In our daily life, we find different kinds of substances. Either they are mixtures or pure substances. Mixtures are made up of two or more two substances. Some mixtures are water-soluble while some of them are not. For example, the salt solution is a water-soluble mixture whereas a mixture of sand and iron nails is a water-insoluble mixture. The iron nails and sand mixture can be easily separated using magnetic separation methods. But in a salt solution, salt and water cannot be separated so easily. The separation techniques like condensation and evaporation are used to extract water and salt separately. Evaporation, is the process by which an element or compound transitions from its liquid state to its gaseous state below the boiling point. Some common examples of evaporation are discussed below: Condensation is the process where the vapour or gaseous state of water is changed to a liquid state. The formation of clouds takes place as a result of condensation. This process is measured by Psychrometry. It measures condensation rate by evaporating into the air moisture at different atmospheric temperatures and pressures. Let us find how evaporation and condensation help in separating salt and water from salt solution with the help of an experiment. Let us take a beaker full of salt solution. Heat it at a high temperature so that water present in the solution evaporates and forms vapours leaving the salt crystals behind. This process of conversion of water into water vapour is known as evaporation. At the top of the beaker at some distance keep a plate so that water condenses. Make the surface of the plate inclined such that condensed water falls on the other beaker kept below. This process of conversion of water vapour into water is known as condensation. Water Cycle:Let us understand the concept of evaporation and condensation in detail with another example of the water cycle process in the atmosphere. During the rainy season, we see that rainwater is collected on top of the roof and corners of the road. But after a few days, it disappears. Now the question is where does it go? Rainwater is heated with the help of sunlight. Water converts into vapour in the air by the process of evaporation. After some time, water vapour in the air gets cooler and converts into water droplets, like clouds. This process is known as the condensation of water vapour. When air cannot hold more amount of water i.e. clouds get heavy, the water falls back on the earth’s surface as rain. This is known as precipitation. The rainwater cycle is a continuous cycle.  To learn more about the separation techniques for water-soluble substances and the rainwater cycle, download BYJU’S – the learning app. Recommended Videos
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out of 0 arewrong out of 0 are correct out of 0 are Unattempted 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.
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.
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.
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.
Specification linksDownload a table of links to exam specifications and the English national curriculum (MS Word or pdf) EvaporationEvaporation 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
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.
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).
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.
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
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.
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 contextIn 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
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
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
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.
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.
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.
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. References
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