, separating iron filings from sand⁵ or breakfast cereal.⁶

You can easily demonstrate the importance of particle sizes by sieving a mixture of rice and caster sugar. Or, show the volatility of substances by using a few drops of propanone on a watch glass in different environments.⁷

Use the context of the formation of saturated solutions of ammonium chloride to discuss the concepts of solution and saturation.⁸ Try this activity both qualitatively and quantitatively, introducing concentration and solubility calculations in later years. The Chemical misconceptions resources⁹ cover the differences between elements, compounds and mixtures.

A relevant context to engage students, and link to social and economic aspects of chemistry, is the separation of metals ores. Rocks are mixtures of different minerals, and must be separated to concentrate and eventually purify the desired metal. Physical processes such as crushing, sifting and flotation separate the components.¹⁰ The recycling industry uses similar processes to separate metals, glasses and plastics in household waste,¹¹ and when recycling cars.

Techniques through the curriculum

Students learn the techniques of evaporation, filtering and crystallisation from primary school through to post-16 education.

Students learn the techniques of evaporation, filtering and crystallisation from primary school through to post-16 education (see table). Download a detailed table of links to the national curriculum and exam specifications from the Education in Chemistry website: rsc.li/EiC418sep2.

A progression of ideas related to evaporation, filtering and crystallisation
Ages 5–7 (Key Stage 1)	Ages 7–11 (Key Stage 2)	Ages 11–16 (Key Stages 3 and 4)	Ages 16–18 (Key Stage 5)
Separation with sieves Formation of limescale (dissolved solids)	Rock cycle – formation of sedimentary rocks Water cycle – filtering of water through rocks	Filtering Evaporating Crystallising	Recrystallising

Specification links

Download a table of links to exam specifications and the English national curriculum (MS Word or pdf)

Evaporation

Evaporation requires heat (or air movement above the sample) to drive off a volatile solvent. If the substance is a solid mixed in a solvent, begin by filtering (or decanting). If the substance is dissolved in a solvent, then use crystallisation.

There are various techniques for heating a substance. For example, direct heating with a Bunsen burner or a sand bath, or by placing an evaporating basin over a beaker of heated water. Students could discuss and evaluate the different methods. For example, direct heating is quicker, but can superheat the solution, potentially degrading the solute and leading to spitting of the solid. You can make links to the difference between distillation via direct heating and steam distillation, and the use of the latter in separating fragile components.²

Students could annotate diagrams of the different methods to consolidate their understanding and help them learn the names and diagrams of the various laboratory apparatus.

Download a set of labelled apparatus diagrams and a worksheet without labels for use in your classroom from the Education in Chemistry website: rsc.li/EiC418sep2.

Download a set of labelled apparatus diagrams (MS Word or pdf) and a worksheet without labels for use in your classroom (MS Word or pdf).

Having a set of questions ready to ask can help focus students on the purpose and practicalities of the techniques (see table 1).

Having a set of questions ready to ask can help focus students on the purpose and practicalities of the techniques.

Table 1: suggested questions to prompt student thinking during separation activities
Technique	Possible questions
Evaporation	What observations tell you the evaporation is complete? What is the best heat source for this mixture? How does this evaporation fit into a larger separation procedure (eg filtration/crystallisation)?
Filtration	Why is the solid residue retained on the filter paper? How successful has the filtration been (how clear is the filtrate)? What is the advantage of a fluted filter paper? Is the residue fully purified once all the filtrate has passed through the filter paper?
Crystallisation	Where and why do crystals first form in the solution? What factors affect the size and quality of the crystals formed? Have solvent molecules formed part of the crystal structure?

Filtration

Filtration requires various pieces of glassware, which can lead to student confusion about what needs placing where, and what is poured where and when. It is worth re-demonstrating and explicitly naming the apparatus to reinforce this important knowledge. Most students will be able to carry out filtration without further guidance, although there is a range of common issues (see table).

Table 2: common issues with separation techniques
Issue	Suggested solution
Slow filtering	Use fluted filter paper Use filtration under reduced pressure Use a different grade filter paper (coarser)
Slow filtering (due to poor reduced pressure)	Check water pressure – use faster taps if available Check efficiency of pumps – check for leaks/service the pumps
Solid in filtrate – due to a hole in the filter paper (usually the student has poked the paper with a glass rod), mixture poured in too quickly (over-topped the filter paper), or filter paper not adhered to base of Buchner funnel	Use a different grade filter paper (finer) Pre-wet the paper with solvent to adhere it to the Buchner funnel

Crystallisation

Crystallisation occurs when the solution solvent evaporates, and the concentration of the solute reaches saturation point. At this stage, the solute begins to precipitate out of solution. Under the right conditions, generally slow evaporation and a clear solution, the solute will crystallise. You can easily demonstrate the principles of crystallisation with saturated solutions of sodium acetate.¹² Teachers commonly use this activity to model the freezing process, and it helps tackle misconceptions around the energetics of freezing, particularly the exothermic nature of freezing (bonds being formed). A similar activity suitable for a student practical is freezing super-cooled sodium thiosulfate.¹³

Recrystallisation is a key practical skill required at A-level. Benzoic acid is a useful substance to demonstrate and practise the technique. Benzoic acid will dissolve in hot water and crystallise when the water cools, potentially reducing the need to carry out recrystallisation using organic solvents.¹⁴ This can be particularly useful if you have limited access to fumehoods.

You can put recrystallisation in the wider context of organic synthesis by synthesising and purifying aspirin¹⁵ or paracetamol.¹⁶ Use videos to give your students some pre-lab support with the technique.¹⁷

Crystallisation could form a project either for extension within class, or perhaps as an activity for a science club. The Royal Society of Chemistry ran a global experiment in 2014 looking at the art of crystallisation. The project suggests various different salts, including potassium nitrate and alum. The resources include a useful crystal shape classification.¹⁸ Further ideas and resources are available from the British Crystallographic Association.¹⁹

Crystallisation could form a project either for extension within class, or perhaps as an activity for a science club. The Royal Society of Chemistry ran a global experiment in 2014 looking at the art of crystallisation. The project suggests various different salts, including potassium nitrate and alum. The resources include a useful crystal shape classification.¹⁸Further ideas and resources are available from the British Crystallographic Association.¹⁹

Bring techniques together in context

In reality, most separation techniques are rarely used in isolation. Separation usually involves multiple steps and techniques to obtain a pure product. However, you can use specific contexts to show how the individual techniques are optimised to obtain the desired results.

The production of sweets is a good example.²⁰ Aqueous sugar solutions at different concentrations have different properties. As a sugar solution is boiled, the water evaporates and the concentration of the sugar increases. As the boiling point is dependent on sugar concentration, the temperature of the solution increases steadily as concentration increases, from the ‘soft ball’ (fudge) concentration (85%) to ‘hard crack’ (toffee, 99%). When a saturated solution of sugar is cooled slowly, sugar crystals will form. The initial crystallisation is slow, but you can speed it up by adding a seed crystal suspended in the solution by a cotton thread. Once crystallisation has started, it proceeds quickly over the next couple of days.²¹

A more complex context, but one that can lead to many interesting discussions, is chocolate. Multiple crystal forms exist, providing a complex phase diagram and many extension opportunities for keen or high attaining students.²² The overlap of chemistry, cooking and business makes studying chocolate a potentially interesting cross-department project.²³

Bringing techniques together in one activity

One large bowl with blue copper sulfate crystals, one watch-glass with smaller crystals

Source: David Paterson

Comparison of crystallisation – in the left hand evaporating basin crystals took two days to completely form. Right hand watch-glass (microscale) – crystals completely formed within ten minutes

Making hydrated copper sulfate crystals from copper oxide and sulfuric acid requires all of the filtration, evaporation and crystallisation techniques.²⁴ React copper oxide with hot sulfuric acid. Filter the mixture to remove unreacted copper oxide, then heat the filtrate to evaporate off about half the water. Leave the concentrated solution in a warm spot to allow the blue hydrated copper sulfate solution time to slowly crystallise. Filter and dry the crystals.

A glass vial with blue liquid in a sand bath

Source: David Paterson

Microscale synthesis of copper sulfate – copper oxide and sulfuric acid in a glass vial in a sand bath

There are some issues with the traditional way this practical is carried out,²⁵ including the need to heat large volumes of acid, the slow evaporation step and slow crystallisation. A microscale alternative allows students to get from reagents to crystals within one lesson.²⁶ Heat the sample using a sand bath, both to increase the rate of the reaction and also to evaporate some solvent from the copper sulfate solution. Filter the sample through mineral wool in a syringe. Aside from the speed advantages, students can work individually, giving them time to focus on improving their manual dexterity.