State the meaning of the term retention factor.

Explain why the value of the retention factor for the same component can be very different if different solvents (eluents) are used for the mobile phase.

If the components of the mixture are coloured then they can be seen with the naked eye. Describe two different ways in which a chromatogram can be developed if the components are colourless.

The ratio between the distance moved by the spot and the distance moved by the solvent front / OWTTE;

Accept this expressed as a correct equation.

\({R_{\text{f}}}\) value depends on the intermolecular forces that the component has with the mobile phase compared to the stationary phase / relative attraction of the component to mobile phase compared to the stationary phase / partition of the component between the mobile phase and the stationary phase / OWTTE;

if polarity of the solvents is different the intermolecular forces/attraction to mobile phase/partition will be different / OWTTE;

Accept “Components have different solubilities in different solvents” / OWTTE.

(viewing under) ultraviolet/UV light;

(staining with) a dye/ninhydrin/potassium manganate(VII);

(exposing to) iodine (vapour);

Accept "staining with a developing (re)agent".

Do not accept just staining.

Many students could correctly define “retention factor” though they frequently used the term “solute” rather than “component” and “solvent” rather than “eluent front”. Very few candidates were capable of describing, in sufficient detail to gain the second mark, how the properties of the eluent, such as its polarity or ability to engage in particular types of intermolecular bonding, affected the relative attraction of a particular component for the stationary and mobile phases, and hence the distance it would travel relative to the eluent front; indeed many said that as the distance travelled by the eluent might vary so the Rf value would vary. About half the candidates were aware of some technique to identify colourless components.

Many students could correctly define “retention factor” though they frequently used the term “solute” rather than “component” and “solvent” rather than “eluent front”. Very few candidates were capable of describing, in sufficient detail to gain the second mark, how the properties of the eluent, such as its polarity or ability to engage in particular types of intermolecular bonding, affected the relative attraction of a particular component for the stationary and mobile phases, and hence the distance it would travel relative to the eluent front; indeed many said that as the distance travelled by the eluent might vary so the Rf value would vary. About half the candidates were aware of some technique to identify colourless components.

Many students could correctly define “retention factor” though they frequently used the term “solute” rather than “component” and “solvent” rather than “eluent front”. Very few candidates were capable of describing, in sufficient detail to gain the second mark, how the properties of the eluent, such as its polarity or ability to engage in particular types of intermolecular bonding, affected the relative attraction of a particular component for the stationary and mobile phases, and hence the distance it would travel relative to the eluent front; indeed many said that as the distance travelled by the eluent might vary so the Rf value would vary. About half the candidates were aware of some technique to identify colourless components.