Purity, Formulations & Chromatography Flashcards

All 23 cards in this deck

What is a pure substance in chemistry?

A single element or compound, not mixed with any other substance.

In everyday language, what does it mean to call something 'pure', e.g. pure milk?

Nothing has been added to it — it is unadulterated and in its natural state.

How do the melting and boiling points of a pure substance differ from those of a mixture?

A pure substance melts and boils at specific (fixed) temperatures; a mixture melts and boils over a range of temperatures.

What are the steps to use melting point data to decide whether a solid is pure?

e.g. a white solid melts between $52\,^\circ\text{C}$ and $58\,^\circ\text{C}$

1. See whether it melts at one specific temperature or over a range: it melts over a range, $52$–$58\,^\circ\text{C}$
2. A range of temperatures means impure; a single specific temperature means pure: so the solid is impure (a mixture)
3. If it were pure, compare its melting point with data book values to identify it.

True or false? A mixture of two pure compounds is still a pure substance.

False. A pure substance must be a single element or compound; any mixture is impure.

What is a formulation?

A mixture that has been designed as a useful product, in which each chemical has a particular purpose.

Why are the components of a formulation mixed in carefully measured quantities?

To ensure the product has the required properties.

Give four examples of formulations.

Any four of: fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods.

Why does a manufacturer always use the same proportions of yellow dye and blue dye in a green ink?

So that the shade of green is always the same — the ink is a formulation.

True or false? Every mixture is a formulation.

False. Only a mixture designed as a useful product, with each component present in a measured quantity for a purpose, is a formulation.

True or false? Paint is a formulation.

True. It is a mixture of components (pigment, binder, solvent, additives) each with a particular purpose, mixed in measured quantities.

In paper chromatography, what is the stationary phase and what is the mobile phase?

Stationary phase: the paper. Mobile phase: the solvent.

In paper chromatography, why does one dye in a mixture travel further up the paper than another?

It is more soluble in the solvent and has a weaker attraction to the paper, so it spends more time in the mobile phase.

What does separation in chromatography depend on?

The distribution of the substances between the stationary phase and the mobile phase.

What is the formula for the $R_f$ value of a spot?

$R_f = \dfrac{\text{distance moved by substance}}{\text{distance moved by solvent (front)}}$ — it has no units.

What are the steps to find the distance moved by the solvent front from an $R_f$ value?

e.g. $R_f = 0.50$ and the dye has moved $4.0$ cm

1. Substitute into $R_f = \dfrac{\text{distance moved by dye}}{\text{distance moved by solvent}}$: $0.50 = \dfrac{4.0}{d}$
2. Rearrange: $d = \dfrac{4.0}{0.50}$
3. $d = 8.0$ cm

Why can the same substance have a different $R_f$ value in a different solvent?

Because its solubility in the new solvent is different, so it moves a different distance relative to the solvent front.

True or false? A pure compound produces a single spot in all solvents.

True. A mixture may separate into two or more spots, and the number of spots can change with the solvent.

In the chromatography practical, why must the baseline be drawn in pencil and not in ink?

Pencil is insoluble in the solvent, so it will not dissolve and separate into spots of its own.

In the chromatography practical, why must the solvent level be below the baseline?

So the spots do not dissolve straight into the solvent in the container instead of travelling up the paper.

What are the steps to calculate an $R_f$ value from a chromatogram?

e.g. a spot has moved $3.0$ cm and the solvent front $12.0$ cm

1. Measure from the baseline to the centre of the spot: $3.0$ cm
2. Measure from the baseline to the solvent front: $12.0$ cm
3. Divide spot distance by solvent distance: $3.0 \div 12.0 = 0.25$ (2 s.f., no units)

Two spots on a chromatogram are very close together. Give two ways to increase the distance between them.

Allow the solvent front to travel further, and use a different solvent.

An ink contains four compounds but only two spots appear on the chromatogram. Suggest two reasons why.

Some of the compounds are colourless in solution, or some compounds have the same $R_f$ value so their spots overlap.