The Objectives of this Website (SIOs)

Learning Objectives of Separation Techniques

(a) name appropriate apparatus for the measurement of time, temperature, mass and volume, including burettes, pipettes, measuring cylinders and gas syringes

(b) suggest suitable apparatus, given relevant information, for a variety of simple experiments, including collection of gases and measurement of rates of reaction

(c) describe methods of separation and purification for the components of the following types of mixtures:

(i) solid-solid

(ii) solid-liquid

(iii) liquid-liquid (miscible and immiscible)

Techniques to be covered for separation and purification include:

(i) use of a suitable solvent, filtration and crystallization or evaporation

(ii) sublimation

(iii) distillation and fractional distillation

(iv) use of a separating funnel

(v) paper chromatography

(d) describe paper chromatography and interpret chromatograms including comparison with ‘known’ samples and the use of R_f values

(e) explain the need to use locating agents in the chromatography of colourless compounds

(f) deduce from the given melting point and boiling point the identities of substances and their purity

(g) explain that the measurement of purity in substances used in everyday life, e.g. foodstuffs and drugs, is important

Quiz (try it before and after you have read through everything)

Laboratory apparatus for measurements

Stopwatch



There are 2 different types of stopwatches, digital stopwatches (top left) and analog stopwatches (top right). They are accurate to 0.01s, and are used to measure speed.


Thermometer


Laboratory Thermometers are usually mercury thermometers, capable of measuring temperatures to the accuracy of 0.1°C. Commonly used to measure boiling points of aqueous substances.


Electronic balance


The electronic scale is a device for the measurement of mass. It is accurate to 0.01 grams, and is used to obtain a specific mass of a reactant.


Burettes



burette is a vertical cylindrical laboratory glassware with a volumetric graduation on its full length and a precision tap at the bottom. It is used to dispense known amounts of a liquid reagent in experiments for which such precision is necessary, such as a titration experiment. Burettes are accurate to 0.1cm3.


Pipette


Pipettes allow the user to measure a volume of solution extremely accurately and then add it to something else. They are commonly used to make laboratory solutions from a base stock as well as prepare solutions for titration. They are typically marked to indicate one single volume in a particular size


Measuring Cylinder



The measuring cylinder is a piece of laboratory equipment used to measure liquids to the accuracy of 0.1cm3


Gas Syringe

A gas syringe is a piece of laboratory glassware used to insert or withdraw a volume of a gas from a closed system, or to measure the volume of gas evolved from a chemical reaction. A gas syringe can also be used to measure and dispense liquids, especially where these liquids need to be kept free from air.

Filtration

Separation of mixtures

Filtration

(i)What is it used for:

This is used to separate a liquid form a solid

(ii)What happens:

The solid is trapped by the filter paper and is called a residue. The liquid goes through the filter paper and is called filtrate.

(iii)How it works:

The filter paper has holes in it called pores. The particles of liquid are small enough to go through the pores, but the bits of solid are too large to go through.

(iv)Experiment: Separation of water and sand

Fold a piece of filter paper and place it in a filter funnel. Next, pour the mixture of sand and water into the filter funnel and have a beaker at the other end to collect the filtrate.

Result: Water is collected in the beaker as filtrate, while sand is trapped in the filter paper as residue.

(v)Notes:

-The dyes in ink go through filter paper because the dye molecules are smaller the pores of in the filter paper and thus go straight through them.

-Salt in sea water cannot be separated from the water by filtration as the sodium and chloride ions are much smaller than the pores in the filter paper.

(vi)Apparatus needed:

Filter funnel, filter paper, beaker

Crystallisation

Crystallisation

(i)What is it used for:

It is used to obtain a solid from a solution of the solid in a liquid. It is often used to obtain crystals of salt from solutions in water.

(ii)What happens:

The solution is put into an evaporating basin and heated. The solution boils. The liquid is boiled much of the water had evaporated (1/3 of liquid left).

The hot solution is then left to cool. Crystals form on cooling. The crystals are dried by pressing them between sheets of filter paper.

(iii)How it works:

The water is evaporated to make the hot solution saturated. This means it can no longer dissolve any extra solid.

(iv)Experiment: Obtaining dry copper(II) sulfate crystals from solution

Put aqueous copper(II) sulfate into an evaporating dish and heat till it boils. Boil the liquid till its saturated (about 1/3 of liquid left). Leave the solution to cool. Blue crystals form on cooling. Dry the crystals by pressing them between sheets of filter paper.

Result: Dry Copper(II) sulfate crystals are obtained.

(v)Notes:

-Some water must usually be left when aqueous solutions of salts are being crystallised as the crystals need water for crystallisation.

-Aqueous sodium chloride solution (salt solution) is unusual. The solution must be heated to evaporate all of the water before salt crystals appear.

(vi)Apparatus needed:

Evaporating dish, Bunsen burner

Simple distillation

Simple distillation

(i)What is it used for:

This is used to separate a liquid from a solution of a solid. The solid is called the solute and the liquid is called the solvent.

(ii)What happens:

The solution is heated in a round-bottomed flask. The solution boils. The solvent evaporates into vapour and rises. It is then cooled when passed through a condenser, which changes back to liquid, called the distillate.

(iii)How it works:

The solvent has a low boiling point and so is easily changed into a gas on heating. The solute has a high melting and boiling point and so does not boil and remains in the flask.

(iv)Experiment: Obtaining pure water from sea water

Heat the sea water in a round-bottomed flask. As the solution boils, water evaporates and become water vapour. The vapour rises and enters the condenser. It is cooled there and condenses into a liquid, and drips out of the other end into a beaker.

Result: Pure water is obtained as a distillate.

(v)Notes:

-The thermometer shows a constant temperature during the distillation as pure solvent is being distilled. Thus, this temperature = boiling point of solvent

-A thermometer is not essential in the apparatus. A cork can be used instead.

(vi)Apparatus needed:

Bunsen burner, round-bottomed flask, thermometer (optional), stoppers, condenser, beaker

Fractional distillation

Fractional distillation

(i)What is it used for:

This is used to separate a mixture of two or more liquids which mix completely with one another (they are said to be miscible).

(ii)What happens:

The mixture of liquids is heated in a flask. The mixture boils. The liquid with the lowest boiling point comes out on top of the fractionating column. It is then cooled by a condenser, which changes it back into liquid, called the distillate. After most of the first liquid has distilled, the liquid with the next lowest boiling point is then distilled.

(iii)How it works:

The liquids must have different boiling points. When the mixture of liquids boil, the fractionating column reduces the percentage of high boiling liquids in the lower boiling point liquids to near 0% using the repeated process of boiling and condensing. This ensures that the resulting distillate is almost pure.

(iv)Experiment: Separating ethanol and water

Heat the mixture of ethanol and water in a round-bottomed flask. The thermometer shows a constant temperature at 78°C. At this point, ethanol starts to boil into vapour, and is distilled. After most of the ethanol has been distilled, the temperature rises to 100°C, where it remains constant again. At this point, water boils into vapour and is distilled.

Result: The mixture has been separated to give pure water and (almost)pure ethanol.

(v)Notes:

-The thermometer shows a constant temperature during distillation as pure liquid is being distilled. This temperature = the boiling point of the liquid being distilled.

-When a mixture of two liquids is being fractionally distilled, the percentage of the higher boiling point liquid remaining in the flask increases.

(vi)Apparatus needed:

Bunsen burner, round-bottomed flask, fractionating column, stoppers, thermometer, condenser, beaker

Paper Chromatography

Paper Chromatography is an analytical chemistry technique for separating and identifying mixtures that are or can be coloured, especially pigments. This can also be used in secondary or primary colors in ink experiments.

Procedure of Paper Chromatography:

1. Draw a pencil line on a piece of chromatography paper about 2cm from the end of paper.

2. Put a tiny spot of sample on the pencil line. Allow the spot to dry. Repeat by dripping the sample on the same spot.

3. Place a chromatography paper inside a glass tank with a small volume of ethanol. Ensure the sample spot is about 1cm above the level of ethanol.

4. Remove the chromatogram when the solvent front reaches nearly to the top of the paper.

5. The components will travel at different rates over paper depending on their solubility in the solver or their ability to be adsorbed on the chromatography paper.

6. In general, when a component is very soluble in the solvent and is weakly adsorbed by the paper, the faster it would move with the solvent and appear further up the paper.

It will be something like this:

As for the R_f (Retention Factor) value, it is a measure of the relative distance travelled by a component with respect to the solvent. It is found by the distance moved by the spot of the substance over distance moved by the solvent font.

If the melting point and boiling point of a substance decrease and increase respectively, this means that the substance is impure. The purity of a substance is very important as it is not safe for human to consume unpure stuffs.

Evaporation to Dryness

Filtration

Sublimation

Simple Distillation

Fractional Distillation