C2.1 Purity and separating mixtures
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C2.1 Purity and separating mixtures
Assessable mathematical learning outcomes:
CM2.1i arithmetic computation, ratio, percentage and multistep calculations permeates quantitative chemistry
CM2.1ii provide answers to an appropriate number of significant figures
CM2.1iii change the subject of a mathematical equation
CM2.1iv arithmetic computation and ratio when determining empirical formulae, balancing equations
Assessable content statements:
C2.1a explain what is meant by the purity of a substance, distinguishing between the scientific and everyday use of the term ‘pure’
C2.1b use melting point data to distinguish pure from impure substances
C2.1c calculate relative formula masses of species separately and in a balanced chemical equation
C2.1d deduce the empirical formula of a compound from the relative numbers of atoms present or from a model or diagram and vice versa
C2.1e explain that many useful materials are formulations of mixtures (to include alloys)
C2.1f describe, explain and exemplify the processes of filtration, crystallisation, simple distillation, and fractional distillation (to include knowledge of the techniques of filtration, crystallisation, simple distillation and fractional distillation)
C2.1g describe the techniques of paper and thin layer chromatography
C2.1h recall that chromatography involves a stationary and a mobile phase and that separation depends on the distribution between the phases (to include identification of the mobile and stationary phases)
C2.1i interpret chromatograms, including measuring Rf values (to include the recall and the use of the formula)
C2.1j suggest suitable purification techniques given information about the substances involved
C2.1k suggest chromatographic methods for distinguishing pure from impure substances (to include paper, thin layer (TLC) and gas chromatography)
To investigate the ingredients for a number of food or other products labelled as ‘pure’.
Learners are requested to collect three or more labels from products claiming to be ‘pure’.
The ingredient panel should then be examined for each product and the ingredients listed in a table.
This section looks at the scientific use of the word ‘purity’.
Chemists are specific in their definition of purity being a single substance, element or compound. All pure materials (elements and compounds) have sharp and unique melting points and boiling points and these can be used to identify pure materials.
If a material is impure, then the melting point will usually be lower and less well defined (for example, salt and ice). Similarly, the boiling point will usually be higher .
While chemists are occupied with compounds and elements and their reactions, they also design mixtures that are vital to our modern society. Alloys such as steel (mainly iron and carbon) and materials like glass (mainly silicon dioxide, sodium oxide and calcium oxide) are mixtures that make up large proportions of most modern buildings. It is important to stress that these are mixtures and not compounds.
One difference between mixtures, and compounds and elements, is that mixtures can be separated. Methods such as filtration, crystallisation and simple chromatography are covered in Key Stage 3 and should be a matter of review in their explanation. More complex mixtures require more complex methods of separation such as fractional distillation, thin layer chromatography (TLC) or gas chromatography (GC).
Rf values are used in chromatographic techniques to identify pure substances, however it needs to be said that it is possible to have two Rfs the same value.
Common misconceptions or difficulties learners may have
Learners are used to seeing retail packaging that uses the word pure in numerous contexts, and therefore they confuse the word ‘pure’ with ‘un-tampered with’ or ‘natural’. They also fail to see that a solution is a mixture, especially if it only contains one solute.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
Separation techniques are used in all facets of industrial chemistry. A good foundation in how these techniques work give the learners a clear idea of why these methods are applied. Fractional distillation of crude oil is one of the most important industrial processes; a clear understanding of the separation of similar liquids by means of differing boiling points caused by differing intermolecular forces offers a number of links to other parts of the specification.
Approaches to teaching the content
Most chemicals have a small number of impurities. Chemists are often required to separate mixtures hence discussion of areas such as synthetic chemistry and forensics are useful in introducing the idea of complex methods of separation. The Practical Activity Groups (PAGs) C3 and C4 cover most of the practical methods available in schools.
To discuss the definitions given for elements, compounds and mixtures.
Complete worksheet as a group (max 4 ).
Video that can be used as a precursor to a possible practical activity.
- Watch the RSC video on the technique of measuring a melting point.
- Prepare the sample by grinding to a powder, if necessary, using a pestle and mortar.
- Fill the capillary to 2-3 mm deep.
- Place in the melting point apparatus and heat at a rate of 2-3 degrees a minute.
- Record the temperature at the first drop of liquid and at complete melting of the solid.
- Check the range against the book value for benzoic acid.
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