Purity and separating mixtures (C2.1) | OCR GCSE Combined Science A (Gateway Science) — Chemistry — Higher (J250)

C2.1 Purity and separating mixtures

Summary In chemical terms elements and compounds are pure substances and mixtures are impure substances. Chemically pure substances can be identified using melting point. Many useful materials that we use today are mixtures. There are many methods of separating mixtures including filtration, crystallisation, distillation and chromatographic techniques. Underlying knowledge and understanding Learners should be familiar with the concept of pure substances. They should have met simple separation techniques of mixtures: filtration, evaporation and distillation. The identification of pure substances in terms of melting point, boiling point and chromatography will also have been met before.

Common misconceptions Learners commonly misuse the word pure and confuse it with natural substances or a substance that has not been tampered with. They think that when a substance dissolves that the solution is pure and not a mixture.

Reference	Mathematical learning outcomes	Maths skills
$CM2.1i$	$arithmetic computation, ratio, percentage and multistep calculations permeates quantitative chemistry$	$M1a, M1c, M1d$

Learning outcomes	To include	Maths	Working scientifically	Practical suggestions
$C2.1a explain what is meant by the purity of a substance, distinguishing between the scientific and everyday use of the term ‘pure’$			$WS1.4a$	$Purification of compounds. (PAG C3, PAG C4)$
$C2.1b use melting point data to distinguish pure from impure substances$		$M1a, M1c, M1d, M2a$		$Measurement of melting point.$
$C2.1c calculate relative formula masses of species separately and in a balanced chemical equation$	$the definition of relative atomic mass, relative molecular mass and relative formula mass$	$M3b, M3c$	$WS1.3c, WS1.4c$
$C2.1d deduce the empirical formula of a compound from the relative numbers of atoms present or from a model or diagram and vice versa$		$M3b, M3c$	$WS1.1b, WS1.4a$
$C2.1e explain that many useful materials are formulations of mixtures$	$alloys$
$C2.1f describe, explain and exemplify the processes of filtration, crystallisation, simple distillation, and fractional distillation$	$knowledge of the techniques of filtration, crystallisation, simple distillation and fractional distillation$		$WS1.2b, WS1.2c, WS2a, WS2b$	$Separation of mixtures and purification of compounds. (PAG C3, PAG C4) Distillation of mixtures (PAG C3)$
$C2.1g describe the techniques of paper and thin layer chromatography$	$using aqueous and non-aqueous solvents and locating agents$		$WS1.2b, WS1.2c, WS1.4a, WS2a, WS2b$	$Paper or thin layer chromatography. (PAG C2)$
$C2.1h recall that chromatography involves a stationary and a mobile phase and that separation depends on the distribution between the phases$	$identification of the mobile and stationary phases$		$WS1.4a$
$_{f}$	$the recall and the use of the formula$	$M3b, M3c$	$WS1.3c, WS1.4a$
$C2.1j suggest suitable purification techniques given information about the substances involved$
$C2.1k suggest chromatographic methods for distinguishing pure from impure substances$	$paper, thin layer (TLC) and gas chromatography$		$WS1.4a$	$Using chromatography to identify mixtures of dyes in an unknown ink. (PAG C2)$

Learning outcomes

To include

Maths

Working scientifically

Practical suggestions

C2.1a explain what is meant by the purity of a substance, distinguishing between the scientific and everyday use of the term ‘pure’

WS1.4a

Purification of compounds. (PAG C3, PAG C4)

C2.1b use melting point data to distinguish pure from impure substances

M1a, M1c, M1d, M2a

Measurement of melting point.

C2.1c calculate relative formula masses of species separately and in a balanced chemical equation

the definition of relative atomic mass, relative molecular mass and relative formula mass

M3b, M3c

WS1.3c, WS1.4c

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

M3b, M3c

WS1.1b, WS1.4a

C2.1e explain that many useful materials are formulations of mixtures

alloys

C2.1f describe, explain and exemplify the processes of filtration, crystallisation, simple distillation, and fractional distillation

knowledge of the techniques of filtration, crystallisation, simple distillation and fractional distillation

WS1.2b, WS1.2c, WS2a, WS2b

Separation of mixtures and purification of compounds. (PAG C3, PAG C4) Distillation of mixtures (PAG C3)

C2.1g describe the techniques of paper and thin layer chromatography

using aqueous and non-aqueous solvents and locating agents

WS1.2b, WS1.2c, WS1.4a, WS2a, WS2b

Paper or thin layer chromatography. (PAG C2)

C2.1h recall that chromatography involves a stationary and a mobile phase and that separation depends on the distribution between the phases

identification of the mobile and stationary phases

WS1.4a

_{f}

the recall and the use of the formula

M3b, M3c

WS1.3c, WS1.4a

C2.1j suggest suitable purification techniques given information about the substances involved

C2.1k suggest chromatographic methods for distinguishing pure from impure substances

paper, thin layer (TLC) and gas chromatography

WS1.4a

Using chromatography to identify mixtures of dyes in an unknown ink. (PAG C2)

Purity and separating mixtures

Next Steps

Learning Objectives