Methods of Separating & Purifying Substances Flashcards

All 31 cards in this deck

In chemistry, what is a pure substance?

A single element or compound, not mixed with any other substance.
e.g. pure water contains only water molecules

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

Everyday 'pure' means natural or with nothing harmful added; chemically it means only one substance is present.

True or false? Bottled drinking water labelled 'pure' is a pure substance in the chemical sense.

False. It contains dissolved solids such as calcium ions, so it is a mixture (impure).

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

A pure substance melts at one sharp temperature; a mixture melts over a range of temperatures.

True or false? Air is a mixture but carbon dioxide is not.

True. Air is a mixture of substances, whereas carbon dioxide is a compound.

Which separation technique separates an insoluble solid from a liquid?

Filtration.
The solid stays on the filter paper as the residue; the liquid passing through is the filtrate.

What are the steps of simple distillation to obtain pure water from a solution?
e.g. separating water from salty water

1. Heat the solution in the flask so the water evaporates.
2. The vapour passes into the condenser and is cooled back to liquid.
3. Pure water is collected in a beaker; the dissolved solid stays in the flask.

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 until it is concentrated (crystals appear at the edge).
2. Leave it to cool so crystals form.
3. Separate the crystals and dry them between filter papers or in a warm oven.

What does fractional distillation separate, and on what property?

A mixture of liquids (miscible), separated by their different boiling points.

Why should the thermometer bulb be level with the side arm in simple distillation?

To measure the temperature (boiling point) of the vapour passing into the condenser.

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

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

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

The mobile phase is the solvent that moves through the paper; the stationary phase is the paper.

Why do substances end up in different places on a chromatogram?

They are carried by the solvent at different rates over the paper.

What is the formula for the $R_f$ value?

$R_f = \dfrac{\text{distance moved by the dye}}{\text{distance moved by solvent front}}$

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

False. A substance cannot travel further than the solvent front, so $R_f$ is never more than 1.

Which technique is used to separate the coloured substances (dyes) in an ink?

Paper chromatography.

Why is the baseline in chromatography drawn in pencil?

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

Why must the baseline be above the level of the solvent in the beaker?

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

In the inks core practical, which technique separates the water (solvent) from the ink?

Simple distillation.

What is potable water?

Water that is safe to drink; it may still contain dissolved impurities, as long as they are harmless.

In what order are the three main processes used to make waste or ground water potable?

Sedimentation → filtration → chlorination.

Why is chlorine added to water during water treatment?

To kill bacteria / microorganisms.

Why is water filtered during water treatment?

To remove insoluble substances / solids.

How is sea water made potable?

By (simple) distillation — the water is boiled off and the vapour condensed, leaving the dissolved salts behind.

Why is tap water not suitable for use in chemical analysis?

It is not pure — dissolved ions (e.g. calcium or chloride ions) are present and would interfere with the test.

On a chromatogram, what does the number of spots produced by an ink tell you?

The number of different dyes (coloured substances) in that ink.
e.g. an ink giving 5 spots contains 5 dyes.

How does a chromatogram show whether a substance is pure or impure?

A pure substance gives only one spot. An impure substance (a mixture) gives more than one spot.

How can paper chromatography be used to identify a substance in a mixture?

Run samples of known pure substances on the same paper next to the mixture. If a spot from the mixture moves the same distance (same $R_f$ value) as a known substance, it may be that substance.

In fractional distillation, which liquid in the mixture is collected first?

The liquid with the lowest boiling point.

Simple distillation only works well when the substances being separated have what kind of boiling points?

Very different boiling points.
Liquids with similar boiling points need fractional distillation instead.

What happens during sedimentation in water treatment?

The water is left to stand so insoluble particles sink to the bottom. Chemicals may also be added so that very fine particles stick together into bigger lumps, which then sink too.