Methods of Separating & Purifying Substances Flashcards

All 35 cards in this deck

In chemistry, what is a pure substance?

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

What is a mixture?

Two or more substances (elements and/or compounds) not chemically joined together.

How does the everyday use of the word 'pure' differ from its chemical meaning?

Everyday 'pure' means natural or with nothing harmful added; in chemistry it means a single element or compound only.

What does the melting point of a pure substance look like?

A sharp melting point — it melts at one fixed temperature.

True or false? A sample that melts between 52 °C and 58 °C is pure.

False. Melting over a range of temperatures shows it is a mixture.

True or false? Pure orange juice with no additives is a pure substance in the chemical sense.

False. It is a mixture of water, sugars, acids and other compounds.

Which separation technique separates an insoluble solid from a liquid?

Filtration.

Which separation technique separates two or more miscible liquids with different boiling points?

Fractional distillation.

Which separation technique is used to obtain the solvent from a solution?

Simple distillation.

What are the steps of simple distillation to obtain pure water from salt water?

e.g. separating water from sea water

1. Heat the solution: the water boils/evaporates to form steam, leaving the salt behind.
2. The steam passes into the condenser.
3. The steam condenses back to liquid and pure water is collected.

What are the steps to obtain crystals of a soluble solid from its solution?

e.g. copper sulfate crystals from copper sulfate solution

1. Heat the solution in an evaporating basin to concentrate it (do not heat to dryness).
2. Leave to cool so crystals form.
3. Filter off the crystals and dry them between filter papers.

In fractional distillation, which liquid is collected first?

The liquid with the lowest boiling point, which evaporates off first and leaves the top of the column.

True or false? Filtration can be used to separate salt from salt solution.

False. Dissolved salt passes through the filter paper; use crystallisation or distillation instead.

Which technique separates a mixture of soluble coloured substances such as dyes in ink?

Paper chromatography.

In paper chromatography, what is the mobile phase?

The solvent that moves through the paper.

In paper chromatography, what is the stationary phase?

The paper, which does not move.

Why do substances in a mixture end up at different heights on a chromatogram?

They move at different rates over the paper, depending on their attraction to the paper and solubility in the solvent.

On a chromatogram, how do you tell that a substance is pure?

It produces only one spot.

How is an unknown substance identified on a chromatogram using reference substances?

Its spot travels the same distance (same $R_f$) as the spot of a known reference substance run on the same paper.

What is the formula for the $R_f$ value?

$R_f = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent}}$

True or false? An $R_f$ value can be greater than 1.

False. The spot can never travel further than the solvent front, so $R_f$ is always between 0 and 1.

What are the steps to find the $R_f$ value of a spot on a chromatogram?

e.g. a spot 2 cm from the start line, solvent front 8 cm from the start line

1. Measure from the start (pencil) line to the centre of the spot: 2 cm.
2. Measure from the start line to the solvent front: 8 cm.
3. Divide: $R_f = 2 \div 8 = 0.25$

In the core practical on inks, which technique is used to separate the solvent from the ink?

Simple distillation.

In the core practical on inks, which technique is used to separate the coloured dyes in the ink?

Paper chromatography.

Why must the start line on a chromatogram be drawn in pencil?

Pencil is insoluble, so it will not dissolve and run up the paper with the solvent.

In the ink distillation, how can the apparatus be improved to collect more of the solvent?

Use a (Liebig) condenser or surround the collecting tube with iced water, to increase the effectiveness of cooling so all the vapour condenses.

True or false? An ink that produces three spots on a chromatogram is a pure substance.

False. Three spots show it is a mixture of three coloured dyes.

Why is a thermometer placed at the neck of the flask in the distillation of ink?

To measure the boiling point of the vapour, showing which solvent is distilling over.

What does 'potable water' mean?

Water that is safe to drink; it may still contain harmless dissolved impurities.

What is sedimentation in water treatment?

Insoluble impurities in the water settle to the bottom of the container.

Why is water filtered during water treatment?

To remove insoluble substances/solids, e.g. sand and grit.

Why is chlorine added to water during water treatment?

To kill bacteria/microorganisms.

How is sea water made potable, and why is this method needed?

By distillation — the dissolved salts cannot be removed by filtration, but water evaporates and condenses leaving the salts behind.

Why must water used in chemical analysis contain no dissolved salts?

Dissolved salts would react in the tests and give false positive results.

True or false? Chlorination removes the insoluble solids from waste water.

False. Chlorination kills bacteria; filtration removes insoluble solids.