Elements, Compounds and Mixtures
6 marks · NormalA scientist wants to investigate the substances present in two unknown samples using paper chromatography. She also wants to classify the samples as elements, compounds, or mixtures.
Describe how the scientist can use the appearance of the chromatogram to decide if each sample is a pure substance or a mixture.
The scientist measures the distances on the chromatogram for one spot as follows: Distance from pencil line to spot centre: 2.4 cm Distance from pencil line to solvent front: 4.0 cm Calculate the Rf value for this spot. Show your working.
Give the definitions for (i) element, (ii) compound, and (iii) mixture.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) One spot indicates a pure substance; more than one spot indicates a mixture. · must show pure substance: one spot, mixture: more than one spot
(b) Rf = 2.4 / 4.0 = 0.60 · must show Rf formula used correctly, Calculation performed and answer given to 2 decimal places
(c) (i) Element: a substance made up of only one type of atom. (ii) Compound: a substance made from two or more elements chemically combined in fixed proportions. (iii) Mixture: two or more substances physically mixed together in any proportion, not chemically bonded. · must show definition of element, definition of compound, definition of mixture
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States a pure substance gives one spot, a mixture gives more than one.
A1 (a) · 1 mark · Links chromatogram appearance correctly to composition.
M2 (b) · 1 mark · Uses correct Rf formula (distance travelled by spot/distance travelled by solvent front).
A2 (b) · 1 mark · Calculates Rf value correctly as 0.60 (to 2 decimal places).
B1 (c) · 1 mark · Gives correct definition of 'element'.
B2 (c) · 1 mark · Gives correct definition of 'compound' and 'mixture'.
Worked solution
a) If the sample gives only one spot on the chromatogram, it is a pure substance; if there is more than one spot, it is a mixture.
b) Rf = (distance moved by the spot)/(distance moved by solvent front)
c) (i) An element is a substance made up of only one type of atom. (ii) A compound is made from two or more elements chemically combined in fixed proportions. (iii) A mixture consists of two or more substances mixed together physically, not chemically bonded.